Non-metals

VidyaarOctober 22, 20260262 views

Non-metals: Carbon and its forms, Hydrogen and its compounds, Sulphur, Nitrogen, Halogens, Inert Gases, and Acids, Bases and Salts.

180. When hydrochloric acid reacts with copper oxide, which product gives a blue-green solution?

[UPSC CDS — 2026]

(A) Copper hydrochloride

(B) Copper(II) chloride

(D) Copper(I) hydride

Answer: (B) Copper(II) chloride

Explanation: Copper(II) oxide reacts with hydrochloric acid to produce copper(II) chloride and water. Copper ions account for the blue-green appearance of the resulting solution. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

179. Why does an aqueous solution of an acid conduct electricity?

[UPSC CDS — 2026]

(A) It contains hydrogen ions

(B) It contains water molecules

(D) Electricity always passes through water

Answer: (A) It contains hydrogen ions

Explanation: Dissolved acid yields mobile ions, including hydrated hydrogen ions, which transport electric charge through water. Molecular water alone conducts extremely poorly. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

178. A chemical species capable of donating a proton is called what?

[UPSC CDS — 2026]

(A) Hydrophilic substance

(B) Protophilic substance

(D) Acid

Answer: (D) Acid

Explanation: A proton donor is a Brønsted–Lowry acid; its conjugate base is left after donation. A proton acceptor is described as a base. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

177. Which statements about bases and alkalis are correct?
1. All bases are water-soluble.
2. Water-soluble bases are alkalis.
3. Alkalis feel soapy and may be corrosive.
4. Diluting alkali can release heat.

[UPSC CDS — 2026]

(A) 1, 2, 3 and 4

(B) 2, 3 and 4 only

(D) 3 and 4 only

Answer: (B) 2, 3 and 4 only

Explanation: Some metal hydroxides are insoluble and therefore not alkalis. Soluble hydroxides give hydroxide ions; their dissolution or dilution may release considerable heat. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

176. Match the acid source: tomato, sour milk, ant sting and vinegar.

[UPSC CDS — 2026]

(A) Tomato–oxalic; milk–methanoic; sting–lactic; vinegar–acetic

(B) Tomato–oxalic; milk–lactic; sting–methanoic; vinegar–acetic

(D) Tomato–acetic; milk–lactic; sting–methanoic; vinegar–oxalic

Answer: (B) Tomato–oxalic; milk–lactic; sting–methanoic; vinegar–acetic

Explanation: The expected examination mapping links tomatoes with oxalic acid, sour milk with lactic acid, ant stings with methanoic acid and vinegar with acetic acid. Natural foods often contain several organic acids. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

175. Which are naturally occurring forms of calcium carbonate: marble, chalk and limestone?

[UPSC CDS — 2026]

(A) Marble and chalk only

(B) Chalk and limestone only

(D) All three

Answer: (D) All three

Explanation: Marble, chalk and limestone are all calcium-carbonate-rich materials, though their textures and geological origins differ. Marble is a metamorphosed limestone. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

174. What is the chemical formula of cooking soda (baking soda)?

[UPSC CDS — 2026]

(A) NaCl

(B) NaOCl

(D) NaHCO3

Answer: (D) NaHCO3

Explanation: Sodium hydrogen carbonate is baking soda, NaHCO3. It reacts with acids to release carbon dioxide, which helps raise bakery dough. The formulas distinguish these common salts and minerals, even when their trade names sound similar.

173. Which pairing of common name and formula is incorrect?

[UPSC CDS — 2026]

(A) Bleaching powder—CaOCl2

(B) Soda ash—Na2CO3

(D) Plaster of Paris—2CaSO4·H2O

Answer: (C) Baking powder—NaHCl2

Explanation: NaHCl2 is not the formula of baking powder. Baking powder is a mixture commonly containing sodium bicarbonate and one or more powdered acids. Water solubility and hydration are often as important as composition in their practical use.

172. An acid releasing H+ and a base releasing OH− in water reflects which acid-base concept?

[UPSC NDA — 2026]

(A) Brønsted–Lowry

(B) Lewis

(D) Conjugate pair

Answer: (C) Arrhenius

Explanation: The Arrhenius definition identifies acids by formation of hydrogen ions in aqueous medium and bases by formation of hydroxide ions. Broader theories address proton transfer and electron pairs. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

171. Which among AlCl3, NH3, BF3 and FeCl3 behave as Lewis acids; how many?

[UPSC NDA — 2026]

(A) One

(B) Two

(D) All four

Answer: (C) Three

Explanation: AlCl3, BF3 and FeCl3 can accept electron pairs, so three qualify as Lewis acids. Ammonia has a lone pair and commonly acts as a Lewis base. Nitrogen compounds are particularly important for agriculture because plants need nitrogen for protein synthesis.

170. When 2 mL of 0.1 N HCl is mixed with 4 mL of 0.1 N NaOH and phenolphthalein, what colour develops?

[UPSC NDA — 2026]

(A) Colourless

(B) Red

(D) Blue

Answer: (C) Pink

Explanation: Twice as many base equivalents as acid equivalents are present. The final solution contains excess hydroxide, turning phenolphthalein pink. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

169. Which statements are true?
I. Thirty percent H2O2 is sold as ten-volume peroxide.
II. All five waters in CuSO4·5H2O are hydrogen-bonded.

[UPSC NDA — 2026]

(A) I only

(B) II only

(D) Neither I nor II

Answer: (D) Neither I nor II

Explanation: Ten-volume peroxide is about 3% rather than 30%. In blue vitriol, four water molecules coordinate to copper while the fifth occupies a different environment. The elemental composition and oxidation state help distinguish related sulphur-containing chemicals.

168. Which are correct?
I. Nettle leaves can cause formic-acid stings.
II. Heating gypsum produces calcium carbonate hemihydrate.
III. Moistening anhydrous copper sulphate produces green dihydrate.

[UPSC NDA — 2026]

(A) I only

(B) III only

(D) I and II

Answer: (A) I only

Explanation: Nettle stings are associated with formic acid. Plaster of Paris is calcium sulphate hemihydrate, not calcium carbonate, and hydrated copper sulphate is blue rather than green. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

167. How is superphosphate of lime commonly prepared?

[UPSC NDA — 2026]

(A) Calcium phosphate + sodium hydroxide

(B) Calcium phosphate + sulphuric acid

(D) Calcium phosphate + nitric acid

Answer: (B) Calcium phosphate + sulphuric acid

Explanation: Treatment of phosphate rock with sulphuric acid produces superphosphate fertiliser. The process converts otherwise poorly soluble calcium phosphates into more plant-available forms. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

166. Which statements about nitrolim (calcium cyanamide) are correct?
1. Made from calcium carbide and nitrogen.
2. Produces ammonia in soil.
3. An NPK fertiliser.
4. Organic fertiliser.

[UPSC NDA — 2026]

(A) 1 and 2 only

(B) 1, 2 and 3

(D) 1, 2 and 4

Answer: (A) 1 and 2 only

Explanation: Calcium cyanamide is produced from calcium carbide and nitrogen. On conversion in moist soil, it provides nitrogen; it is not a balanced NPK or organic fertiliser. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.

165. Which statement about soap micelles is NOT correct?

[UPSC CDS — 2026]

(A) Soap molecules are salts of long-chain fatty acids

(B) Micelles precipitate because of ion-ion repulsion

(D) Soap micelles can scatter light

Answer: (B) Micelles precipitate because of ion-ion repulsion

Explanation: Micelles are colloidal aggregates with hydrophobic tails inward and hydrophilic charged heads outward. Electrostatic repulsion between charged headgroups helps keep micelles dispersed rather than causing precipitation. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

164. Which pollutants are most responsible for acid rain?
a. Ozone;
b. Sulphur dioxide;
c. Carbon monoxide;
d. Nitrogen dioxide.

[MPPSC AE — 2026]

(A) a, b and d

(B) b, c and d

(D) a, b and c

Answer: (C) b and d

Explanation: Sulphur dioxide and nitrogen dioxide produce sulphuric and nitric acids through atmospheric reactions with moisture and oxidants. Ozone and carbon monoxide are not the chief acid-rain precursors in this choice. The resulting acids may also accelerate corrosion of buildings and damage freshwater ecosystems.

163. Mendeleev could not assign an unambiguous position to which element in his periodic table?

[SSC MTS (2025 cycle) — 2026]

(A) Helium

(B) Hydrogen

(D) Chlorine

Answer: (B) Hydrogen

Explanation: Hydrogen resembles group-one alkali metals by its single valence electron and resembles halogens through molecular and chemical behaviour. This duality complicated its placement in the early periodic table. Halogens lie in group 17 and tend to gain electrons when reacting with metals.

162. In CuO + H2 → Cu + H2O, which species undergoes oxidation?

[RRB Group D (2025 cycle) — 2026]

(A) Copper(II) oxide

(B) Water

(D) Copper

Answer: (C) Hydrogen

Explanation: Hydrogen gains oxygen to produce water and is therefore oxidised. Copper(II) oxide loses oxygen to yield copper metal and is reduced in the same redox reaction. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

161. Statements:
I. Alkaline KMnO4 oxidises ethanol into ethanoic acid.
II. Oxidation of ethanol includes removal of hydrogen or addition of oxygen.

Which is correct?

[RRB Group D (2025 cycle) — 2026]

(A) Both correct; II explains I

(B) Both correct; II does not explain I

(D) Only I correct

Answer: (A) Both correct; II explains I

Explanation: Potassium permanganate is a strong oxidising agent that converts a primary alcohol such as ethanol to a carboxylic acid. The oxidation increases bonds to oxygen and removes hydrogen equivalents. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

160. Which air pollutant is most closely linked to the formation of acid rain?

[RRB NTPC — 2026]

(A) Ammonia

(B) Sulphur dioxide

(D) Methane

Answer: (B) Sulphur dioxide

Explanation: Sulphur dioxide released by burning sulphur-bearing fuels undergoes oxidation in air and can form sulphuric acid in precipitation. Nitrogen oxides provide another major route to acid deposition. Both damage sensitive freshwater systems. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.

159. Which set consists entirely of Lewis acids?

[UPSC CDS — 2025]

(A) AlCl3, Co³+, BF3

(B) BCl3, NH3, OH−

(D) BF3, NH3, H2O

Answer: (A) AlCl3, Co³+, BF3

Explanation: Lewis acids accept electron pairs. Aluminium chloride, boron trifluoride and highly charged metal ions can accept electron density; ammonia and hydroxide commonly donate electron pairs. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.

158. An aqueous solution with pH below 7 is classified as what?

[UPSC CDS — 2025]

(A) Neutral

(B) Basic

(D) Hypotonic

Answer: (C) Acidic

Explanation: A pH below seven indicates acid character at approximately 25°C. Lower pH generally means greater hydrogen-ion activity; pH seven represents neutral water at that temperature. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

157. Which oxide is classified as neutral?

[UPSC NDA — 2025]

(A) CO

(B) CO2

(D) MgO

Answer: (A) CO

Explanation: Carbon monoxide is a neutral oxide in ordinary acid-base classification. Carbon dioxide is acidic; sodium and magnesium oxides are typically basic. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

156. Which statement about pH is incorrect?

[UPSC NDA — 2025]

(A) The pH scale reflects hydrogen-ion activity

(B) Higher hydrogen-ion concentration means higher pH

(D) The p relates historically to power

Answer: (B) Higher hydrogen-ion concentration means higher pH

Explanation: pH decreases when hydrogen-ion concentration increases because it is defined by a negative logarithm. More acidic solutions therefore have smaller pH values. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

155. Rain is termed acid rain when its pH falls below roughly what value?

[UPSC NDA — 2025]

(A) 7.0

(B) 6.6

(D) 8.0

Answer: (C) 5.6

Explanation: Natural rainfall is mildly acidic through dissolved carbon dioxide. A pH below approximately 5.6 usually indicates additional acid deposition from pollutants. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.

154. Match washing soda, baking soda, bleaching powder and gypsum to their formulas.

[UPSC NDA — 2025]

(A) CaSO4·2H2O, NaHCO3, CaOCl2, Na2CO3·10H2O

(B) CaSO4·2H2O, CaOCl2, NaHCO3, Na2CO3·10H2O

(D) Na2CO3·10H2O, NaHCO3, CaOCl2, CaSO4·2H2O

Answer: (D) Na2CO3·10H2O, NaHCO3, CaOCl2, CaSO4·2H2O

Explanation: The corresponding formulas are washing soda Na2CO3·10H2O, baking soda NaHCO3, bleaching powder conventionally CaOCl2 and gypsum CaSO4·2H2O. The formulas distinguish these common salts and minerals, even when their trade names sound similar.

153. Which of these mixtures is called water gas?

[UPSC NDA — 2025]

(A) H2O + CO2

(B) NH3

(D) CO + H2

Answer: (D) CO + H2

Explanation: The combination CO and H2 is known as water gas. It differs from producer gas, which contains appreciable nitrogen. Nitrogen compounds are particularly important for agriculture because plants need nitrogen for protein synthesis.

152. Which statement does NOT describe graphene?

[UPSC CDS — 2025]

(A) It is extremely thin

(B) It is nearly transparent

(D) It is a wide-band-gap semiconductor

Answer: (D) It is a wide-band-gap semiconductor

Explanation: Graphene consists of a single atomic layer of carbon in a hexagonal arrangement. It is transparent and highly conductive, but pristine graphene has essentially zero electronic band gap. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

151. Arrange HF, HCl, HBr and HI in increasing acid strength.

[RPSC Senior Teacher (Science) — 2025]

(A) HBr < HI < HCl < HF

(B) HF < HCl < HBr < HI

(D) HF < HBr < HCl < HI

Answer: (B) HF < HCl < HBr < HI

Explanation: For hydrogen halides in water, acidity increases down the group because the hydrogen-halogen bond weakens. Hydrogen fluoride’s particularly strong bond makes it the weakest acid of these four. The chemical properties of group-17 elements follow from their seven outer-shell electrons.

150. Which statements about phosphorus oxyacids are true?
I. H3PO4 is tribasic.
II. All three hydrogens in H3PO3 are ionisable.
III. Orthophosphate salts are used in fertilisers.

[UPPSC LT Grade (Science) — 2025]

(A) I and II

(B) II and III

(D) I, II and III

Answer: (C) I and III

Explanation: H3PO4 has three hydroxyl groups and is tribasic. H3PO3 has only two ionisable hydroxyl hydrogens because its third is bonded directly to phosphorus. Phosphate salts are important fertiliser ingredients. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

149. Which reaction demonstrates acidic behaviour of ethanoic acid?

[RRB Group D — 2025]

(A) Ethanoic acid + ethanol → ester + water

(B) Ethanoic acid + sodium → sodium ethanoate + hydrogen

(D) Ethanoic acid + methanol → methanoic acid + ethane

Answer: (C) Ethanoic acid + sodium hydroxide → sodium ethanoate + water

Explanation: The selected acid-base neutralisation forms sodium ethanoate and water from ethanoic acid and sodium hydroxide. Neutralisation exemplifies reaction of an acid with a base. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

148. Which gas is released when baking soda reacts with vinegar?

[RRB NTPC — 2025]

(A) Carbon dioxide

(B) Carbon monoxide

(D) Hydrogen

Answer: (A) Carbon dioxide

Explanation: Sodium hydrogen carbonate reacts with acetic acid in vinegar to form sodium acetate, water and carbon dioxide. The escaping carbon dioxide produces visible fizzing. Water solubility and hydration are often as important as composition in their practical use.

147. Carbonylation of methanol is an industrial preparation route for which acid?

[RRB NTPC — 2025]

(A) Nitric acid

(B) Citric acid

(D) Hydrochloric acid

Answer: (C) Acetic acid

Explanation: Methanol reacts with carbon monoxide under catalytic conditions to form acetic acid. This carbonylation route is a major industrial production method for ethanoic acid. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

146. Which industrial chemical is manufactured using the Contact Process?

[RRB NTPC — 2025]

(A) Ammonia

(B) Nitric acid

(D) Sulphuric acid

Answer: (D) Sulphuric acid

Explanation: In the Contact Process, sulphur dioxide is catalytically oxidised to sulphur trioxide, commonly with vanadium(V) oxide. Further processing produces concentrated sulphuric acid, an important industrial reagent. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

145. Acid rain is chiefly caused by which two atmospheric gases?

[RRB NTPC — 2025]

(A) Oxygen and hydrogen

(B) Carbon dioxide and carbon monoxide

(D) Sulphur dioxide and nitrogen oxides

Answer: (D) Sulphur dioxide and nitrogen oxides

Explanation: Sulphur dioxide and nitrogen oxides react with atmospheric oxidants and moisture to form sulphuric and nitric acids. Their acid deposition harms soil, lakes, vegetation and exposed buildings. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.

144. Which of these gases is monatomic under normal conditions?

[SSC CGL (2024 cycle, Tier II) — 2025]

(A) Chlorine

(B) Hydrogen

(D) Oxygen

Answer: (C) Xenon

Explanation: Xenon is a group-18 noble gas existing as individual atoms in ordinary gas conditions. Chlorine, hydrogen and oxygen occur as diatomic molecules, so xenon is the only monatomic choice. These group-18 gases usually exist as single atoms rather than bonded diatomic molecules.

143. Which is an amphoteric oxide?

[UPSC CDS — 2024]

(A) MgO

(B) P4O10

(D) Al2O3

Answer: (D) Al2O3

Explanation: Aluminium oxide reacts with both strong acids and alkalis, demonstrating amphoterism. Magnesium oxide and sodium oxide are basic, while phosphorus oxide is acidic. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

142. Vinegar is most accurately described as what?

[UPSC CDS — 2024]

(A) About 5–8% acetic acid in water

(B) About 5–8% carbonic acid in water

(D) About 10–15% propionic acid in water

Answer: (A) About 5–8% acetic acid in water

Explanation: Ordinary vinegar is a dilute aqueous solution of acetic acid, often roughly five to eight percent. Its acidity gives vinegar its characteristic sour taste. Organic acids share acidic behaviour but differ in molecular structure, biological origins and uses.

141. Match bleaching powder, baking soda, washing soda and plaster of Paris to their chemical names.

[UPSC CDS — 2024]

(A) Calcium oxychloride, sodium bicarbonate, sodium carbonate decahydrate, calcium sulphate hemihydrate

(B) Calcium oxychloride, sodium carbonate, sodium bicarbonate, calcium sulphate hemihydrate

(D) Calcium sulphate, sodium bicarbonate, sodium carbonate, calcium oxychloride

Answer: (A) Calcium oxychloride, sodium bicarbonate, sodium carbonate decahydrate, calcium sulphate hemihydrate

Explanation: The named salts have established compositions: bleaching powder is conventionally CaOCl2, baking soda NaHCO3, washing soda Na2CO3·10H2O, and plaster of Paris CaSO4·½H2O. Water solubility and hydration are often as important as composition in their practical use.

140. In what approximate pH range can the human body function?

[UPSC NDA — 2024]

(A) 6.8–7.2

(B) 7.0–7.8

(D) 7.5–8.0

Answer: (B) 7.0–7.8

Explanation: The exam cites a broad physiological range around neutrality and mild alkalinity. In particular, healthy arterial blood is regulated much more tightly, typically about pH 7.35–7.45. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

139. Milk of magnesia consists mainly of what?

[UPSC NDA — 2024]

(A) Magnesium bicarbonate

(B) Magnesium carbonate

(D) Magnesium hydroxide

Answer: (D) Magnesium hydroxide

Explanation: Milk of magnesia is an aqueous suspension of Mg(OH)2. It acts as a mild antacid and, in higher doses, a laxative. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

138. Which straight-chain alkane has molecular formula C9H20?

[SSC CGL — 2024]

(A) Nonane

(B) Nonadecane

(D) Icosane

Answer: (A) Nonane

Explanation: An acyclic saturated alkane has formula CnH2n+2. For nine carbon atoms, that becomes C9H20, called nonane. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

137. Which is a saturated hydrocarbon?

[SSC CGL — 2024]

(A) C2H4

(B) C3H8

(D) C4H8

Answer: (B) C3H8

Explanation: Propane, C3H8, contains only carbon–carbon single bonds and fits the alkane formula CnH2n+2. The others contain unsaturation or aromatic bonding. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

136. Which researchers described the peroxide effect in anti-Markovnikov addition in 1933?

[SSC CGL — 2024]

(A) Liebig and Wöhler

(B) Kharasch and Mayo

(D) Cadogan and Campos

Answer: (B) Kharasch and Mayo

Explanation: Morris Kharasch and Frank Mayo studied radical addition reactions affected by peroxides. In suitable conditions HBr can add to alkenes contrary to Markovnikov orientation. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

135. Which of these carboxylic acids is strongest?

[SSC CGL — 2024]

(A) C3H7COOH

(B) CH3COOH

(D) HCOOH

Answer: (D) HCOOH

Explanation: Formic acid is stronger than the listed alkyl-substituted carboxylic acids because alkyl electron donation weakens acidity. It lacks an electron-donating alkyl group. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

134. Which carbon allotrope was discovered by Curl, Kroto and Smalley in 1985?

[SSC CGL — 2024]

(A) Lonsdaleite

(B) Fullerene

(D) Graphene

Answer: (B) Fullerene

Explanation: The researchers reported buckminsterfullerene C60, a cage-like carbon allotrope, in 1985. The discovery opened extensive fullerene and nanocarbon research. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

133. Palladium deposited on barium sulphate is associated with which named catalyst?

[SSC CGL — 2024]

(A) Aldol catalyst

(B) Cannizzaro catalyst

(D) Hillman catalyst

Answer: (C) Rosenmund catalyst

Explanation: The Rosenmund catalyst is poisoned palladium supported on barium sulphate. It is used to hydrogenate acyl chlorides selectively to aldehydes. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

132. Which carboxylic acid has the formula CH3CH2CH2COOH?

[SSC CGL — 2024]

(A) Methanoic acid

(B) Ethanoic acid

(D) Butyric acid

Answer: (D) Butyric acid

Explanation: The four-carbon straight-chain carboxylic acid is butanoic acid, traditionally butyric acid. It contains one terminal carboxyl group. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

131. Water gas contains which pair of gases?

[UPSC NDA — 2024]

(A) CO2 and H2

(B) CO and H2

(D) CO and H2O

Answer: (B) CO and H2

Explanation: Water gas is mainly carbon monoxide mixed with hydrogen. It is produced by reacting steam with hot carbon, creating a combustible gaseous mixture. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

130. Which fertiliser among these primarily supplies nitrogen?

[UPSC NDA — 2024]

(A) Superphosphate

(B) Potassium sulphate

(D) Urea

Answer: (D) Urea

Explanation: Urea is nitrogen-rich, containing approximately 46% nitrogen by mass. Superphosphate primarily supplies phosphorus, whereas potassium sulphate supplies potassium. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

129. Which combination catalyses hydrogen addition to an unsaturated hydrocarbon?

[UPSC CDS — 2024]

(A) Copper and hydrogen

(B) Iron and hydrogen

(D) Nickel and hydrogen

Answer: (D) Nickel and hydrogen

Explanation: Nickel acts as a catalyst during hydrogenation of unsaturated carbon-carbon bonds. This reaction adds hydrogen across the double or triple bond and produces a more saturated substance. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

128. Which statement is correct about chlorine and fluorine?

[SSC MTS — 2024]

(A) Chlorine has highest electron affinity; fluorine highest electronegativity

(B) Fluorine has highest electron affinity; chlorine highest electronegativity

(D) Fluorine has highest electron affinity; bromine highest electronegativity

Answer: (A) Chlorine has highest electron affinity; fluorine highest electronegativity

Explanation: Fluorine attracts shared bonding electrons most strongly and is the most electronegative element. Chlorine releases slightly more energy on gaining an electron, owing partly to reduced crowding in its larger outer orbital. The chemical properties of group-17 elements follow from their seven outer-shell electrons.

127. 126. Which noble gas produces a characteristic red-orange glow when electricity passes through it?

[SSC CGL — 2024]

(A) Neon

(B) Hydrogen

(D) Helium

Answer: (A) Neon

Explanation: An electric discharge excites neon atoms, whose returning electrons emit strong red-orange spectral lines. Neon is a noble gas and is used in illuminated signs because of its distinctive colour. These group-18 gases usually exist as single atoms rather than bonded diatomic molecules.

125. Which of the following is NOT considered an important greenhouse gas?

[SSC CGL — 2024]

(A) Methane

(B) Carbon dioxide

(D) Nitric oxide

Answer: (D) Nitric oxide

Explanation: Carbon dioxide, methane and nitrous oxide absorb infrared radiation and contribute significantly to greenhouse warming. Nitric oxide is a reactive atmospheric gas, but it is not classified as a major greenhouse gas. Atmospheric gases differ in how strongly they absorb infrared wavelengths emitted by Earth.

124. Which chemical symbol represents an element that is NOT a halogen?

[SSC CGL — 2024]

(A) F

(B) He

(D) Br

Answer: (B) He

Explanation: Helium belongs to group 18, the noble gases. Fluorine, chlorine and bromine belong to group 17, the halogens. Halogens are generally reactive, whereas helium is exceptionally unreactive. These group-18 gases usually exist as single atoms rather than bonded diatomic molecules.

123. What are the elements of group 17 in the periodic table called?

[SSC MTS — 2024]

(A) Noble gases

(B) Alkali metals

(D) Transition metals

Answer: (C) Halogens

Explanation: The group-17 elements are halogens, including fluorine, chlorine, bromine and iodine. They commonly gain one electron to form halide ions and react with metals to form salts. The chemical properties of group-17 elements follow from their seven outer-shell electrons.

122. When diluting concentrated nitric acid, which is correct?
1. Hydronium concentration increases.
2. Water is slowly added to acid.
3. Acid is slowly added to water.

[UPSC CDS — 2023]

(A) 1, 2 and 3

(B) 1 and 2 only

(D) 2 and 3 only

Answer: (C) 3 only

Explanation: Acid must be added slowly to water with stirring because dilution is strongly exothermic. Adding water to concentrated acid may cause violent splashing; dilution reduces hydronium concentration. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

121. Arrange these substances in increasing pH.

[UPSC CDS — 2023]

(A) Coffee < Lemon juice < Milk of magnesia < Blood

(B) Milk of magnesia < Blood < Coffee < Lemon juice

(D) Lemon juice < Coffee < Blood < Milk of magnesia

Answer: (D) Lemon juice < Coffee < Blood < Milk of magnesia

Explanation: Lemon juice is strongly acidic, coffee is mildly acidic, blood is slightly alkaline and milk of magnesia is more alkaline. Increasing pH follows that order. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

120. Which statement about commonly occurring acids is incorrect?

[UPSC CDS — 2023]

(A) Gastric juice contains hydrochloric acid

(B) Vinegar contains acetic acid

(D) Citrus fruits supply citric and ascorbic acids

Answer: (C) Tamarind paste is known mainly for oxalic acid

Explanation: Tamarind is notably rich in tartaric acid rather than oxalic acid. The other statements correctly associate stomach juice, vinegar and citrus fruits with characteristic acids. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

119. Which acid may a goldsmith use for cleaning gold and silver objects?

[UPSC NDA — 2023]

(A) Sulphuric acid

(B) Nitric acid

(D) Phosphoric acid

Answer: (B) Nitric acid

Explanation: Nitric acid is used in metal finishing and can dissolve silver and many base metals, while elemental gold resists nitric acid alone. Controlled treatments require specialist precautions. Solubility and the formation of mobile ions explain several familiar reactions of salts.

118. Why do copper sulphate crystals lose their blue colour when heated?

[UPSC NDA — 2023]

(A) Loss of oxygen

(B) Loss of nitrogen

(D) Loss of hydrogen

Answer: (C) Loss of water

Explanation: Hydrated copper sulphate is blue, while anhydrous copper sulphate is nearly white. Heating removes coordinated and lattice water and changes the environment of copper ions. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

117. Which compound is also known as olefiant gas?

[SSC CGL — 2023]

(A) Ethylene

(B) 1-Pentene

(D) Butane

Answer: (A) Ethylene

Explanation: Olefiant gas is ethene, commonly called ethylene, with formula C2H4. It has a double bond and is an important industrial building block. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

116. Organic compounds with an oxygen atom between two alkyl groups belong to which class?

[SSC CGL — 2023]

(A) Alcohols

(B) Aldehydes

(D) Ethers

Answer: (D) Ethers

Explanation: An ether has the general structure R–O–R′. In contrast, alcohols contain an –OH group, aldehydes a terminal carbonyl and ketones an internal carbonyl. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

115. Which boiling-point statements are correct?
I. Chloroform boils around 334 K.
II. Methane boils around 111 K.

[SSC CGL — 2023]

(A) Only I

(B) Neither

(D) Both I and II

Answer: (D) Both I and II

Explanation: Chloroform boils near 61°C, around 334 K, and methane near −162°C, about 111 K. The large difference reflects molecular mass and intermolecular interactions. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

114. What is the molecular formula of propene?

[SSC CGL — 2023]

(A) C2H4

(B) C3H4

(D) C2H2

Answer: (C) C3H6

Explanation: Propene contains three carbon atoms and one carbon–carbon double bond. Acyclic alkenes follow formula CnH2n, giving C3H6. Carbon forms stable covalent bonds, including single and multiple bonds, across many molecular structures.

113. Complete burning of ethanol in oxygen primarily produces which products?

[SSC CGL — 2023]

(A) Water and heat alone

(B) Carbon dioxide, water and energy

(D) Carbon dioxide alone

Answer: (B) Carbon dioxide, water and energy

Explanation: Ethanol combustion is exothermic: C2H5OH + 3O2 → 2CO2 + 3H2O. The reaction releases heat and may emit light in a flame. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

112. Unsaturated hydrocarbons commonly burn with which flame colour under smoky conditions?

[SSC CGL — 2023]

(A) Blue

(B) Red

(D) Yellow

Answer: (D) Yellow

Explanation: Unsaturated hydrocarbons often burn with a luminous yellow sooty flame when oxygen supply is limited. Glowing carbon particles give the yellow colour. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

111. Which industrial process is widely used for manufacturing ammonia?

[UPSC NDA — 2023]

(A) Ostwald process

(B) Haber–Bosch process

(D) Electrolysis process

Answer: (B) Haber–Bosch process

Explanation: The Haber–Bosch process reacts nitrogen and hydrogen under pressure over an iron-based catalyst. The product ammonia is fundamental to nitrogen fertiliser production. Nitrogen compounds are particularly important for agriculture because plants need nitrogen for protein synthesis.

110. What chemical change can quickly fill certain vehicle airbags with gas?

[UPSC NDA — 2023]

(A) Decomposition of sodium azide to nitrogen

(B) Sublimation of dry ice

(D) Conversion of CO2 to carbon monoxide

Answer: (A) Decomposition of sodium azide to nitrogen

Explanation: Traditional airbag inflators used rapid sodium azide decomposition to generate nitrogen gas. The released gas expands the bag within milliseconds after impact. Nitrogen compounds are particularly important for agriculture because plants need nitrogen for protein synthesis.

109. Which statement correctly compares diamond with graphite?

[UPSC CDS — 2023]

(A) They have similar physical properties

(B) Diamond is hard, whereas graphite is slippery

(D) They possess the same crystal structure

Answer: (B) Diamond is hard, whereas graphite is slippery

Explanation: Although both are carbon allotropes, diamond has a rigid tetrahedral network and is exceptionally hard. Graphite has layered hexagonal sheets that slide over each other readily. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

108. Which chemical is used to disinfect drinking water?

[SSC MTS (2022 cycle) — 2023]

(A) Chlorine

(B) Ethanol

(D) Teflon

Answer: (A) Chlorine

Explanation: Chlorination introduces chlorine-derived disinfecting species, particularly hypochlorous acid, into water. They inactivate many disease-causing microorganisms and can leave a protective residual in distribution systems. Halogens lie in group 17 and tend to gain electrons when reacting with metals.

107. Which compound is commonly used as an antacid for stomach acidity?

[SSC MTS (2022 cycle) — 2023]

(A) Iodoform

(B) Morphine

(D) Magnesium hydroxide

Answer: (D) Magnesium hydroxide

Explanation: Magnesium hydroxide, also called milk of magnesia, reacts with excess stomach hydrochloric acid to form magnesium chloride and water. It is a mild base used for acid neutralisation. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

106. Which statement best describes the behavior of oxides of non-metals with water?

[UPSC CDS — 2022]

(A) They produce basic oxides

(B) They generally give acidic solutions

(D) They always form hydroxides

Answer: (B) They generally give acidic solutions

Explanation: Many non-metal oxides, such as carbon dioxide and sulphur dioxide, yield acids when dissolved in water. The rule has exceptions for neutral oxides like carbon monoxide. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

105. Milk of magnesia, used for indigestion, is a what?

[UPSC CDS — 2022]

(A) Strong base

(B) Mild base

(D) Mild acid

Answer: (B) Mild base

Explanation: Milk of magnesia is a suspension of magnesium hydroxide, a sparingly soluble base. It neutralises excess gastric acid and therefore serves as an antacid. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

104. Litmus indicator is obtained from which organism?

[UPSC CDS — 2022]

(A) Protozoa

(B) Virus

(D) Saccharomyces

Answer: (C) Lichen

Explanation: Litmus is a mixture of natural dyes obtained from certain lichens. In acid it turns red; in alkaline solution it becomes blue. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

103. The fluid injected by stinging nettle hairs is conventionally identified as what?

[UPSC CDS — 2022]

(A) Methanoic acid

(B) Tartaric acid

(D) Sulphuric acid

Answer: (A) Methanoic acid

Explanation: Methanoic or formic acid is the expected school-level answer. Nettle stinging hairs inject irritating compounds into skin and produce a sharp burning sensation. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

102. Methanoic acid is normally associated with which source?

[UPSC CDS — 2022]

(A) Muscles

(B) Urine

(D) Human brain

Answer: (C) Ant stings

Explanation: Methanoic acid is named formic acid from the Latin word for ant. It is associated with the defensive secretions of certain ants. Organic acids share acidic behaviour but differ in molecular structure, biological origins and uses.

101. Which mineral acid is present in human gastric juice?

[UPSC CDS — 2022]

(A) Hydrochloric acid

(B) Lactic acid

(D) Methanoic acid

Answer: (A) Hydrochloric acid

Explanation: Gastric juice contains hydrochloric acid secreted by stomach cells. The acidic medium activates pepsinogen, supports protein digestion and restricts many harmful microorganisms. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

100. Which of the following aqueous solutions cannot conduct electricity appreciably?

[UPSC NDA — 2022]

(A) Copper sulphate

(B) Sodium chloride

(D) Sodium hydroxide

Answer: (C) Sugar

Explanation: Sugar dissolves as neutral molecules and provides almost no charge carriers. The other solutes release mobile ions and behave as electrolytes. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

99. Which acid is usually cited as responsible for an acidic bee sting?

[UPSC NDA — 2022]

(A) Tartaric acid

(B) Citric acid

(D) Methanoic acid

Answer: (D) Methanoic acid

Explanation: Methanoic acid, or formic acid, is the traditional school-text answer. Actual bee venom is a complex mixture including peptides and enzymes. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

98. Which salt lacks water of crystallisation?

[UPSC NDA — 2022]

(A) Potassium permanganate

(B) Blue vitriol

(D) Mohr’s salt

Answer: (A) Potassium permanganate

Explanation: Potassium permanganate KMnO4 crystallises without fixed water of crystallisation. Blue vitriol, washing soda and Mohr’s salt are well-known hydrated crystalline salts. The elemental composition and oxidation state help distinguish related sulphur-containing chemicals.

97. Which statement about urea fertiliser is incorrect?

[UPSC NDA — 2022]

(A) It is a phosphorus fertiliser

(B) It can improve soil fertility

(D) It has high nitrogen content

Answer: (A) It is a phosphorus fertiliser

Explanation: Urea, CO(NH2)2, is a concentrated nitrogen fertiliser, not a phosphorus source. It is produced industrially using ammonia and carbon dioxide. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

96. Which kind of hydrocarbon burns with a yellow, sooty flame more readily?

[UPSC CDS — 2022]

(A) Saturated hydrocarbon

(B) Unsaturated hydrocarbon

(D) Only water-containing fuel

Answer: (B) Unsaturated hydrocarbon

Explanation: Unsaturated hydrocarbons generally have a higher carbon-to-hydrogen ratio and may burn with a luminous smoky flame if oxygen is insufficient. Soot consists of incompletely combusted carbon. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

95. What forms when calcium carbonate reacts with dissolved carbon dioxide and water?

[SSC CGL — 2022]

(A) Calcium oxide

(B) Calcium hydroxide

(D) Calcium hydrogen carbonate

Answer: (D) Calcium hydrogen carbonate

Explanation: Calcium carbonate dissolves more readily in carbon-dioxide-rich water through formation of aqueous calcium hydrogen carbonate, Ca(HCO3)2. This equilibrium contributes to temporary hardness of natural water. Water solubility and hydration are often as important as composition in their practical use.

94. Which statements are correct?
1. All bases are alkalis.
2. Alkalis dissolve in water.
3. Alkalis may be slippery, bitter and corrosive.

[UPSC CDS — 2021]

(A) 1 only

(B) 1 and 3 only

(D) 3 only

Answer: (C) 2 and 3 only

Explanation: Alkalis are bases soluble in water, so every alkali is a base but not every base is an alkali. Concentrated alkali solutions are chemically hazardous. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

93. Which statements about universal indicator are correct?
1. It mixes several indicators.
2. Its color varies with hydrogen ion concentration.
3. It determines exact titration end points for every acid-base system.

[UPSC CDS — 2021]

(A) 1 only

(B) 1 and 2 only

(D) 1, 2 and 3

Answer: (B) 1 and 2 only

Explanation: Universal indicator combines dyes to display a range of pH values. It supplies approximate acidity or alkalinity but does not replace a suitable titration indicator for precise end-point detection. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

92. Which acid is associated with the stinging hairs of nettles?

[UPSC CDS — 2021]

(A) Methanoic acid

(B) Citric acid

(D) Acetic acid

Answer: (A) Methanoic acid

Explanation: Methanoic acid is also called formic acid and is traditionally associated with nettle stings. The irritation results from chemicals delivered by the hairs into the skin. Organic acids share acidic behaviour but differ in molecular structure, biological origins and uses.

91. Which naturally occurring acid is commonly associated with tomatoes?

[UPSC CDS — 2021]

(A) Acetic acid

(B) Citric acid

(D) Tartaric acid

Answer: (C) Oxalic acid

Explanation: This exam associates tomatoes with oxalic acid. Tomatoes also contain substantial citric and malic acids, so the particular exam’s comparison matters. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

90. Which compound is used in a soda-acid fire extinguisher?

[UPSC CDS — 2021]

(A) Sodium chloride

(B) Sodium hydrogen carbonate

(D) Acetic acid

Answer: (B) Sodium hydrogen carbonate

Explanation: Sodium hydrogen carbonate reacts with acid to liberate carbon dioxide. The gas can displace oxygen near flames and assists extinguishing suitable fires. The formulas distinguish these common salts and minerals, even when their trade names sound similar.

89. Which formula corresponds to hypobromous acid?

[UPSC NDA — 2021]

(A) HBrO4

(B) HOBr

(D) HBrO3

Answer: (B) HOBr

Explanation: Hypobromous acid contains bromine in oxidation state +1 and has formula HOBr. It is the bromine analogue of hypochlorous acid. The chemical properties of group-17 elements follow from their seven outer-shell electrons.

88. Which of these is the acid typically associated with tomatoes in basic chemistry?

[UPSC NDA — 2021]

(A) Acetic acid

(B) Tartaric acid

(D) Lactic acid

Answer: (C) Oxalic acid

Explanation: The intended examination answer is oxalic acid. Real tomato acidity also arises importantly from citric and malic acids, so the question reflects textbook conventions. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

87. Approximately what is the pH of milk of magnesia?

[UPSC NDA — 2021]

(A) 0

(B) 7

(D) 14

Answer: (C) 10

Explanation: Magnesium hydroxide suspension is alkaline with pH roughly ten. It reduces acidity in the stomach and is not a neutral solution. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

86. Which is not conventionally manufactured using common salt as a raw material?

[UPSC NDA — 2021]

(A) Bleaching powder

(B) Baking soda

(D) Washing soda

Answer: (C) Plaster of Paris

Explanation: Plaster of Paris derives from gypsum, a calcium sulphate mineral. The chlor-alkali and Solvay routes use common salt for several other commercial chemicals. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

85. How many water molecules are present in ferrous sulphate crystals?

[UPSC NDA — 2021]

(A) 1

(B) 3

(D) 7

Answer: (D) 7

Explanation: Ferrous sulphate commonly crystallises as FeSO4·7H2O, called green vitriol. The seven water molecules are part of its crystal structure. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

84. Which of diamond, graphite and fullerene is a good conductor of electricity?

[UPSC CDS — 2021]

(A) Diamond alone

(B) Diamond and graphite

(D) Diamond and fullerene

Answer: (C) Graphite alone

Explanation: Graphite possesses mobile pi electrons across its layers and therefore conducts electrical current. Diamond has strongly localised covalent bonds; molecular fullerene is not generally a comparable bulk conductor. Different arrangements of the same carbon atoms explain the major contrasts between these materials.

83. Which two acids are principal constituents of acid rain apart from water?

[RPSC RAS / Senior Teacher — 2021]

(A) Hydrochloric and sulphuric acids

(B) Nitric and sulphuric acids

(D) Nitric, hydrochloric and sulphuric acids

Answer: (B) Nitric and sulphuric acids

Explanation: Oxides of sulphur and nitrogen undergo atmospheric oxidation and hydration to form mainly sulphuric and nitric acids. Their deposition lowers the pH of rain and can harm ecosystems. The resulting acids may also accelerate corrosion of buildings and damage freshwater ecosystems.

82. Which compound among these is used as a common food preservative?

[SSC MTS — 2021]

(A) Magnesium silicate

(B) Sodium palmitate

(D) Sodium chloride

Answer: (D) Sodium chloride

Explanation: Common salt, sodium chloride, helps preserve food by lowering water availability for microorganisms, especially in concentrated brines and cured foods. Its ions also contribute to characteristic taste. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

81. Which of the following is not a good conductor of electricity?

[SSC CHSL (2020 cycle) — 2021]

(A) Graphite

(B) Gold

(D) Soil

Answer: (C) Diamond

Explanation: Diamond’s four valence electrons are tied in strong covalent sigma bonds, leaving no freely moving charge carriers. Graphite instead contains delocalised electrons along its carbon layers and conducts electricity. Different arrangements of the same carbon atoms explain the major contrasts between these materials.

80. Which pair of common substances and acids is incorrectly matched?

[UPPCS / UPPSC — 2021]

(A) Vinegar — acetic acid

(B) Bones — calcium

(D) Gastric juice — hydrochloric acid

Answer: (C) Souring of milk — nitric acid

Explanation: Milk becomes sour principally through formation of lactic acid during bacterial fermentation of lactose, not nitric acid. Vinegar contains acetic acid and gastric juice contains hydrochloric acid. Organic acids share acidic behaviour but differ in molecular structure, biological origins and uses.

79. Which element forms the basis of all living organisms and many everyday materials?

[RRB NTPC — 2021]

(A) Antimony

(B) Curium

(D) Barium

Answer: (C) Carbon

Explanation: Carbon can form four covalent bonds, link to itself and form stable chains, rings and multiple bonds. This chemical versatility supports biological molecules and a vast range of materials. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

78. Which of these is a non-metallic mineral?

[RRB NTPC — 2021]

(A) Sulphur

(B) Iron

(D) Platinum

Answer: (A) Sulphur

Explanation: Sulphur is a non-metal that may occur naturally as elemental mineral deposits. Iron, silver and platinum are metals. Sulphur has major industrial uses in sulphuric acid manufacture and vulcanisation. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

77. Silver jewellery turns black upon reaction with which element in the environment?

[RRB NTPC — 2021]

(A) Magnesium

(B) Zinc

(D) Carbon

Answer: (C) Sulphur

Explanation: Silver tarnish develops chiefly when silver reacts with sulphur-containing gases, notably hydrogen sulphide, to form black silver sulphide Ag2S. This surface chemical change differs from rusting of iron. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

76. Which among these is an amphoteric oxide?

[RRB NTPC — 2021]

(A) Sulphur dioxide

(B) Magnesium oxide

(D) Zinc oxide

Answer: (D) Zinc oxide

Explanation: Zinc oxide is amphoteric, reacting with both acids and strong bases. Sulphur dioxide and phosphorus pentoxide are acidic oxides, while magnesium oxide is predominantly basic. The behaviour distinguishes amphoteric compounds. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

75. Which oxide exhibits both acidic and basic behavior?

[UPSC CDS — 2020]

(A) Zinc oxide

(B) Copper oxide

(D) Calcium oxide

Answer: (A) Zinc oxide

Explanation: Zinc oxide is amphoteric: it reacts with acids and with strong alkalis. The other listed metal oxides primarily show basic behavior. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

74. What is a solution with pH equal to zero?

[UPSC CDS — 2020]

(A) Highly alkaline

(B) Highly acidic

(D) Neutral

Answer: (B) Highly acidic

Explanation: The pH is the negative logarithm of hydrogen ion activity. A pH near zero corresponds to very high acidity, unlike neutral water near pH seven at room temperature. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

73. Which acid is produced in the human stomach?

[UPSC CDS — 2020]

(A) Formic acid

(B) Sulphuric acid

(D) Hydrochloric acid

Answer: (D) Hydrochloric acid

Explanation: Stomach parietal cells secrete hydrochloric acid, producing the low pH needed for protein digestion and helping to control ingested microbes. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

72. Which matching of compound to use is correct: boric acid, citric acid, magnesium hydroxide, acetic acid?

[UPSC CDS — 2020]

(A) Antiseptic, food preservative, antacid, pickle

(B) Antiseptic, antacid, food preservative, pickle

(D) Pickle, food preservative, antacid, antiseptic

Answer: (A) Antiseptic, food preservative, antacid, pickle

Explanation: The conventional matches are boric acid as a mild antiseptic, citric acid as a food acidulant, magnesium hydroxide as an antacid, and acetic acid in pickling vinegar. Organic acids share acidic behaviour but differ in molecular structure, biological origins and uses.

71. Match bleaching powder, baking soda, washing soda and slaked lime with their formulas.

[UPSC CDS — 2020]

(A) CaOCl2, NaHCO3, Na2CO3·10H2O, Ca(OH)2

(B) CaOCl2, Na2CO3·10H2O, NaHCO3, Ca(OH)2

(D) Ca(OH)2, NaHCO3, Na2CO3·10H2O, CaOCl2

Answer: (A) CaOCl2, NaHCO3, Na2CO3·10H2O, Ca(OH)2

Explanation: Bleaching powder is conventionally represented by CaOCl2, baking soda by NaHCO3, washing soda by the decahydrate of Na2CO3 and slaked lime by Ca(OH)2. Water solubility and hydration are often as important as composition in their practical use.

70. How many waters of crystallisation are present in blue copper sulphate crystals?

[UPSC CDS — 2020]

(A) 2

(B) 4

(D) 6

Answer: (C) 5

Explanation: Blue vitriol is CuSO4·5H2O and contains five molecules of crystallisation water per formula unit. Heating drives off water and leaves a pale anhydrous solid. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

69. Which mineral acid is generally considered the first to have been discovered?

[UPSC NDA — 2020]

(A) Sulphuric acid

(B) Hydrochloric acid

(D) Phosphoric acid

Answer: (A) Sulphuric acid

Explanation: Sulphuric acid, known historically as oil of vitriol, was among the earliest mineral acids studied by alchemists. Modern manufacturing commonly employs the contact process. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

68. A turmeric stain scrubbed with soap and then rinsed with water changes through which colours?

[UPSC NDA — 2020]

(A) Yellow–pink–blue

(B) Yellow–reddish brown–yellow

(D) Yellow–blue–pink

Answer: (B) Yellow–reddish brown–yellow

Explanation: Turmeric is yellow in neutral or acidic media and reddish brown in alkaline soap. Rinsing removes alkalinity, restoring its usual yellow shade. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

67. What is a common feature of bleaching powder and DDT?

[UPSC NDA — 2020]

(A) Both are inorganic

(B) Both are organic

(D) Both contain calcium

Answer: (C) Both contain chlorine

Explanation: Bleaching powder contains available chlorine, while DDT is an organochlorine compound. Their chemical families and uses differ despite sharing chlorine. The chemical properties of group-17 elements follow from their seven outer-shell electrons.

66. Which of these is used as a fertiliser?

[WBCS Prelims — 2020]

(A) N2

(B) O2

(D) NaNO3

Answer: (D) NaNO3

Explanation: Sodium nitrate supplies nitrogen in the nitrate form, which plants can absorb through roots. Elemental nitrogen gas is abundant in air but cannot directly nourish most plants. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.

65. Liquefied petroleum gas generally consists mainly of which gases?

[WBCS Prelims — 2020]

(A) Methane and butane

(B) Propane and butane

(D) Ethane and butane

Answer: (B) Propane and butane

Explanation: Liquefied petroleum gas is principally propane and butane, stored as liquid under moderate pressure. On opening the valve, the fuel vaporises and mixes with air for combustion. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

64. Graphite, carbon and diamond are described in this WBCS question as what?

[WBCS Prelims — 2020]

(A) Isotopes

(B) Isomers

(D) Allotropes

Answer: (D) Allotropes

Explanation: The intended contrast concerns structurally different carbon forms, especially graphite and diamond. These are allotropes: the same chemical element arranged in different bonding networks, with distinct physical properties. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

63. Which acid is also called vitamin C?

[UPSC CDS — 2019]

(A) Methanoic acid

(B) Ascorbic acid

(D) Tartaric acid

Answer: (B) Ascorbic acid

Explanation: Vitamin C is ascorbic acid, a water-soluble antioxidant that contributes to collagen formation. Citrus fruits and several vegetables are major dietary sources. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

62. Which material will not evolve CO2 when treated with dilute HCl?

[UPSC NDA — 2019]

(A) Limestone

(B) Quicklime

(D) Marble

Answer: (B) Quicklime

Explanation: Quicklime is calcium oxide, not calcium carbonate; it forms calcium chloride and water with hydrochloric acid. Limestone, chalk and marble contain carbonate and liberate carbon dioxide. The formulas distinguish these common salts and minerals, even when their trade names sound similar.

61. Which is not a form of calcium carbonate?

[UPSC NDA — 2019]

(A) Limewater

(B) Limestone

(D) Marble

Answer: (A) Limewater

Explanation: Limewater is a saturated solution of calcium hydroxide, whereas chalk, marble and limestone are chiefly calcium carbonate. The formulas distinguish these common salts and minerals, even when their trade names sound similar.

60. A potassium salt is characteristic of which soap?

[UPSC NDA — 2019]

(A) Soft soap

(B) Hard calcium soap

(D) Neutral glycerine

Answer: (A) Soft soap

Explanation: Soft soaps are generally potassium salts of long-chain fatty acids. Sodium salts usually produce firmer hard soaps because of different solubility properties. Solubility and the formation of mobile ions explain several familiar reactions of salts.

59. In graphite, each carbon atom is bonded to three others in what arrangement?

[UPSC CDS — 2019]

(A) A three-dimensional lattice

(B) A hexagonal arrangement in the same plane

(D) A pentagonal arrangement in the same plane

Answer: (B) A hexagonal arrangement in the same plane

Explanation: Graphite contains flat hexagonal sheets of sp²-bonded carbon. Three sigma bonds surround each atom, while delocalised electrons promote electrical conductivity; weak interlayer forces permit sliding. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

58. Which fertiliser has the highest nitrogen content among those listed?

[WBCS Prelims — 2019]

(A) Urea

(B) Ammonium nitrate

(D) Ammonium phosphate

Answer: (A) Urea

Explanation: Urea has approximately 46% nitrogen by mass, exceeding the nitrogen percentage of the other named fertilisers. It is widely used because its nitrogen concentration is high. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.

57. What is the common nature of toothpaste used to protect teeth against decay?

[WBCS Prelims — 2019]

(A) Acidic

(B) Neutral

(D) Corrosive

Answer: (C) Basic

Explanation: Many toothpastes are mildly alkaline and help neutralise acids produced by oral bacteria. Fluoride and abrasive components provide additional protective cleaning and enamel benefits. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

56. Which listed substance has a pH below seven?

[WBCS Prelims — 2019]

(A) Lemon juice

(B) Lime water

(D) Antacid

Answer: (A) Lemon juice

Explanation: Lemon juice contains citric acid and is acidic. Lime water and antacid solutions are alkaline, while normal human blood is mildly basic, around pH 7.4. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

55. What is the basicity of orthophosphorous acid H3PO3?

[RPSC RAS / Senior Teacher — 2019]

(A) 1

(B) 3

(D) 2

Answer: (D) 2

Explanation: Orthophosphorous acid contains two ionisable hydrogen atoms bonded to oxygen; its third hydrogen is attached directly to phosphorus. Hence it is dibasic even though its formula contains three hydrogen atoms. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

54. Which gas dissolves in water to make an acidic solution?

[UPSC CDS — 2018]

(A) Carbon dioxide

(B) Oxygen

(D) Hydrogen

Answer: (A) Carbon dioxide

Explanation: Carbon dioxide partly forms carbonic acid in water, lowering its pH. The remaining listed gases do not create comparable acidic solutions under normal conditions. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

53. Which one is a tribasic acid?

[UPSC NDA — 2018]

(A) HCl

(B) HNO3

(D) H3PO4

Answer: (D) H3PO4

Explanation: Phosphoric acid can release three ionisable hydrogen ions in stages. Hydrochloric and nitric acids are monobasic, while sulphuric acid is dibasic. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

52. Which reaction with nitric acid can produce nitric oxide?

[UPSC NDA — 2018]

(A) Copper with dilute HNO3

(B) Copper with concentrated HNO3

(D) Copper with dilute HCl

Answer: (A) Copper with dilute HNO3

Explanation: Dilute nitric acid oxidises copper to copper nitrate while nitrate ions are reduced to nitric oxide. Concentrated nitric acid with copper more commonly yields brown nitrogen dioxide. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.

51. What is the normal acid-base nature of toothpaste?

[UPSC NDA — 2018]

(A) Acidic

(B) Neutral

(D) Identical to enamel

Answer: (C) Basic

Explanation: Most toothpastes are formulated to be mildly basic, helping neutralise acids formed by oral bacteria. Ingredients vary among products. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

50. Which of these has the greatest concentration of hydrogen ions under usual conditions?

[UPSC NDA — 2018]

(A) Sodium hydroxide

(B) Milk of magnesia

(D) Gastric juice

Answer: (D) Gastric juice

Explanation: Gastric juice is generally more strongly acidic than lemon juice and releases more hydrogen ions. Sodium hydroxide and milk of magnesia are alkaline. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

49. Which solution typically has pH below 7?

[UPSC NDA — 2018]

(A) NaOH

(B) KCl

(D) NaCl

Answer: (C) FeCl3

Explanation: Iron(III) chloride solution becomes acidic because hydrated Fe3+ ions hydrolyse in water, liberating hydronium. Salts of strong acid and strong base are near neutral. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

48. Brine is a concentrated aqueous solution of which substance?

[UPSC NDA — 2018]

(A) NaCl

(B) NaOH

(D) Na2CO3

Answer: (A) NaCl

Explanation: Brine generally refers to saltwater rich in sodium chloride. Industrial brine electrolysis is used to obtain chlorine, hydrogen and sodium hydroxide. Halogens lie in group 17 and tend to gain electrons when reacting with metals.

47. Which statement about bleaching powder is false?

[UPSC NDA — 2018]

(A) It is principally a reducing agent

(B) It can bleach wood pulp

(D) It bleaches textile linen

Answer: (A) It is principally a reducing agent

Explanation: Bleaching powder is used primarily because of its oxidising and disinfecting action. It is not normally described as a reducing agent. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

46. Which pair of gases is known as synthesis gas or syngas?

[UPSC NDA — 2018]

(A) Carbon and steam

(B) CO and steam

(D) NO2 and hydrogen

Answer: (C) CO and hydrogen

Explanation: Synthesis gas consists mainly of carbon monoxide and hydrogen. It serves as feedstock for methanol synthesis and other industrial chemical reactions. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

45. Which ammonium compound is not commonly used as a fertiliser?

[UPSC NDA — 2018]

(A) Ammonium nitrate

(B) Ammonium sulphide

(D) Ammonium sulphate

Answer: (B) Ammonium sulphide

Explanation: Ammonium nitrate, phosphate and sulphate supply crop nutrients. Ammonium sulphide is not a normal agricultural fertiliser and can liberate toxic hydrogen sulphide. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

44. What is the general molecular formula of an acyclic alkane?

[UPSC CDS — 2018]

(A) CnH2n+2

(B) CnH2n

(D) CnHn

Answer: (A) CnH2n+2

Explanation: Alkanes are saturated hydrocarbons with carbon-carbon single bonds only. For open-chain compounds the hydrogen count is twice the carbon count plus two; cyclic alkanes have another formula. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

43. Which pair of pollutants is identified with the formation of acid rain in this RAS question?

[RPSC RAS / RPSC AEn — 2018]

(A) Carbon dioxide and nitrogen

(B) Carbon monoxide and carbon dioxide

(D) Ozone and carbon dioxide

Answer: (C) Nitrous oxide and sulphur dioxide

Explanation: The archived examination key selects the option mentioning sulphur dioxide and a nitrogen oxide. Chemically, sulphur dioxide and NO/NO2 are major acid-rain precursors; nitrous oxide (N2O) is not a principal cause, so the printed choice is imprecise.

42. In HSO4− + H2O → H3O+ + SO4²−, what role does water perform?

[UPSC CDS — 2017]

(A) Acid

(B) Base

(D) Inert medium

Answer: (B) Base

Explanation: Water accepts a proton from hydrogen sulphate and forms hydronium. This meets the Brønsted–Lowry definition of a base, even though water can act as an acid in other reactions. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

41. Which mineral makes up most of a pearl?

[UPSC CDS — 2017]

(A) Calcium carbonate

(B) Calcium oxide

(D) Calcium sulphate

Answer: (A) Calcium carbonate

Explanation: Pearls are largely crystalline calcium carbonate held in an organic matrix. Mollusks deposit layers of nacre around an irritant or within tissue. Water solubility and hydration are often as important as composition in their practical use.

40. What white powder, mixed with water, is commonly used to immobilise fractured bones?

[UPSC CDS — 2017]

(A) Bleaching powder

(B) Plaster of Paris

(D) Lime powder

Answer: (B) Plaster of Paris

Explanation: Plaster of Paris is calcium sulphate hemihydrate. When mixed with water, it rehydrates and hardens into gypsum, forming a supporting cast. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

39. Which aqueous solution is both basic and electrically conducting?

[UPSC NDA — 2017]

(A) HCl

(B) CH3COOH

(D) NaOH

Answer: (D) NaOH

Explanation: Sodium hydroxide is a strong base that dissociates into sodium and hydroxide ions in water. Those free ions allow its solution to conduct current. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

38. Which acid is conventionally associated with the pain from nettle stings?

[UPSC NDA — 2017]

(A) Acetic acid

(B) Methanoic acid

(D) Hydrochloric acid

Answer: (B) Methanoic acid

Explanation: Methanoic acid is also called formic acid. Traditional chemistry questions attribute nettle-sting irritation to this acid, although natural nettle secretions contain additional active substances. Identifying each acid requires distinguishing its chemical name from the everyday source mentioned.

37. What is a principal industrial application of hydrofluoric acid?

[UPSC NDA — 2017]

(A) Etching glass

(B) Bleaching cloth

(D) Preparing simple nitrates

Answer: (A) Etching glass

Explanation: Hydrofluoric acid attacks the silica network of glass, forming fluorosilicates. It must be handled with exceptional care because severe toxicity can accompany skin exposure. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

36. Which atmospheric gas is a major cause of acid rain?

[UPSC NDA — 2017]

(A) Ozone

(B) Ammonia

(D) Carbon monoxide

Answer: (C) Sulphur dioxide

Explanation: Sulphur dioxide from fuel combustion oxidises and dissolves in atmospheric water to produce acids. Alongside nitrogen oxides, it contributes substantially to acid deposition. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.

35. Which material is also used as a dry lubricant?

[MPPSC / MPPSC AE — 2017]

(A) Cuprite

(B) Graphite

(D) Cryolite

Answer: (B) Graphite

Explanation: Graphite has weak interactions between stacked carbon sheets, allowing the layers to slide past each other. This gives the solid a slippery feel and enables its use as a lubricant. Different arrangements of the same carbon atoms explain the major contrasts between these materials.

34. What is the correct descending order of these atmospheric gases?

[MPPSC / MPPSC AE — 2017]

(A) Nitrogen, oxygen, argon, neon

(B) Oxygen, neon, xenon, argon

(D) Methane, argon, hydrogen, helium

Answer: (A) Nitrogen, oxygen, argon, neon

Explanation: Dry air consists of roughly 78% nitrogen, 21% oxygen and 0.93% argon. Neon is present in trace quantities, so it follows those three gases. Their filled outer electron shells usually account for unusually low reactivity under ordinary conditions.

33. Which acid occurs in acid rain among the following?

[CGPSC / CGPSC Civil — 2017]

(A) Benzoic acid

(B) Acetic acid

(D) Oxalic acid

Answer: (C) Nitric acid

Explanation: Nitrogen oxides from combustion can transform in air into nitric acid, an important constituent of acid rain. Sulphuric acid is another major component produced from sulphur oxides. The resulting acids may also accelerate corrosion of buildings and damage freshwater ecosystems.

32. Which organic acid is particularly abundant in grapes, tamarind and bananas?

[UPPCS / UPPSC — 2017]

(A) Acetic acid

(B) Citric acid

(D) Tartaric acid

Answer: (D) Tartaric acid

Explanation: Tartaric acid occurs naturally in grapes and tamarind and contributes to their tart taste. Its salts include potassium hydrogen tartrate, called cream of tartar. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

31. Why is dolomite powder applied to certain agricultural soils?

[UPSC CDS — 2016]

(A) To raise soil pH

(B) To lower soil pH

(D) To add nitrogen directly

Answer: (A) To raise soil pH

Explanation: Dolomite contains calcium and magnesium carbonates. Their neutralising action reduces excessive soil acidity and can supply calcium and magnesium to crops. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

30. Which alkali metal generally does not form a stable solid hydrogen carbonate?

[UPSC CDS — 2016]

(A) Sodium

(B) Potassium

(D) Lithium

Answer: (D) Lithium

Explanation: Lithium hydrogen carbonate is stable mainly in aqueous solution rather than as an isolable solid, unlike corresponding sodium and potassium hydrogen carbonates. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

29. Which of these is NOT an allotrope of carbon?

[UPSC CDS — 2016]

(A) Coal

(B) Diamond

(D) Graphene

Answer: (A) Coal

Explanation: Coal is a naturally occurring combustible sedimentary rock, generally containing carbon together with impurities and mineral matter. Diamond, graphite and graphene are distinct structural forms of the element carbon. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

28. Which statements about vinegar are correct?
A. Sugarcane juice can be fermented into vinegar.
B. Vinegar contains acetic acid.
C. Benzoic acid is the characteristic acid in vinegar.
D. Vinegar is used as a preservative.

[RPSC RAS / Senior Teacher — 2016]

(A) A, C and D

(B) A, B and D

(D) B, C and D

Answer: (B) A, B and D

Explanation: Vinegar is an aqueous solution of ethanoic acid formed when bacteria oxidise ethanol. It may originate from fermented sugarcane products and preserves food through its acidity; benzoic acid is not its defining constituent. The carbon skeleton and degree of unsaturation influence reactions and observable physical properties.

27. Antacids are generally what kind of chemical substances?

[CGPSC / CGPSC Civil — 2016]

(A) Bases

(B) Acids

(D) Buffers

Answer: (A) Bases

Explanation: Antacids contain mild bases such as magnesium hydroxide or calcium carbonate. They react with excess stomach hydrochloric acid, reducing acidity and relieving symptoms such as heartburn. Water solubility and hydration are often as important as composition in their practical use.

26. Which acid attacks glass and therefore cannot ordinarily be kept in glass containers?

[WBCS Prelims — 2015]

(A) HNO3

(B) HCl

(D) HBr

Answer: (C) HF

Explanation: Hydrofluoric acid reacts with silica, a major component of glass, forming fluorosilicate species. Its special chemistry requires suitable polymer containers and stringent handling precautions. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

25. How much NaOH is required to prepare 100 mL of 0.5 M NaOH solution?

[WBCS Prelims — 2015]

(A) 2 g

(B) 20 g

(D) 1 g

Answer: (A) 2 g

Explanation: Moles required equal molarity multiplied by volume in litres: 0.5 × 0.100 = 0.05 mol. Multiplying by sodium hydroxide molar mass, 40 g/mol, yields 2 g. Solubility and the formation of mobile ions explain several familiar reactions of salts.

24. Aspirin is chemically called what?

[WBCS Prelims — 2015]

(A) Acetylsalicylic acid

(B) Sodium salicylate

(D) Ethyl salicylate

Answer: (A) Acetylsalicylic acid

Explanation: Aspirin is acetylsalicylic acid, a derivative of salicylic acid whose phenolic hydroxyl group has been acetylated. It is commonly used to relieve pain and fever. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

23. How many times more acidic is a solution of pH 3 than one of pH 6?

[RPSC RAS / Senior Teacher — 2015]

(A) 2

(B) 3

(D) 0.5

Answer: (C) 1000

Explanation: Each decrease of one pH unit corresponds to a tenfold increase in hydrogen ion activity. The three-unit difference therefore represents 10 cubed, or one thousand times. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

22. H3PO4 is commonly known as what?

[RPSC RAS / Senior Teacher — 2015]

(A) Pyrophosphorous acid

(B) Hypophosphoric acid

(D) Pyrophosphoric acid

Answer: (C) Orthophosphoric acid

Explanation: H3PO4 is orthophosphoric acid, a triprotic oxoacid of phosphorus. It differs from pyrophosphoric acid H4P2O7 and phosphorous acid H3PO3 in structure and ionisation. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

21. Which noble gas is most abundant in Earth’s atmosphere?

[WBCS Prelims — 2014]

(A) Helium

(B) Neon

(D) Xenon

Answer: (C) Argon

Explanation: Argon accounts for approximately 0.93% of dry air by volume, far more than helium, neon or xenon. It is chemically unreactive under ordinary conditions. Their filled outer electron shells usually account for unusually low reactivity under ordinary conditions.

20. Washing soda is the common name for which chemical?

[WBCS Prelims — 2014]

(A) Sodium carbonate

(B) Calcium carbonate

(D) Sodium hydroxide

Answer: (A) Sodium carbonate

Explanation: Washing soda is sodium carbonate, commonly encountered as the decahydrate Na2CO3·10H2O. Its alkaline solutions help remove grease and soften hard water in cleaning. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

19. What is the most common cause of acidity in natural water?

[CGPSC / CGPSC Civil — 2014]

(A) Carbon dioxide

(B) Oxygen

(D) Chlorine

Answer: (A) Carbon dioxide

Explanation: Carbon dioxide dissolves into water and partly forms carbonic acid. It is a widespread natural contributor to water acidity, although pollution and mineral chemistry can also affect pH. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

18. Muriatic acid used in household applications is dilute what?

[WBCS Prelims — 2013]

(A) Acetic acid

(B) Hydrochloric acid

(D) Aqua regia

Answer: (B) Hydrochloric acid

Explanation: Muriatic acid is a commercial name for hydrochloric acid in water. It is used for masonry cleaning and descaling but may give off irritating hydrogen chloride fumes. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

17. Household phenyl disinfectant is chemically associated with derivatives of what?

[WBCS Prelims — 2013]

(A) Methyl alcohol

(B) Tartaric acid

(D) Anthracene

Answer: (C) Benzene

Explanation: Phenol is hydroxybenzene, an aromatic derivative of benzene. Traditional phenyl-type disinfectants employ phenolic compounds, although modern commercial formulations may contain other active chemicals. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

16. Acid rain chiefly results from atmospheric pollution by which option?

[UPPCS / UPPSC — 2013]

(A) Carbon dioxide

(B) Carbon monoxide

(D) Nitrous oxide and sulphur dioxide

Answer: (D) Nitrous oxide and sulphur dioxide

15. Chlorination purifies water mainly because chlorine forms which disinfecting substance?

[UPSC CSE Prelims — 2010]

(A) Hydrochloric acid in water

(B) Hypochlorous acid in water

(D) Hydrogen in water

Answer: (B) Hypochlorous acid in water

Explanation: Chlorine reacts with water to produce hypochlorous acid, HOCl, an effective oxidising disinfectant. It enters microorganisms and damages essential cellular constituents, helping render drinking water microbiologically safer. Halogens lie in group 17 and tend to gain electrons when reacting with metals.

14. Which carbon allotrope is commonly represented by the fullerene family?

[WBCS Prelims — 2009]

(A) An allotrope of carbon

(B) Another name for carborundum

(D) A single chemical compound of carbon

Answer: (A) An allotrope of carbon

Explanation: Fullerenes are closed-cage molecular forms consisting entirely of carbon atoms. Buckminsterfullerene C60 is a famous example, distinct from silicon carbide, also called carborundum. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.

13. Which compound is known as white vitriol?

[WBCS Prelims — 2007]

(A) FeSO4·7H2O

(B) ZnSO4·7H2O

(D) CuSO4·5H2O

Answer: (B) ZnSO4·7H2O

Explanation: White vitriol is zinc sulphate heptahydrate. For comparison, ferrous sulphate heptahydrate is green vitriol and copper sulphate pentahydrate is blue vitriol. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

12. What is the oxidation number of oxygen in superoxide?

[WBCS Prelims — 2007]

(A) 0

(B) −1/2

(D) +2

Answer: (B) −1/2

Explanation: Superoxide contains the O2− ion, with a total charge of minus one shared by two equivalent oxygen atoms. Each therefore has a fractional average oxidation state of minus one-half. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

11. Which compound can act only as a reducing agent among these?

[WBCS Prelims — 2007]

(A) H2O2

(B) MnO2

(D) H2S

Answer: (D) H2S

Explanation: In hydrogen sulphide the sulphur oxidation number is minus two, its lowest common state. It can be oxidised but not ordinarily reduced further, so it functions as a reducing agent. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

10. Hydrogen emission lines in the ultraviolet region belong to which spectral series?

[WBCS Prelims — 2006]

(A) Lyman series

(B) Balmer series

(D) Paschen series

Answer: (A) Lyman series

Explanation: The Lyman series arises when excited electrons in hydrogen fall to the n = 1 level. These transitions emit ultraviolet radiation, unlike Balmer lines in visible light. The structure and bonding of water-related compounds largely determine their physical and chemical properties.

9. Which molecule is paramagnetic?

[WBCS Prelims — 2006]

(A) H2O

(B) NO

(D) SO2

Answer: (B) NO

Explanation: Nitric oxide has an odd total number of electrons and therefore possesses an unpaired electron. This makes NO paramagnetic, whereas the other molecules listed are normally diamagnetic. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

8. Which molecule has the same total electron count as H2O2?

[WBCS Prelims — 2006]

(A) HCHO

(B) CH3OH

(D) CH2CHO

Answer: (B) CH3OH

Explanation: Hydrogen peroxide contains eighteen electrons: two from hydrogen and sixteen from oxygen. Methanol CH3OH also has eighteen electrons from carbon, oxygen and four hydrogens. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

7. Which is NOT a metal?

[WBCS Prelims — 2005]

(A) Platinum

(B) Mercury

(D) Silicon

Answer: (D) Silicon

Explanation: Silicon is classified as a metalloid, exhibiting properties intermediate between many metals and non-metals. Platinum, mercury and nickel are all metallic elements. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.

6. Which scientist is most commonly credited with the discovery of oxygen in the listed options?

[WBCS Prelims — 2005]

(A) Joseph Priestley

(B) Antoine Lavoisier

(D) Robert Boyle

Answer: (A) Joseph Priestley

Explanation: Joseph Priestley independently isolated oxygen in 1774, while Carl Scheele had prepared it earlier. Lavoisier later explained its role in combustion and named the gas oxygen. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

5. Which statements are correct?
1. Anhydrous sodium carbonate is baking soda.
2. Baking soda is used in fire extinguishers.
3. Bleaching powder is made in a Hasenclever plant.

[UPSC CSE Prelims — 2005]

(A) 1, 2 and 3

(B) 2 and 3

(D) 1 and 2

Answer: (B) 2 and 3

Explanation: Baking soda is NaHCO3, whereas anhydrous sodium carbonate is Na2CO3, so statement one is false. Sodium hydrogen carbonate serves in extinguishers, and the Hasenclever process manufactures bleaching powder. Water solubility and hydration are often as important as composition in their practical use.

4. Which statements are correct?
1. Baking soda is used in extinguishers.
2. Quicklime is used in glassmaking.
3. Gypsum is used to make Plaster of Paris.

[UPSC CSE Prelims — 2004]

(A) 1 and 2

(B) 2 and 3

(D) 1, 2 and 3

Answer: (D) 1, 2 and 3

Explanation: All three uses are established: sodium hydrogen carbonate can release carbon dioxide, calcium oxide stabilises soda-lime glass, and partial dehydration of gypsum produces calcium sulphate hemihydrate, Plaster of Paris. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.

3. Which of the listed substances is a chemical element?

[WBCS Prelims — 2002]

(A) Table salt

(B) Water

(D) Glass

Answer: (C) Graphite

Explanation: Graphite consists solely of carbon atoms and is an allotrope of the element carbon. Table salt and water are compounds, while ordinary glass is a mixture of silicate substances. Different arrangements of the same carbon atoms explain the major contrasts between these materials.

2. Lightning discharges can promote rain formation because they cause what reaction?

[WBCS Prelims — 2002]

(A) Nitrogen and oxygen in air to combine

(B) Only ionisation without reaction

(D) Only photochemical reactions

Answer: (A) Nitrogen and oxygen in air to combine

Explanation: The tremendous energy of lightning enables normally unreactive atmospheric nitrogen and oxygen to combine, forming nitrogen oxides. These can undergo further atmospheric reactions and contribute to nitrate formation. The concentration and mobility of ions help explain the observed chemical behaviour in solution.

1. Assertion: Synthetic detergents lather well in hard water.
Reason: Their calcium and magnesium salts are soluble in water.

Choose the correct relationship.

[UPSC CSE Prelims — 2002]

(A) Both true, and reason explains assertion

(B) Both true, but reason does not explain assertion

(D) Assertion false, reason true

Answer: (A) Both true, and reason explains assertion

Explanation: Hard water contains calcium and magnesium ions, which precipitate ordinary soaps as scum. Synthetic detergents generally avoid such insoluble precipitates, so they remain effective and lather in hard water. Hydrogen participates in many compounds whose reactivity differs markedly with the bonded elements.

180. When hydrochloric acid reacts with copper oxide, which product gives a blue-green solution?

179. Why does an aqueous solution of an acid conduct electricity?

178. A chemical species capable of donating a proton is called what?

177. Which statements about bases and alkalis are correct? 1. All bases are water-soluble. 2. Water-soluble bases are alkalis. 3. Alkalis feel soapy and may be corrosive. 4. Diluting alkali can release heat.

176. Match the acid source: tomato, sour milk, ant sting and vinegar.

175. Which are naturally occurring forms of calcium carbonate: marble, chalk and limestone?

174. What is the chemical formula of cooking soda (baking soda)?

173. Which pairing of common name and formula is incorrect?

172. An acid releasing H+ and a base releasing OH− in water reflects which acid-base concept?

171. Which among AlCl3, NH3, BF3 and FeCl3 behave as Lewis acids; how many?

170. When 2 mL of 0.1 N HCl is mixed with 4 mL of 0.1 N NaOH and phenolphthalein, what colour develops?

169. Which statements are true? I. Thirty percent H2O2 is sold as ten-volume peroxide. II. All five waters in CuSO4·5H2O are hydrogen-bonded.

168. Which are correct? I. Nettle leaves can cause formic-acid stings. II. Heating gypsum produces calcium carbonate hemihydrate. III. Moistening anhydrous copper sulphate produces green dihydrate.

167. How is superphosphate of lime commonly prepared?

166. Which statements about nitrolim (calcium cyanamide) are correct? 1. Made from calcium carbide and nitrogen. 2. Produces ammonia in soil. 3. An NPK fertiliser. 4. Organic fertiliser.

165. Which statement about soap micelles is NOT correct?

164. Which pollutants are most responsible for acid rain? a. Ozone; b. Sulphur dioxide; c. Carbon monoxide; d. Nitrogen dioxide.

163. Mendeleev could not assign an unambiguous position to which element in his periodic table?

162. In CuO + H2 → Cu + H2O, which species undergoes oxidation?

161. Statements: I. Alkaline KMnO4 oxidises ethanol into ethanoic acid. II. Oxidation of ethanol includes removal of hydrogen or addition of oxygen. Which is correct?

160. Which air pollutant is most closely linked to the formation of acid rain?

159. Which set consists entirely of Lewis acids?

158. An aqueous solution with pH below 7 is classified as what?

157. Which oxide is classified as neutral?

156. Which statement about pH is incorrect?

155. Rain is termed acid rain when its pH falls below roughly what value?

154. Match washing soda, baking soda, bleaching powder and gypsum to their formulas.

153. Which of these mixtures is called water gas?

152. Which statement does NOT describe graphene?

151. Arrange HF, HCl, HBr and HI in increasing acid strength.

150. Which statements about phosphorus oxyacids are true? I. H3PO4 is tribasic. II. All three hydrogens in H3PO3 are ionisable. III. Orthophosphate salts are used in fertilisers.

149. Which reaction demonstrates acidic behaviour of ethanoic acid?

148. Which gas is released when baking soda reacts with vinegar?

147. Carbonylation of methanol is an industrial preparation route for which acid?

146. Which industrial chemical is manufactured using the Contact Process?

145. Acid rain is chiefly caused by which two atmospheric gases?

144. Which of these gases is monatomic under normal conditions?

143. Which is an amphoteric oxide?

142. Vinegar is most accurately described as what?

141. Match bleaching powder, baking soda, washing soda and plaster of Paris to their chemical names.

140. In what approximate pH range can the human body function?

139. Milk of magnesia consists mainly of what?

138. Which straight-chain alkane has molecular formula C9H20?

137. Which is a saturated hydrocarbon?

136. Which researchers described the peroxide effect in anti-Markovnikov addition in 1933?

135. Which of these carboxylic acids is strongest?

134. Which carbon allotrope was discovered by Curl, Kroto and Smalley in 1985?

133. Palladium deposited on barium sulphate is associated with which named catalyst?

132. Which carboxylic acid has the formula CH3CH2CH2COOH?

131. Water gas contains which pair of gases?

130. Which fertiliser among these primarily supplies nitrogen?

129. Which combination catalyses hydrogen addition to an unsaturated hydrocarbon?

128. Which statement is correct about chlorine and fluorine?

127.

126. Which noble gas produces a characteristic red-orange glow when electricity passes through it?

125. Which of the following is NOT considered an important greenhouse gas?

124. Which chemical symbol represents an element that is NOT a halogen?

123. What are the elements of group 17 in the periodic table called?

122. When diluting concentrated nitric acid, which is correct? 1. Hydronium concentration increases. 2. Water is slowly added to acid. 3. Acid is slowly added to water.

121. Arrange these substances in increasing pH.

120. Which statement about commonly occurring acids is incorrect?

119. Which acid may a goldsmith use for cleaning gold and silver objects?

118. Why do copper sulphate crystals lose their blue colour when heated?

117. Which compound is also known as olefiant gas?

116. Organic compounds with an oxygen atom between two alkyl groups belong to which class?

115. Which boiling-point statements are correct? I. Chloroform boils around 334 K. II. Methane boils around 111 K.

114. What is the molecular formula of propene?

113. Complete burning of ethanol in oxygen primarily produces which products?

112. Unsaturated hydrocarbons commonly burn with which flame colour under smoky conditions?

111. Which industrial process is widely used for manufacturing ammonia?

110. What chemical change can quickly fill certain vehicle airbags with gas?

109. Which statement correctly compares diamond with graphite?

108. Which chemical is used to disinfect drinking water?

107. Which compound is commonly used as an antacid for stomach acidity?

106. Which statement best describes the behavior of oxides of non-metals with water?

105. Milk of magnesia, used for indigestion, is a what?

104. Litmus indicator is obtained from which organism?

103. The fluid injected by stinging nettle hairs is conventionally identified as what?

102. Methanoic acid is normally associated with which source?

101. Which mineral acid is present in human gastric juice?

177. Which statements about bases and alkalis are correct?
1. All bases are water-soluble.
2. Water-soluble bases are alkalis.
3. Alkalis feel soapy and may be corrosive.
4. Diluting alkali can release heat.

169. Which statements are true?
I. Thirty percent H2O2 is sold as ten-volume peroxide.
II. All five waters in CuSO4·5H2O are hydrogen-bonded.

168. Which are correct?
I. Nettle leaves can cause formic-acid stings.
II. Heating gypsum produces calcium carbonate hemihydrate.
III. Moistening anhydrous copper sulphate produces green dihydrate.

166. Which statements about nitrolim (calcium cyanamide) are correct?
1. Made from calcium carbide and nitrogen.
2. Produces ammonia in soil.
3. An NPK fertiliser.
4. Organic fertiliser.

164. Which pollutants are most responsible for acid rain?
a. Ozone;
b. Sulphur dioxide;
c. Carbon monoxide;
d. Nitrogen dioxide.

161. Statements:
I. Alkaline KMnO4 oxidises ethanol into ethanoic acid.
II. Oxidation of ethanol includes removal of hydrogen or addition of oxygen.

Which is correct?

150. Which statements about phosphorus oxyacids are true?
I. H3PO4 is tribasic.
II. All three hydrogens in H3PO3 are ionisable.
III. Orthophosphate salts are used in fertilisers.

122. When diluting concentrated nitric acid, which is correct?
1. Hydronium concentration increases.
2. Water is slowly added to acid.
3. Acid is slowly added to water.

115. Which boiling-point statements are correct?
I. Chloroform boils around 334 K.
II. Methane boils around 111 K.

94. Which statements are correct?
1. All bases are alkalis.
2. Alkalis dissolve in water.
3. Alkalis may be slippery, bitter and corrosive.

93. Which statements about universal indicator are correct?
1. It mixes several indicators.
2. Its color varies with hydrogen ion concentration.
3. It determines exact titration end points for every acid-base system.