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
(C) Copper(I) 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
(C) It generates electricity
(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
(C) Hydrophobic 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
(C) 1 and 2 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
(C) Tomato–acetic; milk–methanoic; sting–lactic; vinegar–oxalic
(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
(C) Marble 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
(C) Na2CO3
(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
(C) Baking powder—NaHCl2
(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
(C) Arrhenius
(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
(C) Three
(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.