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
(C) Pink
(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
(C) Both I and II
(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
(C) II and III
(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
(C) Calcium phosphate + hydrogen chloride
(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
(C) 4 only
(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
(C) Grease may be trapped within micelles
(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
(C) b 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
(C) Oxygen
(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
(C) Hydrogen
(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
(C) Only II correct
(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.
