90. Which compound is used in a soda-acid fire extinguisher?
[UPSC CDS — 2021]
(A) Sodium chloride
(B) Sodium hydrogen carbonate
(C) Calcium hydroxide
(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
(C) HBr
(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
(C) Oxalic 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
(C) 10
(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
(C) Plaster of Paris
(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
(C) 5
(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
(C) Graphite alone
(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
(C) Nitric and hydrochloric 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
(C) Dipropylene glycol
(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
(C) Diamond
(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.
