30. Which alkali metal generally does not form a stable solid hydrogen carbonate?
[UPSC CDS — 2016]
(A) Sodium
(B) Potassium
(C) Caesium
(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
(C) Graphite
(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
(C) A, B and C
(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
(C) Salts
(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
(C) HF
(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
(C) 4 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
(C) Methyl 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
(C) 1000
(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
(C) Orthophosphoric 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
(C) Argon
(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.
