40. What white powder, mixed with water, is commonly used to immobilise fractured bones?
[UPSC CDS — 2017]
(A) Bleaching powder
(B) Plaster of Paris
(C) Zinc oxide
(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
(C) CH3OH
(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
(C) Sulphuric 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
(C) Oxidising metals
(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
(C) Sulphur dioxide
(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
(C) Hematite
(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
(C) Oxygen, argon, neon, helium
(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
(C) Nitric 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
(C) Lactic 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
(C) To add phosphorus directly
(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.
