60. A potassium salt is characteristic of which soap?
[UPSC NDA — 2019]
(A) Soft soap
(B) Hard calcium soap
(C) Metal-free detergent
(D) Neutral glycerine
Answer: (A) Soft soap
Explanation: Soft soaps are generally potassium salts of long-chain fatty acids. Sodium salts usually produce firmer hard soaps because of different solubility properties. Solubility and the formation of mobile ions explain several familiar reactions of salts.
59. In graphite, each carbon atom is bonded to three others in what arrangement?
[UPSC CDS — 2019]
(A) A three-dimensional lattice
(B) A hexagonal arrangement in the same plane
(C) A square arrangement in the same plane
(D) A pentagonal arrangement in the same plane
Answer: (B) A hexagonal arrangement in the same plane
Explanation: Graphite contains flat hexagonal sheets of sp²-bonded carbon. Three sigma bonds surround each atom, while delocalised electrons promote electrical conductivity; weak interlayer forces permit sliding. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.
58. Which fertiliser has the highest nitrogen content among those listed?
[WBCS Prelims — 2019]
(A) Urea
(B) Ammonium nitrate
(C) Potassium nitrate
(D) Ammonium phosphate
Answer: (A) Urea
Explanation: Urea has approximately 46% nitrogen by mass, exceeding the nitrogen percentage of the other named fertilisers. It is widely used because its nitrogen concentration is high. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.
57. What is the common nature of toothpaste used to protect teeth against decay?
[WBCS Prelims — 2019]
(A) Acidic
(B) Neutral
(C) Basic
(D) Corrosive
Answer: (C) Basic
Explanation: Many toothpastes are mildly alkaline and help neutralise acids produced by oral bacteria. Fluoride and abrasive components provide additional protective cleaning and enamel benefits. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.
56. Which listed substance has a pH below seven?
[WBCS Prelims — 2019]
(A) Lemon juice
(B) Lime water
(C) Human blood
(D) Antacid
Answer: (A) Lemon juice
Explanation: Lemon juice contains citric acid and is acidic. Lime water and antacid solutions are alkaline, while normal human blood is mildly basic, around pH 7.4. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.
55. What is the basicity of orthophosphorous acid H3PO3?
[RPSC RAS / Senior Teacher — 2019]
(A) 1
(B) 3
(C) 4
(D) 2
Answer: (D) 2
Explanation: Orthophosphorous acid contains two ionisable hydrogen atoms bonded to oxygen; its third hydrogen is attached directly to phosphorus. Hence it is dibasic even though its formula contains three hydrogen atoms. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
54. Which gas dissolves in water to make an acidic solution?
[UPSC CDS — 2018]
(A) Carbon dioxide
(B) Oxygen
(C) Nitrogen
(D) Hydrogen
Answer: (A) Carbon dioxide
Explanation: Carbon dioxide partly forms carbonic acid in water, lowering its pH. The remaining listed gases do not create comparable acidic solutions under normal conditions. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
53. Which one is a tribasic acid?
[UPSC NDA — 2018]
(A) HCl
(B) HNO3
(C) H2SO4
(D) H3PO4
Answer: (D) H3PO4
Explanation: Phosphoric acid can release three ionisable hydrogen ions in stages. Hydrochloric and nitric acids are monobasic, while sulphuric acid is dibasic. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
52. Which reaction with nitric acid can produce nitric oxide?
[UPSC NDA — 2018]
(A) Copper with dilute HNO3
(B) Copper with concentrated HNO3
(C) Zinc with concentrated HNO3
(D) Copper with dilute HCl
Answer: (A) Copper with dilute HNO3
Explanation: Dilute nitric acid oxidises copper to copper nitrate while nitrate ions are reduced to nitric oxide. Concentrated nitric acid with copper more commonly yields brown nitrogen dioxide. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.
51. What is the normal acid-base nature of toothpaste?
[UPSC NDA — 2018]
(A) Acidic
(B) Neutral
(C) Basic
(D) Identical to enamel
Answer: (C) Basic
Explanation: Most toothpastes are formulated to be mildly basic, helping neutralise acids formed by oral bacteria. Ingredients vary among products. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
