UPSC Prelims 2014— Question 70

Correct answer: Option B. Crystallization of sodium chloride and melting of ice are physical changes because the substances retain their chemical identity. Souring of milk is a chemical change involving biochemical transformations that produce new substances and alter the milk's properties. Therefore only statement 3 is a chemical change. Option A: Incorrect. This option does not match the verified answer because its proposition, location, institution, sequence, or combination conflicts with the relevant fact tested in the question. The option reads: “1 and 2 only” Option B: Correct. This option is consistent with the governing fact or the valid combination identified in the question. The option reads: “3 only” Option C: Incorrect. This option does not match the verified answer because its proposition, location, institution, sequence, or combination conflicts with the relevant fact tested in the question. The option reads: “1, 2 and 3” Option D: Incorrect. This option does not match the verified answer because its proposition, location, institution, sequence, or combination conflicts with the relevant fact tested in the question. The option reads: “None” The key Prelims lesson is to test each proposition independently before selecting the combination. Where the question uses words such as 'only', 'all', 'none', 'always' or a precise institutional/geographical association, a single inaccurate component is sufficient to eliminate the option. The answer recorded here follows the official answer key for the question-paper series represented by the uploaded source and has been cross-checked against an independent published key. The explanation is intended for conceptual revision rather than as a reproduction of any third-party solution.