Which of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Souring of milk Select the correct answer using the code given below.
- (a) 1 and 2 only
- (b) 3 only ✓ UPSC's answer
- (c) 1, 2 and 3
- (d) None
Why the answer is (b)
• A chemical change produces new substances; a physical change alters only state or form.
• Souring of milk (3) is chemical: bacteria convert lactose into lactic acid, forming new compounds irreversibly.
• Crystallisation of salt (1) and melting of ice (2) are physical changes — the substance stays NaCl or H2O and the change is reversible.
• Hence 3 only, option (b).
Why the other options are wrong
- (a) 1 and 2 only
- Crystallisation and melting are physical changes.
- (c) 1, 2 and 3
- Only souring of milk is chemical.
- (d) None
- Souring of milk is a chemical change.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2014, held on 24 August 2014. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.