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.

[UPSC Civil Services Exam – 2014 Prelims]

(a) 1 and 2 only

(b) 3 only

(c) 1, 2 and 3

(d) None

Answer: (b)        

Explanation:

• Crystallization of sodium chloride – It is a physical change since sodium chloride does not transform into some other chemical element after crystallization.
• Melting of ice – It is not a chemical change since the composition of water molecules stays the same before and after melting.
• Souring of milk – Lactic acid bacteria ferment lactose at room temperature and turn it into lactic acid. As a re­sult, the acid­i­ty of the milk in­creas­es, which in its turn caus­es the pro­tein to fold and liq­uid to sep­a­rate into whey and a vis­cous mass. The soured milk has new elements and it cannot be reversed. Hence, it is a chemical change.