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.

The correct answer is B, since only souring of milk is a chemical change. Souring happens because bacteria such as Lactobacillus convert lactose (milk sugar) into lactic acid. This produces a new substance with a different chemical composition, and the change cannot be reversed by simply cooling or heating. Formation of a new substance and irreversibility are the usual markers of a chemical change.

Why the others fail:

• Statement 1, crystallization of sodium chloride: Salt dissolved in water comes out as crystals with the same NaCl composition. Only the physical state and arrangement change, so this is a physical change.
• Statement 2, melting of ice: H2O changes from solid to liquid, but the molecules remain H2O. This is a physical change.

The concept connects to the Class 8 and 10 chemistry basics that UPSC tests repeatedly, and in Mains it helps in discussing food processing, fermentation and dairy technology. Note that 'chemical' here does not mean 'harmful'; the question only asks whether the composition of the substance changes.

Trap: Aspirants see crystals forming or a solid turning into a liquid and call it chemical because it looks dramatic. Ask whether a new substance forms. If the chemical identity is unchanged, it is a physical change.