UPSC CSE Prelims 2014
Souring of milk is an example of a chemical change. This process involves the conversion of lactose (milk sugar) into lactic acid by microorganisms. New substances with different chemical properties are formed, and the change is generally irreversible, indicating an alteration in chemical composition.
Crystallization of sodium chloride is a physical change. It involves the formation of solid crystals from a solution. The chemical composition of sodium chloride (NaCl) remains unchanged; only its physical state or arrangement is altered. This process is reversible.
Melting of ice is a physical change. It represents a change in the state of matter from solid (ice) to liquid (water). The chemical composition of water (H₂O) remains identical in both states. This change is reversible by freezing.
Based on the above, only statement 3 describes a chemical change. Statements 1 and 2 describe physical changes. Therefore, options that include statements 1 or 2 as chemical changes, or exclude statement 3, are incorrect.