Which of the following is a chemical change?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Rusting of iron
A chemical change forms a new substance with new properties and is not easily reversed; a physical change alters only the state, shape or size, and the substance stays the same.
In rusting, iron reacts with the oxygen of the air in the presence of water and forms hydrated iron(III) oxide, \(\ce{Fe2O3.xH2O}\), the brown flaky rust. Rust is a new substance with properties quite unlike iron, and it cannot be turned back into iron by a simple physical step.
Why the other options are incorrect
Option (1) Dissolving of salt in water — Incorrect. No new substance forms; evaporating the water gives the salt back.
Option (3) Boiling of water — Incorrect. Water only turns into steam, which condenses back into water: a change of state.
Option (4) Melting of ice — Incorrect. Solid ice becomes liquid water, still the same substance, and it freezes back on cooling.