Which one of the following processes involves chemical reaction ?

A chemical reaction involves the formation of new substances with different chemical properties. Option D (Burning of magnesium ribbon in air) is a chemical reaction because magnesium reacts with atmospheric oxygen to produce a new substance, magnesium oxide (MgO). The chemical equation is: 2Mg + O2 → 2MgO. This process is irreversible by physical means and is accompanied by the evolution of heat and light.

• Evaporation of petrol: This is a physical change involving a change of state from liquid to gas without altering chemical composition.
• Dissolution of salt in water: This is a physical change; the salt can be recovered through evaporation, and no new chemical compound is formed.
• Storing oxygen under pressure: This is a physical process involving the compression of gas molecules into a cylinder without any change in the chemical nature of oxygen.