Which of the following is an example of a chemical reaction?

The combustion of methane gas resulting in carbon dioxide and water is indeed an example of a chemical reaction. In this process, the methane (CH₄) reacts with oxygen (O₂) in the presence of heat. This reaction breaks the chemical bonds of the reactants and forms new products: carbon dioxide (CO₂) and water (H₂O). The transformation involves a change in the chemical composition of the substances involved, leading to the release of energy in the form of heat and light.

In contrast, the melting of ice into water, the dissolving of salt in water, and the freezing of liquid water into ice are all physical changes rather than chemical reactions. These processes do not alter the fundamental chemical structure of the substances involved; instead, they change the state of matter (solid, liquid, gas) or the physical form without altering their chemical identity. Therefore, they do not involve the formation of new substances.