Which sort of change is combustion?

Combustion is classified as a chemical change because it involves the transformation of substances through a chemical reaction. During combustion, a fuel reacts with an oxidizer, typically oxygen, resulting in the production of new substances, most commonly carbon dioxide and water. This process is characterized by the breaking of chemical bonds in the reactants and the formation of new bonds in the products, indicating a fundamental change at the molecular level.

In addition to producing different chemical substances, combustion also releases energy in the form of heat and light, which is often observable as flames. This energy release further emphasizes that a chemical reaction is taking place rather than a mere physical change, where the substance's chemical composition remains unchanged.

Physical changes, on the other hand, do not alter the chemical structure of the substance, such as state changes (solid to liquid) or dissolving in water. Biological changes typically refer to processes related to living organisms, such as metabolism or decomposition, and thermal changes pertain to variations in temperature without necessary changes in chemical composition. Therefore, the classification of combustion as a chemical change is essential in understanding the nature of the reactions and products involved in this important process.