Reactivity is classified as a chemical property. This is because reactivity refers to the ability of a substance to undergo a chemical change when it interacts with another substance. During these interactions, the original substances can transform into entirely new substances, which is characteristic of chemical reactions.
For example, when hydrogen reacts with oxygen, they form water, demonstrating that a new substance is created as a result of their interaction. Although the atoms remain the same, their arrangement changes, leading to new properties and behaviors. In contrast, physical properties are associated with changes that do not alter the chemical composition of a substance, such as phase changes or temperature variations. Therefore, since reactivity involves the formation of new substances, it is considered a chemical property.