a chemical reaction is a specific thing within the realm of chemistry, rather than attempt to define it myself and get it wrong, I will quote Oxford Languages (via google) here:

chemical reaction: process that involves rearrangement of the molecular or ionic structure of a substance, as distinct from a change in physical form or a nuclear reaction.

So, a chemical reaction is not just what you would colloquially describe as a reaction of something that belongs to the realm of chemistry, it has a specific definition. A reaction is H + O2 = H2O (omitting concerns of stoichiometry for simplicities sake). A reaction is not e.g. salt dissolving in water, because salt stays salt (NaCl) and water stays water (H2O)