A substance that forms a chemical reaction is called a reactant.

Here's a simple explanation:

* Reactants are the starting materials in a chemical reaction. They are the substances that will change and form new substances.

* Products are the substances that are formed as a result of the chemical reaction.

For example, in the reaction of hydrogen and oxygen to form water:

* Reactants: Hydrogen (H2) and Oxygen (O2)

* Product: Water (H2O)