Substances that change when they become involved in a chemical reaction are called reactants.

Here's why:

* Chemical reactions involve the breaking and forming of chemical bonds. This means the atoms within the original substances rearrange themselves, resulting in new substances with different properties.

* Reactants are the starting materials in a chemical reaction. They undergo changes to become the products.

* Products are the substances formed as a result of a chemical reaction. They have different chemical compositions and properties than the reactants.

Example:

In the reaction of hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O), the reactants are hydrogen and oxygen, and the product is water.

H₂ + O₂ → H₂O