The substance that undergoes a change in a chemical reaction is called a reactant.

Here's why:

* Reactants are the starting materials in a chemical reaction.

* During the reaction, they break apart and rearrange their atoms to form new substances called products.

Example:

In the reaction of hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O):

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

* Product: Water (H₂O)

The chemical equation for this reaction is:

2H₂ + O₂ → 2H₂O