The substance that a catalyst works on is called the reactant or substrate.

Here's why:

* Catalyst: A substance that speeds up a chemical reaction without being consumed in the process.

* Reactant: A substance that participates in a chemical reaction and is transformed into a product.

* Substrate: In biological reactions, the specific reactant that an enzyme (a biological catalyst) acts on.

Think of it this way: The catalyst helps the reactant(s) to transform into the product(s) more quickly. The catalyst itself remains unchanged throughout the reaction.