Question

When two substances react to form products, the reactant which is used up is called what?

Answer 1

The reactant that is completely used up (while some amount of the others may still remain) is called the limiting reactant.

Explanation:

The limiting reactant is named what it is because it is what limits the chemical reaction. Since it gets used up first, it must produce the least amount of product, and adding more would allow for more of the reaction to occur.