What is an excess reactant?

• A. The reactant that is completely used up
• B. The product formed
• C. The unreacted reactant which has not been used
• D. The catalyst

Answer: (C)

In a chemical reaction, reactants are used in fixed proportions. If there’s more of one reactant than is needed, the extra amount doesn’t react and remains unused. That leftover is what we call the excess reactant. It’s the material that stays behind once the reaction has gone as far as the other reactants allow.

So the unreacted substance that remains unused matches the idea of an excess reactant. The option about the reactant being completely used up describes the limiting reactant, not the excess. The product formed is the outcome of the reaction, not an excess reactant. A catalyst speeds up the reaction without being consumed, so it’s not an excess reactant either.