Excess reactant is defined as

• A. Unreacted reactant which has not been used
• B. The product formed
• C. The catalyst
• D. The limiting reagent

Answer: (A)

Excess reactant is the amount of a reactant that remains unreacted after the reaction has gone as far as the amounts of the other reactants allow. In any reaction, one reactant is used up first—the limiting reagent—and once it’s used up, the reaction stops. Any extra quantity of the other reactants that wasn’t needed to react with the limiting reagent stays behind as excess. For example, in the reaction 2H2 + O2 → 2H2O, if you start with 5 units of hydrogen and 3 units of oxygen, hydrogen runs out first (you’d need 6 units of hydrogen to use all the oxygen), so some oxygen remains unreacted. That leftover oxygen is the excess reactant.