Excess Reactant
An excess reactant is a substance in a chemical reaction that is present in a larger amount than necessary to completely react with the limiting reactant. The limiting reactant is the substance that is completely consumed first, determining the maximum amount of product that can be formed.
In a typical reaction, once the limiting reactant is used up, the excess reactant remains unreacted. This leftover material can be important for understanding reaction efficiency and yield. For example, in the reaction between hydrogen and oxygen to produce water, if there is more hydrogen than needed, it becomes the excess reactant.