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Chemical reactions will stop when one or more of the reactants is completely consumed. The reactant that is completely used up is the limiting reactant. It determines how much product is formed in the reaction.
A reactant that is left over when the reaction stops is called the excess reactant.
In the diagram, the same amount of hydrochloric acid is used in each flask, but (from left to right) increasing amounts of magnesium metal are added. When the magnesium is competely reacted (it is the limiting reactant), the production of hydrogen gas ends. The amount of product is indicated by the size of the balloons. Hydrochloric acid still remains in the flask. It is the excess reactant.