A limiting reactant is the substance in a chemical reaction that is completely consumed first, preventing any further reaction from occurring. It determines the maximum amount of product that can be formed. When reactants are mixed, the one that runs out first limits the quantity of the final product. In a balanced chemical equation, the amounts of each reactant are compared to find the limiting reactant. For example, in the reaction between hydrogen and oxygen to produce water, if there is not enough hydrogen to react with all the oxygen, then hydrogen is the limiting reactant.