In many situations, an excess of one or more substance is available for chemical reaction. Some of these excess substances will therefore, be left over when the reaction is complete. The reaction stops immediately as soon as one of the reactants is totally consumed. Consider a chemical reaction given below initiated by passing a spark through a reaction vessel containing 10 mol of H₂ and 7 mol of O₂. Since 2 mol H₂ = 1 mol O₂, thus

                       2H₂ (g) + O₂ (g) → 2H₂O (g)

Moles before

reaction          10             7              0

Moles after

reaction           0              2              10

It is thus evident that the reaction stops only after consumption of 5 moles O₂ since no further amount of H₂ is left to react with unreacted O₂. The substance (here H₂) that is completely consumed in the reaction is called limiting reagent because it determines or limits, the amount of product. The other reactants present in excess are called as excess reagents.