LIMITING REACTANT
A reactant which is completely consumed earlier leaving behind the other reactant in excess is called limiting reactant.
A limiting reactant
(i) Is consumed earlier
(ii) Is in lesser than stoichiometric amount
(iii) Limitize (controls) the chemical reaction.
(iv) Limitize (controls) the amount of product.
(v) Gives leas amount of product.
Example:

If two moles of hydrogen (4g) and two moles of oxygen (64g) are made to react, 2moles of water will be formed.
Actually, 2moles of O2 requires 4moles of H2 for complete reaction. To react with 2moles of O2, H2 is in (2moles) lesser than stoichiometric (required) amount (4moles). Hence, will be consumed earlier and will be the limiting reactant.
Note: One of the reactant will be the limiting reactant only when it is in lesser than stoichiometric amount.
Significance of Limiting Reactant:
The significance of limiting reactant is as follows.
(i) Expensive reactant is taken as limiting to ensure that it is completely used up in a chemical reaction.
(ii) Sometimes one of the reactant is taken as limiting reactant to make the reaction faster e.g. Taking oxygen in excess to burn a substance makes the burning faster.
(iii) To determine the effect of concentration on the rate of reaction.
Identification of limiting reactant:
To identify a limiting reactant, the following three steps are performed:
1. Calculate the number of moles from the given amount of reactant.
2. Find out the number of moles of product with the help of a balanced chemical equation.
3. Identify the reactant which produces the least amount of product as limiting reactant.