The process by which a particular substance or different substances are transformed to another is called as Chemical Reaction. In a chemical reaction the starting compounds which are being reacted with each other are called as Reactants. While, those compounds which are being formed and produced from reactants after reaction are called as Products.
The concentration of reactants decreases as the reaction proceeds and at last their concentration comes to an end. Hence, the reaction stops too. If two reactants are being reacted, then the reactant which is in limited amount will consume first and will result in the termination of reaction.