Concentration: It is define as the amount of substance per unit volume.
Or
Concentration is defined as the ratio of solute in a solution to either solvent or total solution.
Concentration usually is expressed in terms of mass per unit volume.
When the amount of a substance in a given volume is increased then it alter the concentration of reactant.When the concentration of reactant is increase then the rate of reaction increases.Because when the concentration is increases then more ions or molecules interact to form new compounds therefore, the rate of reaction increases.When the concentration of reactant decrease then less number of ions or molecules interact to form new compounds therefore, the rate of reaction decreases.
