Diluting a concentrated acid :
The process of dissolving an acid in water is a highly exothermic reaction. Care must be taken while mixing concentrated acids (specifically, strong acids) with water. Always add acid to water and not the other way round. The acid must always be added slowly to water with constant stirring.
But, if water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause bums. The glass container may also break due to excessive heating.