Dilution of concentrated acid is known to be an exothermic process. If water is added to a concentrated acid, the heat or energy liberated may cause the mixture to splash out and cause burns. When the acid is added to water slowly with constant stirring, the mixture will not splash out, thus will not burn or harm us. That is why it is preferred to add acid to water slowly.
Note:
Exothermic Process: The reaction or process in which energy is liberated in the form of heat or light is called the exothermic process.