Reaction in which energy is released either in heat or light form. Hence, this reaction releases energy in the surrounding rather than taking energy from surroundings.
An exothermic reaction is defined in thermochemistry as "a reaction in which the overall standard enthalpy change is negative."
The reason for water never being added to acid:
When we add water to acid, we get a concentrated solution that produces heat.
This solution may boil and spill strong acid from the container.
This is an exothermic reaction, and it is safer to add acid to water rather than water to acid.
