What is produced when an acid reacts with water?
Heat and an a more dilute acid
Always add the acid to the water, no the water to the acid.
The amount of heat that can be produced by entropy as the concentrated acid is diluted by the water is significant.
putting water in the acid releases the energy all at once and can be dangerous.
Adding a small amount of acid to the water slowly allows the heat produced to be released slowly and safely.