Why is acid added to water and not water to acid when diluting?

When diluting acid, it's safer to add acid to water than to add water to acid because heat is released when strong concentrated acids are mixed with water. When water is added directly to the acid a very concentrated solution initially forms and the heat that's released can cause boiling and splashing of the solution at the surface. It can also give off acidic steam. This creates a chemical contact hazard for the person preparing the solution.

If instead the concentrated acid is added to water, it initially creates a dilute solution. Heat is evolved more slowly because less acid is being diluted at once. The heat is absorbed by the water.

Chemical splash goggles, protective gloves and a lab apron should be worn when dispensing concentrated acid.