Concentrated acid dilution is an exothermic process.When water is added to a concentrated acid, the heat produced may cause the mixture to splash out, causing burns.The mixture will not splash out if the acid is slowly added to water while stirring constantly.An exothermic process is one in which energy is released in the form of light or heat.As a result, rather than withdrawing energy from the environment as an endothermic reaction does, an exothermic reaction transfers energy into the environment.In an exothermic process, the change in enthalpy (H) is negative.Since the hydrogen ion in acid is so small, a huge amount of charge is concentrated in a small area, attracting the polar water molecules forcefully.The development of a covalent link to one of the waters, as well as a huge number of strong hydrogen bonds, makes this 'hydration' of the hydrogen ion a highly exothermic process.This results in a significant exothermic reaction when a strong acid is mixed with water.