While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Water splits and acid into its constituents cation and anion. The process is highly exothermic.
HA + H2O ---> H3O+ + A-+ Δ
If we add water to a concentrated acid the reaction will be spontaneous and may cause violent splashing. However, if we add acid slowly to a water solution the heat evolved will be gradual and there will be no spillage or pilferage.
As a result, we do add acid to water but not water to acid.
Next: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?