Question

Why we should always mix acid in water but not water in acid ?why?

Answer 1

The answer is very simple.
On adding water to acid, exothermic reaction takes place as the acid is initially concentrated but later it causes explosion due to sudden change in the enthalpy. It cause violent reaction splashing acid out of the vessel. (Shown in figure)
While in case of adding acid to water, water being absorbable easily assimilate acid content being poured without splashing.