Question

why during dilution acid is added in water not water in acid

Answer 1

o much heat is released that the solution may boil very violently, splashing concentrated acid out of the container and all this because the reaction is exothermic. On the other hand, adding acid to water, the solution that forms is verydilute and the small amount of heat released is not enough to vaporize it.

Answer 2

if you will add water in acid large amount of heat will be released splashing concentrated acid out of the container because of exothermic reaction
whereas in adding acid in water the solution that is formed is very dilute and the small amount of heat released is not enough to vaporize it.