Question

while diluting an acid why it us recommended that acid should be added to water but not water to acid

Answer 1

Adding water to acid causes excessive local heating which may cause the test tube to crack. 

Answer 2

Explanation:

Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.