While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
The process of dissolving an acid in water is highly exothermic. It produces a large amount of heat. When acid is added to water slowly with constant stirring, the heat is distributed throughout the large volume of water, keeping the temperature under control.
However, if water is added to concentrated acid, the heat generated causes the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Therefore, it is recommended that the acid should always be added to water and not water to the acid.
Explanation
This answer is based on the textbook explanation in the section discussing dilution of acids. The key concept is that dissolving acid in water is exothermic. Adding acid to water ensures the heat is absorbed by the larger volume of water, whereas the reverse causes localized intense heating leading to splashing and potential breakage of the container. This is a standard safety precaution mentioned in NCERT.