posted by defa on .
Why must one add water first and then add the acid rather than add the acid first and then water?
Water added to some acids creates so much heat that the solution actually boils and spatters. You don't want the spattering acid to erode your skin, your clothes, or the surroundings. Therefore, you pour the acid into the water. This way, the solution is the more dilute and working toward a more concentrated solution. The other way it is the most concentrated and working toward more dilute. Adding it the way it should be added still produces a great amount of heat but its more gradual and the solution never actually boils (nor spatters). By the way, this is the most important for concentrated sulfuric acid. Most other acids don't have the problem to that extent; but adding acid to water is a good rule to remember.
I meant, I was wondering that too, so thanks even though it isn't my question.