Why do you add Acid to water or aqueous solution but not vice versa?

Question

I am taking an online Lab Safety Course for school before I start next week and on one of the slides it had said "Always Add Acid (AAA) to the water or aqueous solution, never add water to acid!"

I've never heard of this expression and I was wondering why do you add acid to water/aqueous solution but not vice versa?

Thanks :)

Answer 1

You always add acid to water. this is because the ionising that happens i.e. HCl to H+ and CL- and H2SO4 to 2H+ and (SO4)2- is highly exothermic. adding acid to water means you can control the process (water doesn't boil) and splash everywhere. If you do it the other way around it will spit and bubble profusely especially if you add the water slowly (ie carefully).

It can be very dangerous to add water to acid and visa verse. Care must be taken at all times while handling the products.

Answer 2

Water disperses the acid which prevents localized overhearing possibly leading to boiling and splashing.  This is also why the water ought to be stirred when you add the acid.  Sulfuric acid is especially prone to his problem and can be violent when mixed with water improperly.