Acid Into Water, Never Water Into Acid: Why the Order Is Not a Preference

Diluting a strong acid RELEASES HEAT — a great deal of it. Concentrated sulfuric acid and water together can reach boiling point in seconds, and what boils is not the acid but the small amount of water that got there first. That is the whole reason for the order. Pour acid into a large volume of water and the heat is spread through all of it, raising the temperature a little. Pour water into acid and the first drops land in a pool of acid, boil instantly, and spit concentrated acid back out of the vessel. The rule people remember it by is crude and effective: **do as you oughta, add acid to water.** It applies to every strong acid and to concentrated alkali solutions too, and it is not a laboratory formality — it is the difference between a warm beaker and a face.