Question

In an experiment, Ann mixes concentrated sulfuric acid with water in a reaction test tube. The walls of the test tube turn hot. What type of reaction has occurred?

Answer 1

I hope Ann got the order of addition right! Always add acid to water. An hydration reaction has occurred.

Explanation:

Upon addition to water concentrated acids become hydrated, and both anion and cation become solvated by water molecules to form `H_3O^+(aq)` and `SO_4^(2-)(aq)`, each aquated by (solvated by) water molecules. Because bonds are formed between water and `H^+` this reaction is reasonably exothermic and releases heat.

The addition of water to acid results in local heating of the added water. The acid would spit at you. (If you spit in acid it spits back!). This is always something to know when you use concentrated acids in the laboratory.