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?

Upon addition to water concentrated acids become hydrated, and both anion and cation become solvated by water molecules to form ${H}_{3} {O}^{+} \left(a q\right)$ and $S {O}_{4}^{2 -} \left(a q\right)$, each aquated by (solvated by) water molecules. Because bonds are formed between water and ${H}^{+}$ this reaction is reasonably exothermic and releases heat.