Hydronium ions (H₃O⁺) react with hydroxide ions (OH⁻) to form water (H₂O) and heat. This is a neutralization reaction:

H₃O⁺ + OH⁻ → 2H₂O + heat

Here's why:

* Hydronium ions represent the proton (H⁺) from an acid dissolved in water.

* Hydroxide ions represent the basic component of a solution.

* When they combine, the proton from the hydronium ion bonds with the hydroxide ion, forming water.

* The reaction releases heat, indicating it is exothermic.