When hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) combine, they form water (H₂O) and heat. This is a neutralization reaction:

H₃O⁺ (aq) + OH⁻ (aq) → 2H₂O (l) + heat

Here's how it works:

* Hydronium ion (H₃O⁺): A proton (H⁺) attached to a water molecule.

* Hydroxide ion (OH⁻): A water molecule that has lost a proton.

When they combine, the proton from the hydronium ion bonds with the hydroxide ion, effectively reforming two water molecules. This reaction releases heat, making it an exothermic reaction.