The product of a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻) is water (H₂O) and heat.

This reaction is the basis of the neutralization reaction in chemistry:

H₃O⁺ + OH⁻ → 2H₂O + heat

Here's why:

* Hydronium ions (H₃O⁺) are formed when a proton (H⁺) from an acid combines with a water molecule (H₂O).

* Hydroxide ions (OH⁻) are formed when a base releases a hydroxide ion.

* When these ions meet, the proton from the hydronium ion combines with the hydroxide ion to form a water molecule.

This reaction releases heat, making it an exothermic reaction.