The substance always produced when an Arrhenius acid reacts with an Arrhenius base in aqueous solution is water (H₂O).

Here's why:

* Arrhenius acids release hydrogen ions (H⁺) in water.

* Arrhenius bases release hydroxide ions (OH⁻) in water.

When these ions meet, they combine to form water:

H⁺ + OH⁻ → H₂O

This reaction is a fundamental neutralization reaction.