The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is a neutralization reaction. The substance always produced in this reaction is water (H₂O).

Here's the balanced chemical equation:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

In this reaction, the hydrogen ion (H⁺) from the acid combines with the hydroxide ion (OH⁻) from the base to form water. The other product is sodium chloride (NaCl), a salt.