When an acid is added to water, hydroxide ions (OH⁻) react with the hydrogen ions (H⁺) released by the acid, forming water (H₂O). This process is part of acid-base neutralization.
In a more general sense, adding an acid to water increases the concentration of hydrogen ions in the solution, while hydroxide ions tend to decrease. However, in pure water, the concentration of hydroxide ions remains relatively constant, but the equilibrium can shift depending on the concentration of the added acid.
Overall, the reaction can be represented as follows:
\[ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} \]
In summary, when an acid is added to water, hydroxide ions can neutralize some of the hydrogen ions, resulting in the formation of water.