# What happens to acid when put in water?

It is conceived to increase the concentration of the hydronium ion, ${H}_{3} {O}^{+}$, the characteristic cation of water.
The lower hydrogen halides, $H - X$, $\left(X \ne F\right)$, are all room temperature gases. They are all quite soluble in water, and in aqueous solution they are conceived to protonate the water molecule:
$H X + {H}_{2} O \rightarrow {H}_{3} {O}^{+} + {X}^{-}$
The $\text{hydronium}$ or $\text{acidium ion}$ is a cluster of 3-4 water molecules with an extra ${H}^{+}$ added. We use representations such as ${H}^{+}$ or ${H}_{3} {O}^{+}$ for simplicity.