Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
In my textbook, it is given that the ionic product of water $K_\mathrm{w}$ remains constant even when a non-neutral solute such as an acid is added to it.
$$K_\mathrm{w} = \ce{[H3O+][OH-]}$$
When a strong acid is added to water, the concentration of $\ce{[H3O+]}$ increases. Yet the $K_\mathrm{w}$ must remain constant. But how? How does the concentration of $\ce{[OH-]}$ decrease?
$K_w$ is the equilibrium constant of this reaction: $$\ce{2H2 O <=> H3O+ +OH-}.$$ So, it's constant at a fixed temperature. It equals $10^{-14}$ at $25~^\circ\mathrm{C}$. At this temperature $\ce{[H3O+]=[OH^{-}]}=10^{-7}~\mathrm{mol/L}$.
If you add acid to water, the above reaction equilibrium is displaced in order to counteract the change in the concentration of ion hydronium, i.e. the reaction is displaced to the left. This means that ions hydroxide will react with the excess of ions hydronium to give water. As a result: the concentration of ion hydroxide will be less than $10^{-7}~\mathrm{mol/L}$, and the concentration of ion hydronium will be more than $ 10^{-7}~\mathrm{mol/L}$. But their product is constant. $\ce{[H3O+] \cdot[OH^{-}]}=10^{-14}$.
In Layman’s views, you can also think of it this way: initially, you have water, a small amount of $\ce{H3O+}$ and a small amount of $\ce{OH-}$. If you add an external proton source, we can consider what will happen to each of these:
\begin{array}{lccc}\hline \text{time point} & \text{acid} & \text{neutral} & \text{base} \\ \hline \text{before a.} & \ce{H3O+} & \ce{H2O} & \ce{OH-} \\ \text{after a.} & \text{no change} & \ce{H3O+} & \ce{H2O} \\ \hline \end{array}
In words: Nothing will happen to $\ce{H3O+}$ — at least nothing notable; $\ce{H4O^2+}$ is too unstable in aquaeous solution to contribute meaningly. Water will be protonated to $\ce{H3O+}$ and hydroxide ions will also be protonated (statistically) by the incoming acid to give $\ce{H2O}$. Thus, you can consider the addition of acid to both increase oxonium ion concentration and reduce hydroxide ion concentration.
The equilibrium constant and equation is just a more formal and scientifically more correct way of putting this.
