# Why is water neutral in pH despite the fact that it produces hydrogen and hydroxide ions?

Dec 3, 2015

Because at neutrality, the concentration of hydroxide ion is equal to the concentration of protium ion.

#### Explanation:

As you know, water undergoes the equilibrium reaction:

${H}_{2} O r i g h t \le f t h a r p \infty n s {H}^{+} + O {H}^{-}$

(These days, it's a little bit more common to speak of the hydronium ion, ${H}_{3} {O}^{+}$: $2 {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + O {H}^{-}$) Both equations represent the autoprotolysis of water. Now, this equilibrium has been extensively studied, and under standard conditions; the equilibrium constant for the reaction, ${K}_{w} = {10}^{- 14}$ $=$ $\left[{H}^{+}\right] \left[O {H}^{-}\right]$.

If the solution is neutral, then $\left[{H}^{+}\right] = \left[O {H}^{-}\right]$ (and $p H = p O H = 7$, where $p H = - {\log}_{10} \left[{H}^{+}\right]$ and $p O H = - {\log}_{10} \left[O {H}^{-}\right]$).

${K}_{w}$ is quoted under standard conditions of $1$ $a t m$ and $298 K$. What do you think would happen under non-standard conditions, say at a temperature of $350$ $K$; would $p {K}_{w}$ go up or go down? (Be careful with the negative sign!)