# On the pH scale, which numbers indicate a basic solution? An acidic solution? Why?

Oct 2, 2016

On the $p H$ scale in aqueous solution, values of $p H > 7$ indicate a basic solution.........

#### Explanation:

And of course values of $p H < 7$ indicate an acidic solution. $p H$, literally $\text{pouvoir hydrogen}$, measures the concentration of ${H}_{3} {O}^{+}$, conceived to be the acidic entity in aqueous solution. Extensive measurement has confirmed the following equilibrium in water under standard conditions:

$2 {H}_{2} O \left(a q\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H {O}^{-}$

And this reaction can be quantified by taking the ion product:

$\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{- 14}$

If we take $- {\log}_{10}$ of both sides we get,

$- {\log}_{10} \left[{H}_{3} {O}^{+}\right] - {\log}_{10} \left[H {O}^{-}\right] = 14$

But under our standard defintion of $p H$ and $p O H$,

$p H + p O H = 14$

And thus values of $p H > 7$ indicate a preponderance of hydroxide ions, and $p H < 7$ indicates a preponderance of hydronium ions.

What is $p H$ at neutrality? Why?