# What pH level is considered to be acidic and basic?

Dec 6, 2016

In aqueous solution, for $p H$ $<$ $7$, an acidic solution is specified...........

#### Explanation:

And for $p H$ $>$ $7$, a basic solution is described. For $p H = 7$, we describe a NEUTRAL solution.

But what is $p H$?

$p H$ is a measure of the concentration of hydronium ion, $\left[{H}_{3} {O}^{+}\right]$, i.e. $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$. Equivalently, $p H = - {\log}_{10} \left[{H}_{^} +\right]$. ${H}_{3} {O}^{+}$ and ${H}^{+}$ are used to represent the $\text{acidium ion}$ in aqueous solution.

The use of logarithms is a bit old-fashioned, but back in the day A level students used to used log tables, before the advent of cheap electronic calculators.

For water:

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

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

Now ${K}_{w}$ $=$ $\left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right]$ $=$ ${10}^{- 14} \text{ at 298K}$.

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

$= - {\log}_{10} \left({10}^{-} 14\right)$ $=$ $+ 14$

And thus $p O H + p H = 14$ under standard conditions.

You should review the logarithmic function if you have any problem with this treatment. You should review it anyway, because I might have made a mistake.