# As the pH number of an acid decreases, what happens to the strength of the acid?

Apr 20, 2018

As $p H$ increases, the acidity of the solution decreases.

#### Explanation:

First, let's begin with some definitions:

• In the Bronsted-Lowry definition, acids are donors of ${H}^{+}$ ions.
• Strong acids are those that almost completely ionise in aqueous solutions to form these ${H}^{+}$ ions.
• $p H$ is the negative $\log$ of the concentration of hydrogen cations ($\left[{H}^{+}\right]$) in an aqueous solution.

$p H = - \log \left[{H}^{+}\right]$

Let's plug some numbers in for $\left[{H}^{+}\right]$ and try to notice trends!:

$p H = - \log \left(1.0 \times {10}^{- 2}\right) = 2.00$
$p H = - \log \left(1.0 \times {10}^{- 3}\right) = 3.00$
$p H = - \log \left(1.0 \times {10}^{- 4}\right) = 4.00$

We can see that, as $\left[{H}^{+}\right]$ increases, the $p H$ value decreases.

As the $p H$ value increases, $\left[{H}^{+}\right]$ decreases.

We defined a strong acid as one that almost completely ionises in aqueous solutions to form ${H}^{+}$ ions. So, in strong acids, $\left[{H}^{+}\right]$ will be high and $p H$ will be low.

Therefore, as the $p H$ of a solution increases, $\left[{H}^{+}\right]$ would decrease and the acidity of the solution would also decrease as a result. :)