# Which pH is more acidic, a pH of 3, a ph of 7, or a ph of 9?

The solution whose $p H = 3$ is MOST acidic.
$p H$ $=$ $- {\log}_{10} \left[{H}_{3} {O}^{+}\right]$.
And thus $\left[{H}_{3} {O}^{+}\right] = {10}^{- p H}$. Clearly ${10}^{- 3} > {10}^{- 7} > {10}^{- 9}$; we see here orders of magnitude difference.