The pH Scale

The pH turns a concentration into an easy number:

$$\mathrm{pH} = -\log[\mathrm{H_3O^+}]$$

[H₃O⁺] = 10⁻³ M is pH 3. Back again: $[\mathrm{H_3O^+}] = 10^{-\mathrm{pH}}$.

Below 7 acidic, 7 neutral, above 7 basic.

The pH scale

One pH step is a factor of 10 in [H₃O⁺]: lower pH, more H₃O⁺.