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Acids differ in their ability to donate H⁺. Stronger acids, such as HCl, react almost completely with water, whereas weaker acids, such as acetic acid (CH₃COOH), react only slightly. The exact strength of a given acid HA in water solution is described using the acidity constant ($K_a$) for the acid-dissociation equilibrium.

Example:$\begin{array}{l}{\text{HA+H}}_2\text{O}⇄{\text{A}}^{−}{\text{+H}}_{\text{3}}{\text{O}}^{−}\\ K_a\underset{}{}\text{=}\underset{}{}\frac{{\text{[H}}_{\text{3}}{\text{O}}^{+}{\text{][A}}^{−}\text{]}}{\left[HA\right]}\end{array}$

Acid strengths are normally using $\text{p}{K}_a$values rather than $K_a$values, where the$\text{p}{K}_a$is the negative common logarithm of$K_a$:

$\text{p}K\text{a=-log}{K}_a$

The stronger acid has a smaller $\mathbf{p}{K}_a$, and a weaker acid has a larger $\mathbf{p}{K}_a$

The substance with the largest value of pK_a is the least acidic. So, the order of increasing acidity is shown below:

Least acidic most acidic

Order of acidity