Acids are substances that dissociate in water to release hydrogen ions (H⁺) and anions. The extent of this dissociation determines whether an acid is classified as strong or weak.

When acids dissolve in water, they undergo a process called dissociation, where the acid molecules split into hydrogen ions (H⁺) and negative ions (anions). The general dissociation of an acid can be represented as:

Here, HX represents the acid, H⁺ is the hydrogen ion, and X⁻ is the anion. The degree to which an acid dissociates into ions determines its strength.

Strong acids are those that completely dissociate in water. This means that every molecule of the acid separates fully into its ions. As a result, the concentration of H⁺ ions in the solution is high, leading to a low pH.

In strong acids, the dissociation is complete, which is why the concentration of hydrogen ions is directly proportional to the concentration of the acid itself.

Weak acids only partially dissociate in water, meaning that only a small fraction of the acid molecules release hydrogen ions. The dissociation of weak acids is reversible, and an equilibrium is established between the undissociated acid and the ions:

The pH of an acid solution is determined by the concentration of H⁺ ions. As the concentration of H⁺ ions increases, the pH decreases. For every tenfold increase in the concentration of H⁺ ions, the pH decreases by 1 unit.

This relationship can be summarised by the formula: Factor change in \ce{[H⁺]=10^−^X}

Where X is the change in pH. For example, if the pH of an acid increases from 1 to 3, the concentration of H⁺ ions decreases by a factor of 100.

It's important to distinguish between an acid's strength and its concentration: