How It Works: H⁺ Ions Make Solutions Acidic, OH⁻ Make Them Alkaline

⚙️ How It Works: H⁺ Ions Make Solutions Acidic, OH⁻ Make Them Alkaline

The pH of a solution is determined by the concentration of hydrogen ions (H⁺) in that solution. In pure water, a small proportion of water molecules ionise spontaneously: H₂O ⇌ H⁺ + OH⁻. At 25°C, this produces equal, tiny concentrations of H⁺ and OH⁻ — so water is neutral with pH 7.

When an acid dissolves in water, it releases extra H⁺ ions. This tips the balance — there are now more H⁺ ions than OH⁻ ions, and the pH falls below 7. The stronger the acid and the more concentrated the solution, the more H⁺ ions are produced, and the lower the pH.

When an alkali dissolves in water, it releases OH⁻ ions. These react with the H⁺ ions already present (H⁺ + OH⁻ → H₂O), reducing the H⁺ concentration. With fewer H⁺ ions, the pH rises above 7. A strongly alkaline solution has very few H⁺ ions and a high OH⁻ concentration.

This is also why the pH scale is logarithmic (though GCSE does not require you to calculate this): each whole number change represents a ten-fold change in H⁺ concentration. A solution at pH 3 has ten times more H⁺ ions than one at pH 4.