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.
This how it works covers How It Works: H⁺ Ions Make Solutions Acidic, OH⁻ Make Them Alkaline within Acids and Alkalis for GCSE Chemistry. Revise Acids and Alkalis in Chemical Changes for GCSE Chemistry with 25 exam-style questions and 21 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 4 of 12 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Acids and Alkalis
Which ion do acids produce when dissolved in water?
Explain the difference between a strong acid and a concentrated acid.