How It Works: H⁺ Ions Make Solutions Acidic, OH⁻ Make Them Alkaline
Part of Acids and Alkalis — GCSE Chemistry
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 20 exam-style questions and 20 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. 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.
Topic position
Section 4 of 12
Practice
20 questions
Recall
20 flashcards
⚙️ 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.