The pH scale and neutralisation — AQA GCSE Chemistry
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The pH scale and neutralisation explained
When an acid dissolves in water, it ionises to release hydrogen ions, H⁺.
Read the full explanation
These are the species responsible for acidic properties such as turning universal indicator red or orange, reacting with metals to produce hydrogen gas, and reacting with bases to form salts and water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which contains H⁺(aq) and Cl⁻(aq); the H⁺ ions are produced because the polar water molecules pull the HCl molecule apart. The subscript (aq) shows that the ions are dissolved in water. In aqueous solution, H⁺ ions are strongly attracted to water molecules, but for GCSE it is sufficient to represent them as H⁺(aq). The concentration of H⁺ ions determines the pH: a higher concentration of H⁺ gives a lower pH value.
Your focus
- State that acids produce hydrogen ions, H⁺, in aqueous solution.
- Explain how the concentration of H⁺ ions relates to the pH of a solution.
- Write ionic equations that include H⁺(aq) for reactions of acids.
The pH scale and neutralisation exam tips
Marking Points
- State that acids produce hydrogen ions, H⁺, when dissolved in water.
- Use the term aqueous or the symbol (aq) to show that the ions are in solution.
- Link the presence of H⁺ ions to acidic properties such as low pH, reaction with metals, and neutralisation.
- Explain that the concentration of H⁺ ions determines the pH value, with more H⁺ giving a lower pH.
- Write balanced ionic equations for acid reactions, showing H⁺(aq) as a reactant where appropriate.
Examiner Tips
- 💡When explaining acidic properties, always refer to H⁺(aq) rather than just 'hydrogen' to show precise chemical understanding.
- 💡If asked why a solution is acidic, link your answer to the presence and concentration of H⁺ ions.
- 💡In ionic equations, balance both atoms and charge, and include state symbols where they are given or required.
Common Mistakes
- Writing the hydrogen ion without a charge or with the charge in the wrong place: students may write H or H+ instead of H⁺. Correction: always show the positive charge as a superscript, H⁺.
- Confusing hydrogen ions with hydrogen atoms or hydrogen molecules: students may say acids contain hydrogen gas. Correction: acids produce H⁺ ions in aqueous solution; hydrogen gas, H₂, is only produced when some acids react with metals.
- Omitting the aqueous state symbol: students may write H⁺ without (aq). Correction: include (aq) to show the ion is dissolved in water, because acids only produce H⁺ ions in aqueous solution.