GCSE Chemistry Revision — pH and Neutralisation
Revise pH and Neutralisation for GCSE Chemistry with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is pH and Neutralisation?
The pH scale measures the acidity or alkalinity of a solution. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. Neutralisation is a chemical reaction between an acid and a base, which produces a salt and water. In this reaction, hydrogen ions (H⁺) from the acid react with hydroxide ions (OH⁻) from the alkali to form water (H₂O).
Board notes: Neutralisation and the pH scale are fundamental concepts for all boards. For higher tiers, you will need to understand the difference between strong and weak acids and alkalis in terms of their ionisation in water.
Step-by-step explanationWorked examples
Worked example
When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the H⁺ ions from the acid and the OH⁻ ions from the alkali combine to form water. The Na⁺ and Cl⁻ ions are left in solution, forming the salt sodium chloride. The overall equation is HCl + NaOH → NaCl + H₂O.
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Common mistakes
- 1Thinking that neutralisation always results in a solution with a pH of exactly 7. This is only true for the reaction of a strong acid with a strong alkali.
- 2Forgetting the ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l). This is the key chemical change that occurs.
- 3Confusing the terms 'strong' and 'weak' with 'concentrated' and 'dilute'. Strength refers to the degree of ionisation, while concentration refers to the amount of substance per unit volume.
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Frequently asked questions
What is a weak acid?
A weak acid is an acid that only partially ionises in water. This means that in a solution of a weak acid, only a small proportion of the acid molecules have released their H⁺ ions. Acetic acid (in vinegar) is a common example.
How do you measure pH?
You can measure pH using a chemical indicator, like universal indicator, which changes colour at different pH values. For a more accurate measurement, a digital pH probe can be used.