The pH Scale and Neutralisation
Master The pH Scale and Neutralisation for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The pH scale runs 0–14; acids release H⁺ ions and alkalis release OH⁻ ions, which combine to form water during neutralisation.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The pH scale measures how acidic or alkaline a solution is, running from 0 to 14. A pH below 7 is acidic, 7 is neutral, and above 7 is alkaline.
Acids produce hydrogen ions, H⁺, in aqueous solution. Alkalis produce hydroxide ions, OH⁻. Neutralisation occurs when these combine: H⁺ + OH⁻ → H₂O.
pH can be measured with universal indicator, which gives a range of colours and therefore an approximate value, or with a pH probe and meter, which gives a precise numerical value. Indicators such as litmus give only a broad acid or alkali result, not a pH.
Revision notes
The scale
pH runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral, above 7 is alkaline.
The lower the pH the more acidic the solution; the higher the pH the more alkaline.
Ions in solution
Acids produce hydrogen ions, H⁺, when dissolved in water.
Alkalis produce hydroxide ions, OH⁻. In neutralisation these combine to form water: H⁺ + OH⁻ → H₂O. This ionic equation is worth memorising.
Measuring pH
Universal indicator changes through a range of colours, giving an approximate pH.
A pH probe attached to a meter gives an accurate numerical value. Litmus only shows whether a solution is acidic or alkaline, not how strongly.
Key points
- pH runs from 0 to 14.
- Below 7 is acidic and above 7 is alkaline.
- pH 7 is neutral.
- Acids produce H⁺ ions in solution.
- Alkalis produce OH⁻ ions in solution.
- H⁺ + OH⁻ → H₂O in neutralisation.
Worked examples
Example 1
State the ions produced by acids and by alkalis in aqueous solution. [2 marks]
Model answer
Acids produce hydrogen ions, H⁺ (1 mark). Alkalis produce hydroxide ions, OH⁻ (1 mark).
Example 2
Write the ionic equation for neutralisation. [2 marks]
Model answer
H⁺ + OH⁻ → H₂O. Correct reactants (1 mark). Correct product (1 mark).
Example 3
Explain why a pH probe gives a better measurement than universal indicator. [2 marks]
Model answer
A pH probe gives a precise numerical value (1 mark), whereas universal indicator only gives an approximate value based on matching a colour to a chart (1 mark).
Common mistakes
Saying pH 0 is neutral.
pH 7 is neutral; 0 is strongly acidic.
Confusing H⁺ and OH⁻.
Acids give hydrogen ions; alkalis give hydroxide ions.
Saying litmus gives a pH value.
It only shows whether a solution is acidic or alkaline.
Writing the neutralisation equation without charges.
The ions must be written as H⁺ and OH⁻.
Exam tips
- Memorise the ionic equation for neutralisation.
- State the ion and its formula when asked.
- Distinguish approximate indicators from precise pH probes.
- Remember pH 7 is the neutral point.
Key terms
- pH scale
- A measure of acidity from 0 to 14.
- Hydrogen ion
- H⁺, produced by acids in solution.
- Hydroxide ion
- OH⁻, produced by alkalis in solution.
- Universal indicator
- An indicator giving an approximate pH from its colour.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.