AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The pH scale and neutralisation: A solution with pH 7 is neutral. Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7.
The pH scale runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The pH scale runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is. A solution with pH 7 is neutral, for example pure water at room temperature. Aqueous solutions of acids have pH values below 7: a strong acid such as hydrochloric acid may have pH 1, while a weak acid such as ethanoic acid may have pH 3. Aqueous solutions of alkalis have pH values above 7: sodium hydroxide solution may have pH 13, while ammonia solution may have pH 10. The further the pH is from 7, the more acidic or alkaline the solution. Universal indicator or a pH probe can measure pH, and the colour or reading is compared with a scale.
What earns the marks
- State that pH 7 is neutral, using pure water as the standard example.
- State that acids in aqueous solution have pH values below 7, and link lower values to greater acidity.
- State that alkalis in aqueous solution have pH values above 7, and link higher values to greater alkalinity.
- Use the pH scale from 0 to 14 and place given examples correctly, such as pH 1 for a strong acid and pH 13 for a strong alkali.
- Describe how universal indicator or a pH probe is used to measure pH and how the result is interpreted.
Common mistakes and how to put them right
Saying that all acids have pH 7 or that neutral means pH 0.
Put it right by stating that neutral is pH 7 and acids are below 7.
Confusing strong and weak with concentrated and dilute.
Put it right by explaining that strong or weak describes the degree of ionisation, while concentrated or dilute describes the amount of substance dissolved.
Believing that alkalis are always harmless or that all bases dissolve in water.
Put it right by stating that alkalis are soluble bases and can still be corrosive.
Exam tips
- Quote pH values with the correct inequality, for example pH < 7 for acids and pH > 7 for alkalis.
- When asked to compare solutions, give the pH value and state whether it is acidic, neutral or alkaline.
- If describing a test, name the indicator or probe and state the colour or reading that shows acid, neutral or alkali.
Try the lesson
The start of the The pH scale and neutralisation: A solution with pH 7 is neutral. Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7. lesson, made from checked teaching notes for this specification point.
Have a go first
A student tests four aqueous solutions with universal indicator. Which solution is neutral?
Listen
The pH scale runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is. A solution with pH 7 is neutral, for example pure water at room temperature. Aqueous solutions of acids have pH values below 7: a strong acid such as hydrochloric acid may have pH 1, while a weak acid such as ethanoic acid may have pH 3. Aqueous solutions of alkalis have pH values above 7: sodium hydroxide solution may have pH 13, while ammonia solution may have pH 10. The further the pH is from 7, the more acidic or alkaline the solution. Universal indicator or a pH probe can measure pH, and the colour or reading is compared with a scale. A strong answer earns marks for these points. State that pH 7 is neutral, using pure water as the standard example. State that acids in aqueous solution have pH values below 7, and link lower values to greater acidity. State that alkalis in aqueous solution have pH values above 7, and link higher values to greater alkalinity. Use the pH scale from 0 to 14 and place given examples correctly, such as pH 1 for a strong acid and pH 13 for a strong alkali. A common slip: Saying that all acids have pH 7 or that neutral means pH 0. Instead, stating that neutral is pH 7 and acids are below 7. Exam tip: Quote pH values with the correct inequality, for example pH 7 for alkalis.
See it
Key words
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The pH scale
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Neutral solutions
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Acids and alkalis
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See every definition
- The pH scale
- Runs from 0 to 14 and measures how acidic or alkaline an aqueous solution is.
- Neutral solutions
- A solution with pH 7 is neutral, for example pure water at room temperature.
- Acids and alkalis
- Aqueous solutions of acids have pH values below 7, while aqueous solutions of alkalis have pH values above 7.
Label the diagram
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1Box 1
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3Box 3
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