AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The pH scale and neutralisation: The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.
The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A pH below 7 is acidic, pH 7 is neutral, and pH above 7 is alkaline. Universal indicator is a mixture of dyes that gives a colour for each pH value, such as red for strong acids and purple for strong alkalis. A pH probe gives a numerical reading and is more precise, so it is better when you need an exact value. For example, a solution with pH 2 is acidic, while one with pH 12 is alkaline. The scale is logarithmic, so each change of one pH unit represents a tenfold change in hydrogen ion concentration, though at GCSE you mainly need to read and interpret the scale.
What earns the marks
- States that the pH scale runs from 0 to 14 and measures acidity or alkalinity.
- Identifies pH values below 7 as acidic, pH 7 as neutral and pH values above 7 as alkaline.
- Describes universal indicator as a indicator that gives a colour related to pH.
- Describes a pH probe as an instrument that gives a numerical pH reading and is more precise.
- Compares the two methods, noting that a pH probe gives a more accurate value than colour matching.
- Interprets given pH values correctly as acidic, neutral or alkaline.
Common mistakes and how to put them right
Saying that pH 7 is always neutral for every liquid.
Put it right: pH 7 is neutral in aqueous solutions at standard conditions, but the statement applies to solutions.
Confusing universal indicator with litmus.
Put it right: Litmus only shows acidic or alkaline, while universal indicator gives a range of colours across the pH scale.
Reading the pH scale backwards, for example calling pH 12 acidic.
Put it right: Lower numbers are acidic and higher numbers are alkaline.
Exam tips
- Learn the colour range of universal indicator, from red at low pH through green at pH 7 to purple at high pH.
- When comparing methods, mention that a pH probe gives a numerical value and avoids subjective colour judgement.
- Check whether a question asks for a description or a comparison, and include both methods if it asks you to compare.
Try the lesson
The start of the The pH scale and neutralisation: The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe. lesson, made from checked teaching notes for this specification point.
Have a go first
A solution is tested and found to have a pH of 2. What does this pH value show about the solution?
Listen
The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A pH below 7 is acidic, pH 7 is neutral, and pH above 7 is alkaline. Universal indicator is a mixture of dyes that gives a colour for each pH value, such as red for strong acids and purple for strong alkalis. A pH probe gives a numerical reading and is more precise, so it is better when you need an exact value. For example, a solution with pH 2 is acidic, while one with pH 12 is alkaline. The scale is logarithmic, so each change of one pH unit represents a tenfold change in hydrogen ion concentration, though at GCSE you mainly need to read and interpret the scale. A strong answer earns marks for these points. States that the pH scale runs from 0 to 14 and measures acidity or alkalinity. Identifies pH values below 7 as acidic, pH 7 as neutral and pH values above 7 as alkaline. Describes universal indicator as a indicator that gives a colour related to pH. Describes a pH probe as an instrument that gives a numerical pH reading and is more precise. Exam tip: Learn the colour range of universal indicator, from red at low pH through green at pH 7 to purple at high pH.
See it
Key words
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pH scale
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Universal indicator
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pH probe
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Acidic
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See every definition
- pH scale
- Runs from 0 to 14 and is a measure of the acidity or alkalinity of a solution.
- Universal indicator
- A mixture of dyes that gives a colour for each pH value, such as red for strong acids and purple for strong alkalis.
- pH probe
- Gives a numerical reading and is more precise, so it is better when you need an exact value.
- Acidic
- Describes a solution with a pH below 7, for example a solution with pH 2.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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