AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a…
Full specification wording
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.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 6
- 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 tells you how acidic or alkaline a solution is. A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is alkaline. The scale is logarithmic, so each change of one pH unit represents a tenfold change in the concentration of H⁺ ions; for example, pH 3 is ten times more acidic in terms of H⁺ concentration than pH 4. Universal indicator is a mixture of dyes that gives a colour for each pH, which you compare with a colour chart; it is quick but only gives an approximate value. A pH probe is an electronic sensor connected to a meter; it gives a numerical reading and is more precise and objective.
For example: Testing a solution with universal indicator might show orange, suggesting pH 4, while a pH probe could give a reading of 4.2.
What earns the marks
- State that the pH scale runs from 0 to 14 and is a measure of acidity or alkalinity.
- Identify that pH below 7 is acidic, pH 7 is neutral, and pH above 7 is alkaline.
- Describe how universal indicator is used, including matching the colour to a pH colour chart to estimate pH.
- Describe how a pH probe is used, including reading a numerical value from a meter for a more precise measurement.
- Compare the two methods, recognising that universal indicator is approximate while a pH probe is more precise and gives a numerical value.
Common mistakes and how to put them right
Thinking that pH 0 means no acid or that pH 14 means the strongest possible alkali in all contexts.
Put it right: pH is a logarithmic measure of H⁺ concentration, and the scale simply runs from 0 to 14 for the solutions studied.
Believing universal indicator gives an exact pH value.
Put it right: Universal indicator gives an approximate pH based on colour matching; a pH probe gives a more precise numerical reading.
Confusing the colour of universal indicator with the colour of a named indicator such as litmus.
Put it right: Universal indicator has a range of colours across the pH scale, while litmus is mainly red in acid and blue in alkali.
Exam tips
- When describing a measurement, name the method and state whether the result is approximate or precise.
- Use the pH scale correctly: state acidic, neutral or alkaline and give a pH range or value where asked.
- If comparing methods, give a clear advantage of each, such as speed for universal indicator and precision for a pH probe.
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
What is the range of the pH scale?
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 a pH above 7 is alkaline. The scale is logarithmic, so each change of one pH unit represents a tenfold change in the concentration of H⁺ ions; for example, pH 3 is ten times more acidic in terms of H⁺ concentration than pH 4. Universal indicator is a mixture of dyes that gives a colour for each pH, which you compare with a colour chart; it is quick but only gives an approximate value. A pH probe is an electronic sensor connected to a meter; it gives a numerical reading and is more precise and objective. For example, testing a solution with universal indicator might show orange, suggesting pH 4, while a pH probe could give a reading of 4.2. A strong answer earns marks for these points. State that the pH scale runs from 0 to 14 and is a measure of acidity or alkalinity. Identify that pH below 7 is acidic, pH 7 is neutral, and pH above 7 is alkaline. Describe how universal indicator is used, including matching the colour to a pH colour chart to estimate pH. Describe how a pH probe is used, including reading a numerical value from a meter for a more precise measurement. A common slip: Thinking that pH 0 means no acid or that pH 14 means the strongest possible alkali in all contexts: students may treat the scale as linear. Instead, pH is a logarithmic measure of H⁺ concentration, and the scale simply runs from 0 to 14 for the solutions studied. Exam tip: When describing a measurement, name the method and state whether the result is approximate or precise.
See it
Key words
Say what each word means before you turn the card.
pH scale
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Universal indicator
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pH probe
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See every definition
- pH scale
- A scale from 0 to 14 that is a measure of the acidity or alkalinity of a solution.
- Universal indicator
- A mixture of dyes that gives a colour for each pH, which you compare with a colour chart; it is quick but only gives an approximate value.
- pH probe
- An electronic sensor connected to a meter that gives a numerical reading and is more precise and objective.
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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