AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
This reaction can be represented by the equation: Students should be able…
Full specification wording
The pH scale and neutralisation: This reaction can be represented by the equation: Students should be able to: describe the use of universal indicator or a wide range indicator to measure the approximate pH of a solution use the pH scale to identify acidic or alkaline solutions.
The neutralisation reaction between an acid and an alkali can be represented by the ionic equation H⁺ + OH⁻ → H₂O. To measure the approximate pH of a solution, you can use universal indicator or a…
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The neutralisation reaction between an acid and an alkali can be represented by the ionic equation H⁺ + OH⁻ → H₂O. To measure the approximate pH of a solution, you can use universal indicator or a wide range indicator. These indicators contain a mixture of dyes and change through a range of colours as pH changes. A few drops are added to the solution, or a piece of indicator paper is dipped into it, and the colour is compared with a colour chart. The chart links each colour to an approximate pH value. Red and orange colours indicate acidic solutions with pH below 7; green indicates a neutral solution around pH 7; blue and purple indicate alkaline solutions with pH above 7. The indicator gives an approximate value, not an exact one, so it is used to identify whether a solution is acidic or alkaline and to estimate its pH.
What earns the marks
- States that the neutralisation reaction can be represented by the ionic equation H⁺ + OH⁻ → H₂O.
- Describes adding universal indicator or a wide range indicator to a solution and observing the colour change.
- Explains that the colour is compared with a colour chart to obtain an approximate pH value.
- Uses the pH scale to identify a solution as acidic if its pH is below 7, neutral if pH 7, and alkaline if above 7.
- Recognises that universal indicator gives an approximate pH rather than an exact value.
- Interprets common indicator colours, such as red or orange for acids and blue or purple for alkalis.
Common mistakes and how to put them right
Treating universal indicator as a precise measuring instrument.
Put it right by describing it as giving an approximate pH that must be compared with a colour chart.
Confusing the colour of universal indicator with the colour of litmus.
Put it right by using the wide range of colours from red through green to purple for universal indicator.
Thinking that a green colour always means the solution is acidic.
Put it right by linking green to neutral, around pH 7.
Exam tips
- When describing the use of universal indicator, include the steps: add a few drops to the solution, observe the colour, compare with a colour chart, and read the approximate pH.
- If asked to identify whether a solution is acidic or alkaline, quote the pH value and state whether it is below, equal to or above 7.
- Use the ionic equation H⁺ + OH⁻ → H₂O when explaining neutralisation, and keep the charges and state symbols correct.
Try the lesson
The start of the The pH scale and neutralisation: This reaction can be represented by the equation: Students should be able to: describe the use of universal indicator or a wide range indicator to measure the approximate pH of a solution use the pH scale to identify acidic or alkaline solutions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which indicator is used to measure the approximate pH of a solution by giving a range of colours?
Listen
The neutralisation reaction between an acid and an alkali can be represented by the ionic equation H⁺ + OH⁻ → H₂O. To measure the approximate pH of a solution, you can use universal indicator or a wide range indicator. These indicators contain a mixture of dyes and change through a range of colours as pH changes. A few drops are added to the solution, or a piece of indicator paper is dipped into it, and the colour is compared with a colour chart. The chart links each colour to an approximate pH value. Red and orange colours indicate acidic solutions with pH below 7; green indicates a neutral solution around pH 7; blue and purple indicate alkaline solutions with pH above 7. The indicator gives an approximate value, not an exact one, so it is used to identify whether a solution is acidic or alkaline and to estimate its pH. A strong answer earns marks for these points. States that the neutralisation reaction can be represented by the ionic equation H⁺ + OH⁻ → H₂O. Describes adding universal indicator or a wide range indicator to a solution and observing the colour change. Explains that the colour is compared with a colour chart to obtain an approximate pH value. Uses the pH scale to identify a solution as acidic if its pH is below 7, neutral if pH 7, and alkaline if above 7. A common slip: Treating universal indicator as a precise measuring instrument. Instead, describing it as giving an approximate pH that must be compared with a colour chart. Exam tip: When describing the use of universal indicator, include the steps: add a few drops to the solution, observe the colour, compare with a colour chart, and read the approximate pH.
See it
Key words
Say what each word means before you turn the card.
universal indicator
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wide range indicator
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neutralisation
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See every definition
- universal indicator
- A mixture of dyes that changes through a range of colours as pH changes, used to measure the approximate pH of a solution.
- wide range indicator
- An indicator that can be used to measure the approximate pH of a solution, added as a few drops or as a piece of indicator paper dipped into it.
- neutralisation
- A reaction between an acid and an alkali that can be represented by the ionic equation H⁺ + OH⁻ → H₂O.
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