2.6.1Inorganic Chemistry
Indicators — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 26 of 52
Part 1 of 5Common Indicators
An indicator is a substance that changes colour depending on whether it is in an acid or an alkali.
Litmus
- Litmus is red in acid and blue in alkali, and it is useful as paper for testing solutions or gases.
Phenolphthalein
- It is colourless in acid and pink in alkali, and it gives a sharp end-point in acid-alkali titrations.
Methyl orange
- It is red in acid and yellow in alkali, and it also gives a sharp colour change in titrations.
Universal indicator
- It is a mixture of plant indicators that gives a range of colours across the pH scale for estimating pH.
| Indicator | Colour in acid | Colour in alkali | Suitable for titration? |
|---|---|---|---|
| Litmus | Red | Blue | No, colour change is not sharp |
| Phenolphthalein | Colourless | Pink | Yes |
| Methyl orange | Red | Yellow | Yes |
| Universal indicator | Red to orange | Blue to violet | No, gradual colour change |
Part 2 of 5The pH Scale
The pH of a solution is a measure of its hydrogen ion concentration.
1
A solution with a pH below 7 is acidic, a solution with a pH of exactly 7 is neutral, and a solution with a pH above 7 is alkaline.
2
Each fall of one pH unit means the hydrogen ion concentration has increased by a factor of ten.
3
Universal indicator colours are matched against a chart to estimate the pH of an unknown solution to the nearest whole number.
4
A strongly acidic solution has a pH close to 0, while a strongly alkaline solution has a pH close to 14.
Part 3 of 5Choosing an Indicator
Choosing an Indicator
End-point
- The end-point is the point at which the indicator changes colour, showing that the acid and alkali have just reacted completely.
Sharpness
- A sharp colour change is needed, so a gradual change such as that of universal indicator is not suitable for titrations.
Litmus
- Litmus changes through a purple shade near neutral, which makes it hard to judge the exact end-point.
Synthetic indicators
- Phenolphthalein and methyl orange change colour sharply over a narrow range, so they suit titrations.
Part 4 of 5Worked Example
Question
- A student titrates hydrochloric acid with sodium hydroxide solution.
- Which indicator should be chosen, and why?
- Identify the reaction as strong acid with strong alkali, so the end-point is close to pH 7.
- Check the colour change of each candidate: litmus changes gradually, universal indicator is not sharp,
- while phenolphthalein and methyl orange change sharply.
- Choose one of the sharp indicators, phenolphthalein or methyl orange, and state its colour change.
Answer
- Phenolphthalein (colourless to pink at the end-point) or methyl orange (red to yellow).
- Either gives a sharp change at the neutralisation point.
Part 5 of 5Exam Checklist & Tips
2.6.1 – Key Points for the Exam
- Indicator A substance that changes colour depending on the acidity or alkalinity of a solution.
- pH A scale from 0 to 14 measuring how acidic or alkaline a solution is.
- End-point The point in a titration at which the indicator changes colour.
- Universal indicator A mixture of indicators that shows a range of colours to estimate pH.
Common Mistakes
- Using universal indicator in a titration; its colour change is too gradual to locate the exact end-point.
- Saying litmus is suitable for titrations; it changes through purple, so the end-point is unclear.
- Reversing the colours, such as saying phenolphthalein is pink in acid.
- Stating that pH 7 is acidic; pH 7 is neutral, and values below 7 are acidic.
Last updated:
\n