Lesson focus: indicatorA substance that changes colour to show whether something is an acid or an alkali. provide chemical evidence about acidity and alkalinity, while pH estimates the hydrogen-ion conditions of an aqueous solution.
Use litmusAn indicator that turns red in acids and blue in alkalis., universal indicatorAn indicator that shows a range of colours, from red to purple, to tell you the pH. and pH measurements appropriately and evaluate their resolution.
Build on earlier learning
Acids and alkalisAcid and alkali definitions now connect to observable indicator evidence.
Practical
Indicators and pH
- Add a few drops of universal indicator to each solution.
- Match the colour to the pH chart.
- Use the model on this page to see how pH changes as acid and alkali are mixed.
Record: A table of solution, colour, pH and whether it is acidic, neutral or alkaline.
Safety: Wear eye protection.
Universal indicator
Choose a solution to see the colour universal indicator turns and where it sits on the pH scale.
Solution
You will be successful when you can…
- Describe litmus results.
- Interpret the pH scale.
- Use universal indicator colours cautiously.
- Compare indicator and probe resolution.
Words that carry the science
- indicator
- A substance whose colour depends on chemical conditions such as pH.
- litmus
- An indicator distinguishing acidic from alkaline conditions.
- universal indicator
- A mixture producing a range of colours across pH.
- pH
- A logarithmic measure related to hydrogen-ion concentration.
- neutral
- Neither acidic nor alkaline; at 25 °C pure water is pH 7.
Different indicators answer different questions
01Different indicators answer different questions
Blue litmusAn indicator that turns red in acids and blue in alkalis. turns red in acid; red litmus turns blue in alkali. Litmus classifies but does not provide a detailed pH.
universal indicatorAn indicator that shows a range of colours, from red to purple, to tell you the pH. gives an approximate pH range from its colour. Natural indicatorA substance that changes colour to show whether something is an acid or an alkali. such as red-cabbage pigment also change colour but need calibration.
02pH is a logarithmic scale
At typical classroom temperatures, pH below 7 is acidic, pH 7 is neutralNeither acid nor alkali, with a pH of 7, like pure water. and pH above 7 is alkaline.
A change of one pH unit corresponds to a tenfold change in hydrogen-ion concentration. The usual 0–14 chart is common but pH values can exist outside it.
03Measurement has limits
Colour comparison is subjective, coloured samples can mask the endpoint and universal indicatorAn indicator that shows a range of colours, from red to purple, to tell you the pH. gives only approximate values.
A calibrated pH probe gives numerical readings, but it must be rinsed, allowed to stabilise and checked against buffers.
Choose an indicator method
A student must distinguish pH 5 from pH 6 in a dark purple sample.
- Litmus only classifies both as acidic.
- Universal indicator colours are close and may be masked.
- A calibrated pH probe gives a numerical reading.
- Rinse between samples and repeat measurements.
How to make the reasoning visible: The probe is the most suitable method for this comparison.
Indicator investigation
Use the observations and identify measurement limits.
| Sample | Blue litmus | Red litmus | Universal indicator | Probe pH |
|---|---|---|---|---|
| A | red | red | red-orange | 3.1 |
| B | blue | blue | blue-purple | 11.4 |
| C | blue | red | green | 7.0 |
| D | red | red | yellow | 5.8 |
Classify A–D.
Why is the probe evidence more precise?
Check your understanding
Answer from memory, then check against the suggested answer.
Blue litmus in acid?
What pH is neutral water near 25 °C?
Why is pH logarithmic?
Can universal indicator distinguish pH 6.0 from 6.2 securely?
Why can two solutions of the same acid have different pH?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student uses universal indicator to compare two weakly acidic coloured drinks.
Give two things universal indicator can show here.
(2)Give two problems with the method and one improvement.
(3)Total for this question: 5 marks
Command word: Balance usefulness against limitations and finish with a justified improvement.