Indicators and pH
Test solutions with litmus, universal indicator and a pH meter, and classify them as acidic, neutral or alkaline.
On this page
Food Chemistry · about 25–35 min
What you need to understand
Observation: blue litmus turned red. Conclusion: the solution is acidic.
Definitions
- indicator
a substance whose colour gives evidence about acidity or alkalinity
For example: Universal indicator estimates a pH range.
Explanation
An indicator changes colour according to acidity or alkalinity. Test separate small portions of a sample so one test does not contaminate another. With litmus, record both the starting and final colour before drawing a conclusion.
| Classification | pH | Blue litmus | Red litmus |
|---|---|---|---|
| Acidic | Below 7 | Turns red | Stays red |
| Neutral | 7 | Stays blue | Stays red |
| Alkaline | Above 7 | Stays blue | Turns blue |
An alkali is a base that dissolves in water to produce an alkaline solution. A base can neutralise an acid; the next lesson compares these reactions. An unchanged litmus strip is incomplete evidence: red litmus staying red could indicate an acidic or neutral sample. Litmus has no numerical scale, so it cannot give an exact pH.
Use universal indicator with its chart
Put a small portion of the sample in a clean container and add the indicator as directed by your teacher. Compare its colour with the chart supplied for that indicator, using good lighting. Record the observed colour and the estimated pH separately. Different indicator charts need not use exactly the same colours at every pH; use the chart given in the question or practical.
Use a calibrated pH meter
A pH meter gives a numerical measurement. Use a meter calibrated as directed, rinse its sensing end with distilled water, and gently blot away excess water. Place it in a fresh sample, wait for a stable reading and record the pH. Rinse between samples to avoid transferring one solution into another. Follow the instrument’s handling instructions; a numerical display does not remove uncertainty or errors from poor calibration or contamination.
Pause and say it: Observation: blue litmus turned red. Conclusion: the solution is acidic.
Common mistake
Tempting wrong idea: Litmus gives an exact numerical pH.
Why it fails: Litmus changes between acidic and alkaline colours; it has no numbered pH scale. Two acidic samples can both turn blue litmus red while having different pH values.
Use this instead: Litmus indicates acidic or alkaline behaviour but does not give an exact pH number.
Worked Example
Modelled example 1
What do the colours establish?
Problem
Separate portions of each sample give these observations. Use this supplied universal-indicator chart: red ≈ pH 2, orange ≈ pH 4, green ≈ pH 7, blue ≈ pH 10 and purple ≈ pH 12. Colour matches are estimates.
| Sample | Blue litmus | Red litmus | Universal indicator |
|---|---|---|---|
| A | turns red | stays red | orange |
| B | stays blue | turns blue | blue |
| C | stays blue | stays red | green |
Classify the samples. Which observations justify the conclusions, and can litmus alone give their exact pH?
Study the worked solution
Separate the observation from the conclusion
Method
Use the change of litmus colour to classify A and B.Reason
An acid changes blue litmus to red; an alkali changes red litmus to blue.Working
A is acidic because blue litmus turns red. B is alkaline because red litmus turns blue. C changes neither strip, and the green indicator supports a neutral result.Use the supplied chart
Method
Compare each universal-indicator colour with its chart.Reason
The chart links observed colours to approximate pH; litmus has no numerical scale.Working
A is approximately pH 4, B approximately pH 10 and C approximately pH 7. Litmus alone cannot supply these numbers. Use a calibrated pH meter if a numerical measurement is required.
Guided practice
Try it with support
Sample A changes blue litmus to red. Sample B changes red litmus to blue. State each observation and conclusion.
Keep what was seen separate from what it means.
Check the guided answer
Answer: A: blue to red, so acidic. B: red to blue, so alkaline.
Check: These results do not give exact pH values.
Practise and continue
Practise this
A sample leaves red litmus red. A learner calls it neutral. Is that conclusion justified?
Need a hint?
Could another class of solution give the same observation?
Check your answer
Answer: No. An acid also leaves red litmus red. Test blue litmus too, or use universal indicator with its chart.
Check: Unchanged red litmus alone cannot distinguish acid from neutral.
Think like a scientist
Why is a calibrated pH meter stronger evidence for a numerical pH than litmus paper?
Check the reasoning
The meter gives a numerical measurement; litmus gives a colour response classifying acidic or alkaline conditions.
Remember: Universal indicator estimates pH by comparison with its supplied chart.
One-minute check
Describe the litmus changes that indicate acidity and alkalinity.
How do universal indicator and a calibrated pH meter give different kinds of pH evidence?
Correct this common mistake: “Litmus gives an exact numerical pH.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: