Indicators and pH
Observation: blue litmus turned red. Conclusion: the solution is acidic.
Continue where you stopped
The core idea
Food Chemistry · Lesson 4 of 5 · 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 alkalinityFor example: Universal indicator estimates a pH range.
Explanation
Test separate small portions with blue and red litmus. An acid turns blue litmus red; an alkali turns red litmus blue. Record the starting and final colours before drawing a conclusion. Red litmus remaining red alone cannot distinguish acid from neutral water.
Compare universal indicator colour with the chart supplied for that indicator to estimate pH. On the scale used here, below 7 is acidic, 7 is neutral and above 7 is alkaline. A calibrated pH meter gives a numerical measurement. Litmus cannot distinguish two acidic solutions by exact pH.
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: An unchanged strip is not automatically a neutral result. Use both litmus colours or further pH evidence.
Use this instead: Litmus indicates acidic or alkaline behavior 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
- Hide the page and explain indicators and ph in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “Litmus gives an exact numerical pH.”