Interpret hydrogencarbonate indicator in aquatic-plant studies
Use a reference colour comparison without calling it an oxygen test.
Section B6.1 · Photosynthesis
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chloroplasts, enzymes and the difference between photosynthesis and respiration.
- Use the stated observations, quantities and controls; diagrams are schematic.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gas-related change does the supplied indicator comparison track?
What you will learn
- Use a reference colour comparison without calling it an oxygen test.
- Use the stated evidence to explain the method and its limits.
Read the stated carbon dioxide comparison
In the supplied standard hydrogencarbonate-indicator comparison, higher carbon dioxide is associated with yellow, the starting atmospheric comparison with red or orange-red, and lower carbon dioxide with purple. Use the supplied reference chart and starting condition; indicator response depends on pH and the actual preparation.
This indicator is not an oxygen test. Carbon dioxide affects solution acidity and hence the indicator colour. Other changes in acidity or alkalinity can interfere, so avoid adding unrelated chemicals or inferring an exact gas concentration from a named colour without calibration.
Use matched tubes and controls
For an aquatic plant, compare light and dark treatments using similar healthy material, matched indicator volume and starting colour, temperature, duration and sealed conditions. Include matched no-plant tubes in the relevant light and dark conditions to check environmental or reagent changes.
Read the change relative to the control. A purple plant tube with a red no-plant comparison supports reduced carbon dioxide in that plant treatment; a yellow tube supports increased carbon dioxide under the working conditions. Neither colour directly counts oxygen molecules or proves that respiration stopped.
Sources for this lesson
Pause and explain
Why is a matched no-plant tube useful?
Worked example
A matched lit plant tube turns purple while its no-plant comparison stays red. What does the supplied indicator key support?
Show the worked solution
- Purple corresponds to lower carbon dioxide than the starting comparison.
- The no-plant control retains the reference colour.
- State a carbon dioxide change rather than an oxygen measurement.
Answer The plant treatment shows a carbon dioxide decrease relative to the working comparison; oxygen concentration is not directly measured.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Using the supplied key, state the interpretation of yellow relative to a red starting comparison.
Give me a hint
Use the carbon dioxide direction.
Compare my reasoning
- Yellow is the higher-carbon-dioxide side of the key.
- Limit the interpretation to working matched conditions.
Carbon dioxide increased relative to the red comparison under these indicator conditions.
Look for these in your work
- Correct direction.
- Do not call it an oxygen measurement.
Both a plant tube and its no-plant comparison turn yellow. Why is a plant-only explanation weak?
Give me a hint
The control changed too.
Compare my reasoning
- The reference condition did not remain stable.
- A shared environmental or chemical change is possible.
- Check and repeat the indicator comparison.
A change unrelated to the plant may be affecting both tubes; the control does not support assigning the change to plant respiration alone.
Look for these in your work
- Use the control observation.
- Specify checking the shared conditions.
A student adds an acid to one tube and treats its yellow colour as proof of greater plant respiration. Evaluate.
Give me a hint
The indicator responds to acidity.
Compare my reasoning
- Added acid can alter the colour directly.
- The tube has an extra chemical treatment.
- Use matched unaltered indicator conditions.
The added acid confounds the carbon dioxide interpretation; yellow alone is not clean evidence of greater respiration.
Look for these in your work
- Name pH interference.
- Retain the need for matched controls.
Common mistakes
- Use the chart and starting colour for the actual indicator.
- Unrelated pH changes can interfere.
What can you explain now?
Why is a matched no-plant tube useful?
Compare with the explanation
It checks colour changes unrelated to the plant.
Temperature, lighting or reagent changes can affect the comparison.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Retrieve the supplied colour directions and two reasons to use controls.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.