Calculate gas-release rates and recognise proxy limits
Use change in collected volume, repeated means and honest measurement labels.
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
What is the collected change from 0.3 to 2.7 cm³?
What you will learn
- Use change in collected volume, repeated means and honest measurement labels.
- Use the stated evidence to explain the method and its limits.
Use a change rather than the final reading alone
Average gas-release rate is change in collected volume divided by elapsed time. If an apparatus begins with a nonzero reading, subtract the initial reading before division. Match the time units and give a rate such as cm³/min.
For equally timed repeats, calculate the mean of the measured rates or use the mean collected change divided by that common interval. If intervals differ, calculate the individual rates first or explicitly define a pooled total-volume over total-time estimate; these are not always the same statistic.
Recognise what the proxy measures
Bubble counts record the number of bubbles, not their volume. Larger bubbles can give more gas even with fewer bubbles. Measuring collected volume addresses that issue but still reflects apparatus capture, dissolved gas, respiration and other experimental limitations.
Do not label a measured net gas-release rate as an exact total photosynthesis rate without the additional evidence. No visible bubbles do not prove photosynthesis is absent: gas can dissolve or be used, and the collection method can fail. Written calculations are practice, not a practical-skills qualification.
Sources for this lesson
Pause and explain
Can unequal bubble sizes support an exact count-to-volume comparison?
Worked example
A collector reads 0.3 cm³ initially and 2.7 cm³ after 4 min. Calculate average collected gas release.
Show the worked solution
- Subtract 2.7 − 0.3 = 2.4 cm³.
- Divide by 4 min.
- Report 0.6 cm³/min as collected gas release.
Answer 0.6 cm³/min of collected gas release.
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.
A collector increases from 0.5 to 2.0 cm³ in 3 min. Calculate the average collected rate.
Give me a hint
Subtract first, then divide.
Compare my reasoning
- 2.0 − 0.5 = 1.5 cm³.
- 1.5 ÷ 3 = 0.5 cm³/min.
0.5 cm³/min.
Look for these in your work
- Subtract the starting volume.
- Correct rate units.
Three equally timed trials give rates of 0.4, 0.6 and 0.8 cm³/min. Calculate their mean.
Give me a hint
Use all three rates.
Compare my reasoning
- 0.4 + 0.6 + 0.8 = 1.8.
- 1.8 ÷ 3 = 0.6.
0.6 cm³/min.
Look for these in your work
- Use the arithmetic mean.
- Retain the units.
Plant A gives 20 bubbles/min, each measured as 0.01 cm³; B gives 15 bubbles/min, each 0.02 cm³. Compare volume rates.
Give me a hint
Multiply count rate by bubble volume.
Compare my reasoning
- A: 20 × 0.01 = 0.20 cm³/min.
- B: 15 × 0.02 = 0.30 cm³/min.
- B has fewer bubbles but more measured volume per minute.
A releases 0.20 cm³/min and B 0.30 cm³/min; count alone reverses the volume-rate comparison.
Look for these in your work
- Calculate both rates.
- Explain the proxy limitation.
Common mistakes
- Do not divide the final reading without subtracting the initial value.
- Bubbles/min and cm³/min are different units.
What can you explain now?
Can unequal bubble sizes support an exact count-to-volume comparison?
Compare with the explanation
No; bubble volume needs measurement or calibration.
A count does not specify each bubble’s volume.
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Make it stick
Calculate a rate from two readings and explain one limit of bubble counting.
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.