Edexcel IGCSE Biology · Spec 2.50
Investigating Carbon Dioxide in Exhaled Air
Covers compare carbon dioxide in inhaled and exhaled air, Use a suitable indicator and control and Design a safe and repeatable method.
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Investigating Carbon Dioxide in Exhaled Air
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Explained
Showing that breathing out adds carbon dioxide
Every cell respires, and respiration produces carbon dioxide. The blood carries it to the lungs and it leaves in the air you breathe out. This experiment makes that invisible difference visible.
The point to hold on to is that the air you breathe in is not carbon dioxide free. Ordinary air already contains a small amount, roughly four hundredths of one per cent. Exhaled air contains about a hundred times more. So the comparison is between a little and a lot, not between none and some, and an answer that says inhaled air has no carbon dioxide is wrong before it starts.
The apparatus
Two boiling tubes or flasks, each containing the same volume of hydrogencarbonate indicator, connected by tubing to a central mouthpiece with a T junction.
The tubing is arranged so that when you breathe in, air is drawn through tube A, and when you breathe out, air is pushed through tube B. One tube therefore sees inhaled air and the other sees exhaled air, and both see the same volume of air passing through the same volume of indicator.
Limewater can be used in place of hydrogencarbonate indicator. It turns milky or cloudy with carbon dioxide, and it turns cloudy faster the more carbon dioxide there is, so the measurement becomes the time taken to go cloudy rather than a colour.
Reading the indicator
Hydrogencarbonate indicator starts orange or red when it is in equilibrium with ordinary air.
More carbon dioxide turns it yellow. Less carbon dioxide turns it purple. Those two directions are the whole of the reading, and knowing which way is which is worth more than any amount of detail about the tubing.
So the expected result is that the tube receiving exhaled air turns yellow, and the tube receiving inhaled air stays close to its starting colour. The tube that barely changes is doing as much work as the one that does, because it is the comparison that proves the point.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme sets a hydrogencarbonate indicator experiment with two containers and asks what was being measured. It credits the colour change, the change in gas composition, the change in carbon dioxide, the carbon dioxide concentration, the ratio of gases or gas exchange, and its note adds that levels or amounts of carbon dioxide are accepted.
A wide range of wordings, then, but every one of them names carbon dioxide or names the colour. Saying that you measured the indicator, without saying what about it, names the equipment rather than the variable.
The next part asks for an explanation of the results, and the marks are laid out as paired comparisons. In A the colour stayed orange and in B it went yellow. In A there was no change in carbon dioxide. In B carbon dioxide increased, or more was released than removed. Its guidance accepts the phrasing colour stayed the same in A and went yellow in B.
Notice the structure. Almost every marking point mentions both containers, because the question is about a difference between them. Describing only the tube that changed answers half the question, and it is the half the examiner already knows.
Controls and fair testing
Use the same volume of indicator in each tube, the same starting colour, tubes of the same size, and the same temperature throughout.
Breathe at a steady rate for a fixed time, so that the same volume of air passes through each tube. If you breathe out harder than you breathe in, more air goes through one tube than the other, and any difference could be a difference in volume rather than in composition.
Repeat with several people and compare, or repeat with the same person and take a mean. One breath is not a result.
A control tube left open to the room, with no air drawn through it, shows whether the indicator changes on its own over the time of the experiment. If it does, the changes in the other tubes cannot be attributed to breathing alone.
Safety
The one real hazard here is drawing liquid into your mouth.
Hydrogencarbonate indicator and limewater are both irritants, and limewater is alkaline. Use a mouthpiece that cannot be shared, keep the liquid levels well below the top of the tubes, and check before you start that the tubing is connected the right way round, so that breathing in draws air rather than liquid.
Anyone who becomes dizzy should stop. Breathing deeply and repeatedly through tubing for several minutes is enough to make some people light headed.
Extending it
The obvious extension is exercise. Repeat the readings after a period of activity and the exhaled air tube changes colour faster, because respiration has increased to supply the muscles and more carbon dioxide is being produced.
To turn colour into a number, time how long each tube takes to reach a matched colour, or use a colorimeter and record absorbance. A time or an absorbance can be plotted and averaged; a colour cannot.
Spec 2.50
What you need to know
- Compare carbon dioxide in inhaled and exhaled air
- Use a suitable indicator and control
- Design a safe and repeatable method
Active recall
Quick check
Answer each question before opening the answer.
Why does exhaled air contain more carbon dioxide?
Carbon dioxide is produced by respiration and transported to the lungs
Name two variables that must be controlled
Any two from indicator volume, air volume, flow rate, temperature and starting colour
What colour change is expected with more carbon dioxide?
Hydrogencarbonate indicator changes towards yellow
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