Compare oxygen and carbon dioxide in inspired and expired air
Use approximate teaching percentages while recognising that expired air still contains oxygen.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall proportions and gas mixtures.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does 16% mean in a mixture comparison?
What you will learn
- Compare the two mixtures
- Read differences carefully
Compare oxygen and carbon dioxide in inspired and expired air
| Gas | Inspired / % | Expired / % |
|---|---|---|
| Oxygen | 21 | 16 |
| Carbon dioxide | 0.04 | 4 |
Compare the two mixtures
Inspired air generally contains more oxygen and less carbon dioxide than expired air. For the supplied approximate classroom comparison, oxygen is 21% in inspired air and 16% in expired air; carbon dioxide is 0.04% and 4% respectively. These are example values for describing the pattern, not fixed values for every person, breath or environment.
Both samples remain mixtures of gases. Expired air still contains oxygen, and inspired air already contains some carbon dioxide. A gas-exchange comparison must not turn into the claim that one sample is pure oxygen and the other pure carbon dioxide. In many ordinary comparisons the nitrogen proportion changes little, but the required contrasts here concern oxygen and carbon dioxide.
Read differences carefully
For oxygen, 21 − 16 = 5 percentage points. For carbon dioxide, 4 − 0.04 = 3.96 percentage points. Percentage-point change is the subtraction of percentages, whereas a relative percentage change divides the difference by the starting value; the question must specify which quantity it asks for.
Composition describes proportions rather than the total air volume breathed. A larger breath can contain a larger amount of a gas at the same percentage, so do not infer a measured oxygen amount from percentages alone. The Extended explanation later connects these composition changes to diffusion and cellular respiration.
Sources for this lesson
Pause and explain
Which comparison is correct for the supplied data?
Worked example
Find the oxygen difference between 21% inspired and 16% expired air.
Show the worked solution
- Subtract the percentages.
- Use percentage points for the difference.
Answer 5 percentage points less oxygen in expired air.
Try a topic test
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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.
State the oxygen and carbon-dioxide differences in words.
Give me a hint
Compare inspired with expired air.
Compare my reasoning
- Oxygen is lower in expired air.
- Carbon dioxide is higher in expired air.
Expired air contains less oxygen and more carbon dioxide than inspired air.
Look for these in your work
- Compare both gases
- State the direction
Calculate the percentage-point differences for oxygen and carbon dioxide using the supplied values.
Give me a hint
Subtract 16 from 21 and 0.04 from 4.
Compare my reasoning
- Oxygen difference: 5.
- Carbon-dioxide difference: 3.96.
O₂: 5 percentage points lower. CO₂: 3.96 percentage points higher in expired air.
Look for these in your work
- Calculate both
- Use percentage points
Does 16% oxygen prove that every expired breath contains exactly 160 mL oxygen?
Give me a hint
A proportion needs a total volume.
Compare my reasoning
- 160 mL follows only if the relevant total is 1000 mL.
- No fixed total volume is given for every breath.
No; the amount depends on total expired volume, and the example percentage is approximate.
Look for these in your work
- Identify the missing volume
- Keep the approximation
Common mistakes
- Saying expired air has no oxygen.
- Confusing percentage points with relative percent change.
What can you explain now?
Which comparison is correct for the supplied data?
Compare with the explanation
Expired air has less O₂ and more CO₂
It matches both changes.
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Make it stick
State both changes and explain why a percentage alone is not an amount.
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.