Compare water vapour in inspired and expired air
Describe the usual moisture difference and keep humidity conditions explicit.
Section B11.1 · Gas exchange in humans
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 evaporation and condensation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is condensation?
What you will learn
- Air gains moisture in the respiratory passages
- Keep the evidence tied to moisture
Compare water vapour in inspired and expired air
Air gains moisture in the respiratory passages
Expired air usually contains more water vapour than inspired air in the ordinary classroom comparison. Air passing through moist respiratory surfaces becomes humidified. Water vapour is gaseous water; visible droplets formed on a cooler surface are liquid produced by condensation, not the invisible vapour itself.
The comparison depends on temperature and the humidity of the inspired air. Very humid incoming air may already contain substantial water vapour, so more in expired air is the usual stated pattern rather than a universal fixed percentage. Describe the conditions supplied by the question instead of assigning a constant water percentage to every breath.
Keep the evidence tied to moisture
A supplied observation of condensation on a cool surface can support moisture in expired air, but the amount condensed depends on cooling and collection conditions. It does not measure carbon-dioxide concentration, and limewater cloudiness is not a direct water-vapour test. Separate these observations before comparing air composition.
For this Core description, state that expired air is generally moister alongside its lower oxygen and higher carbon dioxide. The Extended sequence explains how evaporation from moist linings contributes. A written comparison or supplied demonstration does not ask the learner to use unsupervised chemicals or infer a clinical condition from ordinary condensation.
Sources for this lesson
Pause and explain
Which description fits the usual inspired–expired comparison?
Worked example
Why is a visible mist on a cooler surface not identical to water vapour?
Show the worked solution
- Vapour is gaseous water.
- Cooling can make liquid droplets.
Answer The visible droplets are condensed liquid water; water vapour itself is invisible.
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.
Describe the usual water-vapour difference between inspired and expired air.
Give me a hint
Use a comparative statement.
Compare my reasoning
- Air passes moist respiratory surfaces.
- Expired air is generally moister in the stated comparison.
Expired air usually contains more water vapour than inspired air.
Look for these in your work
- State the direction
- Keep usual conditions explicit
Distinguish water vapour from droplets condensed on a cool surface.
Give me a hint
Compare physical states.
Compare my reasoning
- Vapour is gaseous water.
- Condensation forms liquid droplets.
Water vapour is gas; visible droplets are liquid formed by condensation.
Look for these in your work
- Name both states
- Name the change
Can a faster limewater cloudiness result establish a higher water-vapour percentage?
Give me a hint
Check what that test detects.
Compare my reasoning
- Limewater cloudiness indicates carbon dioxide.
- A separate suitable moisture method is needed.
No; that test compares carbon dioxide, not water-vapour percentage.
Look for these in your work
- Separate gas tests
- State the evidence limit
Common mistakes
- Calling visible liquid droplets a gas.
- Using limewater as the water-vapour test.
What can you explain now?
Which description fits the usual inspired–expired comparison?
Compare with the explanation
Expired air generally contains more water vapour
Moist respiratory surfaces humidify the air.
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Make it stick
Compare the moisture pattern and distinguish vapour from droplets.
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.