Explain how moist surfaces change expired air
Link evaporation from respiratory linings with humidification without confusing it with carbon-dioxide testing.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall the usual water-vapour comparison.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which change is evaporation?
What you will learn
- Explain moisture gain
- Follow phase changes and evidence
Explain how moist surfaces change expired air
Explain moisture gain
Air passing through the respiratory system is warmed and humidified by moist surfaces. Water evaporates from those surfaces into the air, contributing to the greater water-vapour content usually described in expired air. This is different from carbon dioxide diffusing out of the blood: two gases can change for different reasons.
Aerobic respiration produces water in cells, but it is inaccurate to attribute every molecule of expired water vapour to newly made water inside the lungs. The syllabus composition explanation should include evaporation from moist respiratory linings. Incoming humidity and temperature affect the size of the difference.
Follow phase changes and evidence
Evaporation transfers liquid water into gaseous water vapour. If the outgoing air meets a sufficiently cooler surface, some vapour may condense into visible liquid droplets. Neither change turns water into carbon dioxide or establishes oxygen concentration. Keep the biological source explanation and the physical-state observation distinct.
For a supplied comparison, cold-surface droplets support the presence of moisture, while faster limewater cloudiness supports more carbon dioxide under matched conditions. The two outcomes need separate methods and controls. A classroom observation explains a mechanism; it does not quantify all body water loss or diagnose a respiratory condition.
Pause and explain
Which explanation fits the usual moisture difference?
Worked example
Explain the ordinary water-vapour increase without calling it carbon dioxide.
Show the worked solution
- Moist linings supply water by evaporation.
- The passing air is humidified.
Answer Water evaporates from moist respiratory surfaces into the passing air.
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.
Explain why expired air usually contains more water vapour.
Give me a hint
Name the moist surface and phase change.
Compare my reasoning
- Air passes moist respiratory linings.
- Water evaporates into it, increasing humidity.
Evaporation from moist respiratory surfaces humidifies passing air, usually increasing expired water vapour.
Look for these in your work
- Name evaporation
- Identify the surfaces
Explain why visible droplets on a cooler surface do not contradict water leaving the airways as vapour.
Give me a hint
Follow cooling after humidification.
Compare my reasoning
- Water leaves in the gas phase.
- Cooling can condense some vapour into liquid droplets.
Vapour can subsequently condense on a cooler surface, producing visible liquid droplets.
Look for these in your work
- Give the phase sequence
- Separate gas and liquid
A student uses limewater timing to quantify total body water loss. Evaluate the method.
Give me a hint
Identify both the gas test and missing whole-body information.
Compare my reasoning
- Limewater tests CO₂, not water amount.
- Total body water loss includes other routes and needs suitable measurements.
The method is invalid for that conclusion: it tests CO₂ and does not measure total body water loss.
Look for these in your work
- State the test mismatch
- Retain the whole-body limit
Common mistakes
- Attributing all expired moisture to lung-cell respiration.
- Confusing evaporation with condensation.
What can you explain now?
Which explanation fits the usual moisture difference?
Compare with the explanation
Evaporation from moist respiratory linings humidifies air
It supplies the mechanism.
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Make it stick
Explain the source and phase changes without using a CO₂ test as a moisture measure.
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.