Topic 11.1 · Core
Inspired and Expired Air
Inspired air is the air you breathe in; expired air is the air you breathe out. For this syllabus, the comparison Cambridge wants is limited to three components — oxygen, carbon dioxide and water vapour — and it rewards knowing both the direction of each change and, at Supplement level, the reason behind it.
What actually changes
| Component | Inspired air | Expired air |
|---|---|---|
| Oxygen | More | Less |
| Carbon dioxide | Less | More |
| Water vapour | Less / variable | More |
Those three relative differences are the required Core knowledge, and they matter more than any exact figure. If you find approximate values useful for scale, oxygen falls from around 21% inspired to roughly 16% expired, and carbon dioxide rises from around 0.04% inspired to roughly 4% expired — but the current content objective asks for the direction of each difference, not memorised percentages. Nitrogen is commonly shown as roughly unchanged at around 78% in comparison tables; it’s useful supporting knowledge, but it sits outside this objective’s three-component limit. It’s also worth remembering that expired air still contains substantial oxygen — your body doesn’t remove all of it in a single breath.
Why expired air has less oxygen
Oxygen diffuses from alveolar air into the blood at the exchange surface described in gas exchange surfaces and alveoli. That blood then carries the oxygen to tissues around the body, where it’s used in aerobic respiration. Because some of the oxygen that entered the lungs has diffused away into the blood, less remains in the air you eventually breathe out.
Why expired air has more carbon dioxide
Respiring cells throughout the body produce carbon dioxide as a waste product. Blood transports that carbon dioxide back towards the lungs, where it diffuses from the blood into the alveoli — the reverse direction to oxygen — and is then removed from the body the next time you breathe out. The higher carbon dioxide concentration in expired air is simply this process showing up as a measurable difference.
Why expired air has more water vapour
The airway and gas-exchange surfaces lining your breathing system are moist. As air passes over them on its way to and from the alveoli, water evaporates from those moist surfaces and enters the air, so the air you breathe out is more humidified than the air you breathed in. It’s worth being precise about the mechanism here: the extra water vapour comes from evaporation off moist surfaces, not from water produced directly by respiration — that’s a subtly different claim, and Cambridge wants the surface-evaporation explanation specifically.
Description versus explanation
| Type | Example |
|---|---|
| Description | “Expired air has less oxygen.” |
| Explanation | “Oxygen diffuses from alveolar air into the blood, so less remains in expired air.” |
A Core answer only needs the description. A Supplement answer needs the causal link as well — stating that the gases differ is not, on its own, an explanation of why.
Investigating the difference: the limewater test
Cambridge requires a practical investigation comparing inspired and expired air using limewater as a test for carbon dioxide. Limewater is colourless and clear, and carbon dioxide reacts with it to turn it cloudy or milky — the more carbon dioxide passing through a sample under comparable conditions, the faster and more strongly it clouds.
A workable apparatus directs inspired air through one sample of limewater and expired air through a separate sample, arranged so the two air routes stay distinguishable and neither sample of limewater can be drawn back into the mouth. To make the comparison fair, keep the conditions as similar as possible between the two samples: a similar volume and concentration of limewater in each, a comparable duration or amount of airflow through each, and a clear, correct identification of which route is inspired and which is expired.
Under those conditions, the limewater exposed to expired air turns cloudy noticeably faster than the limewater exposed to inspired air. The conclusion follows directly: expired air contains a higher concentration of carbon dioxide than inspired air. It’s worth keeping the observation and the conclusion as two separate statements rather than blurring them together — “this limewater went cloudy faster” is the observation; “this air contains more carbon dioxide” is the conclusion drawn from it, and a complete practical answer states both, in that order.
It’s also worth noting what this investigation doesn’t show: it’s a test specifically for carbon dioxide, and it says nothing directly about the oxygen or water-vapour differences covered above. Inspired air isn’t carbon-dioxide-free either — it normally produces a much weaker, slower limewater change, not no change at all.