Comparing inhaled and exhaled air – Structure and Functions of the Human Gas Exchange System (Senior 1 Biology) | Ganzaa

Structure and Functions of the Human Gas Exchange System

Lessons

Senior 1 · Biology

Structure and Functions of the Human Gas Exchange System

5 lessons

Lesson 4 of 5

Comparing inhaled and exhaled air

25 min

What does your breath take away, and what does it add?

By the last lesson of a hot afternoon, a closed classroom with fifty students in it feels stuffy and warm, and the windows may mist up. Fifty people have been breathing in the same air and breathing it out again. What have they changed in it?

In Lesson 1 you saw that in the alveoli oxygen leaves the air and carbon dioxide enters it. If that is true, the air you breathe out should contain less oxygen and more carbon dioxide than the air you breathe in. In this lesson you test that prediction.

Definition

Inhaled air: the air you breathe in, from your surroundings.

Exhaled air: the air you breathe out, after it has been in your lungs.

Limewater: a clear, colourless solution of calcium hydroxide, $\mathrm{Ca(OH)_2}$ , in water. It turns milky when carbon dioxide is bubbled through it, so it is the standard test for $\mathrm{CO_2}$ .

Precipitate: an insoluble solid that forms in a solution during a reaction, making it cloudy. Example: the white calcium carbonate that makes limewater milky.

Experiment: inhaled and exhaled air with limewater

Aim: to find out whether exhaled air contains more carbon dioxide than inhaled air.

Figure 1: The apparatus. Blue arrows: the path of air when you breathe in. Orange arrows: the path of air when you breathe out.

Method

Pour the same volume of fresh limewater, about 3 cm deep, into two boiling tubes, A and B. Check that both are clear.
Fit the bungs and tubes exactly as in Figure 1. In A the long tube is open to the room; in B the long tube is joined to the mouthpiece.
Breathe gently in and out through the mouthpiece. When you breathe in, room air is drawn down the long tube of A and bubbles through its limewater. When you breathe out, your breath goes down the long tube of B and bubbles through its limewater.
Count your breaths. After every 5 breaths, look at both tubes against a dark background, and record whether each is clear or milky.

Warning

Safety

Limewater is an alkali and irritates the eyes: wear eye protection, and wash any splash off your skin with water.
Breathe gently. If you breathe in hard, limewater can be sucked up a tube towards your mouth. If you see liquid rising in a tube, stop at once.
Each student uses a clean mouthpiece, and it is disinfected afterwards. Never share one without cleaning it.
Do not keep bubbling for many minutes: with a lot of extra $\mathrm{CO_2}$ the milkiness can slowly clear again, which could confuse your result.

Table 1: Sample results (sample data, for illustration)

Tube	Air bubbled through it	After 5 breaths	After 20 breaths
A	inhaled air (from the room)	clear	clear
B	exhaled air	milky	very milky

Conclusion: exhaled air turns limewater milky far more quickly than inhaled air, so exhaled air contains much more carbon dioxide. Tube A is the control: it is set up in exactly the same way, so any difference between A and B must be caused by the air that passed through them.

What happens in the tube

Carbon dioxide reacts with the calcium hydroxide in the limewater to make calcium carbonate and water:

calcium hydroxide + carbon dioxide $\rightarrow$ calcium carbonate + water

\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}

Calcium carbonate is insoluble in water. It forms as millions of tiny white particles, a precipitate, which make the liquid look milky. (It is the same compound as chalk and limestone.)

Calcium hydroxide, a white powder, on a watch glass. Stirred into water and filtered, it gives clear limewater. It is an alkali, so it is handled with a spatula, never with bare hands. Photo: Walkerma, public domain, via Wikimedia Commons

Other differences you can test

Water vapour. Breathe onto a cold mirror or window: a mist of tiny water droplets forms. Or breathe onto dry blue cobalt chloride paper, held with forceps: it turns pink, showing water. (Cobalt chloride is toxic: do not touch it, and wash your hands afterwards.)
Temperature. Hold a thermometer in the room air, then breathe gently onto its bulb for a minute: the reading rises, because exhaled air has been warmed inside your body.

Table 2 brings the differences together. The figures are approximate: they vary from person to person and from breath to breath.

Table 2: The approximate composition of inhaled and exhaled air

Component	Inhaled air	Exhaled air
Nitrogen	about 78%	about 78% (the body does not use it)
Oxygen, $\mathrm{O_2}$	about 21%	about 16%
Carbon dioxide, $\mathrm{CO_2}$	about 0.04%	about 4%
Water vapour	varies with the weather	a lot, about 5 to 6%: the air is saturated
Temperature	the same as the surroundings	warmer, close to body temperature

Warning

"We breathe in oxygen and breathe out carbon dioxide." This is only partly true. Both inhaled and exhaled air are mostly nitrogen. Exhaled air still contains about 16% oxygen: the body takes out only about a quarter of the oxygen in each breath. And inhaled air already contains a little carbon dioxide; exhaled air contains about 100 times as much.

Worked example 1: explaining the results

Uwase's tube B turned milky after 5 breaths, but tube A was still clear after 20. Explain the difference, and explain why tube A was needed.

Observation: the limewater that exhaled air passed through went milky much sooner.
Link to the test: limewater turns milky with $\mathrm{CO_2}$ , so the faster it turns, the more $\mathrm{CO_2}$ the air contains.
Explanation: exhaled air contains much more $\mathrm{CO_2}$ (about 4%) than inhaled air (about 0.04%), because $\mathrm{CO_2}$ made by respiration in the body's cells diffuses from the blood into the alveoli and is breathed out.
Why tube A: it is the control. It shows what happens to the same limewater, in the same apparatus, with ordinary air. Without it, Uwase could not show that the milkiness came from her breath and not, say, from old limewater.

Worked example 2: how much oxygen does one breath deliver?

A student breathes in $500\,\mathrm{cm^3}$ of air and breathes out the same volume. Using Table 2, calculate (a) the volume of $\mathrm{O_2}$ the body keeps from this breath and (b) how many times more $\mathrm{CO_2}$ is breathed out than in.

$\mathrm{O_2}$ breathed in: $\frac{21}{100} \times 500 = 105\,\mathrm{cm^3}$ .
$\mathrm{O_2}$ breathed out: $\frac{16}{100} \times 500 = 80\,\mathrm{cm^3}$ .
(a) $\mathrm{O_2}$ kept: $105 - 80 = 25\,\mathrm{cm^3}$ : roughly a quarter of the oxygen breathed in.
(b) $\frac{4}{0.04} = 100$ : about 100 times as much $\mathrm{CO_2}$ leaves as enters.

Note: the volumes breathed in and out are not exactly equal in real life, and Table 2 is approximate, so these answers are estimates.

Using what you know

Rescue breaths. Because exhaled air still contains about 16% oxygen, a trained first aider can breathe into the lungs of a person who has stopped breathing, and deliver enough oxygen to help keep them alive until help arrives. In Rwanda, call 912 for an ambulance (SAMU), as you learned in Unit 1.

Fresh air in classrooms. Every breath out adds carbon dioxide, water vapour and warmth to a room. In a crowded classroom with the windows shut, they build up, and the room feels stuffy. Opening windows on opposite sides of the room lets fresh air flow through.

Science and Maths with Patrick Maye: "To Investigate the Carbon Dioxide Levels of Inhaled and Exhaled Air - Limewater Test". Watch which tube turns milky first, and check that the apparatus matches Figure 1.

Tip

Two numbers to remember: carbon dioxide goes up about 100 times (0.04% to 4%); oxygen goes down by about a quarter (21% to 16%). Nitrogen stays the same.

✦

Practice: inhaled and exhaled air

What is limewater, and which gas does it test for?
Write the word equation for the reaction that makes limewater milky.
In Figure 1, why does limewater B turn milky much sooner than limewater A?
Give two differences between inhaled and exhaled air other than carbon dioxide, and a test for one of them.
Why must you breathe gently through the mouthpiece?
A breath of $400\,\mathrm{cm^3}$ contains 21% oxygen going in and 16% oxygen coming out. Calculate the volume of oxygen the body keeps.
A friend says: "We breathe in oxygen and breathe out carbon dioxide." Use Table 2 to explain why this is only partly true.

Show answersHide answers

A clear solution of calcium hydroxide in water; it tests for carbon dioxide (it turns milky).
Calcium hydroxide + carbon dioxide $\rightarrow$ calcium carbonate + water.
Exhaled air, which passes through B, contains about 4% $\mathrm{CO_2}$ , about 100 times as much as inhaled air, which passes through A.
Any two: exhaled air contains less oxygen, more water vapour, and is warmer. Tests: breathe on a cold mirror (mist) or on blue cobalt chloride paper (turns pink) for water; a thermometer for temperature.
Breathing in hard can suck limewater up a tube towards your mouth; limewater is an alkali and must not be swallowed.
$\frac{21}{100} \times 400 = 84\,\mathrm{cm^3}$ in; $\frac{16}{100} \times 400 = 64\,\mathrm{cm^3}$ out; kept $= 84 - 64 = 20\,\mathrm{cm^3}$ .
Both inhaled and exhaled air are mostly nitrogen (about 78%). Exhaled air still contains about 16% oxygen, and inhaled air contains a little carbon dioxide (about 0.04%). The true statement is that exhaled air has less oxygen and much more carbon dioxide.

Key points

Limewater turns milky with carbon dioxide: $\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O}$ .
Exhaled air turns limewater milky much faster than inhaled air, because it contains about 100 times as much $\mathrm{CO_2}$ .
Compared with inhaled air, exhaled air has less $\mathrm{O_2}$ (about 16% instead of 21%), more water vapour, and is warmer; the nitrogen is unchanged.

Sources and credits

Composition of inhaled and exhaled air: Breathing, Wikipedia, section "Composition" (read 2026-09-28). Figures only, rounded by Ganzaa.
7.7.2: Systems of Gas Exchange, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the oxygen content of air. Adapted by Ganzaa.
Photo: Calcium hydroxide, Walkerma, public domain, via Wikimedia Commons.
Video: To Investigate the Carbon Dioxide Levels of Inhaled and Exhaled Air - Limewater Test, Science and Maths - Patrick Maye, embedded from YouTube.

Figure 1, the experiment, Tables 1 and 2, the worked examples and the practice questions are written by Ganzaa.

1What does your breath take away, and what does it add?

2Experiment: inhaled and exhaled air with limewater

3What happens in the tube

4Other differences you can test

5Using what you know

What does your breath take away, and what does it add?

Experiment: inhaled and exhaled air with limewater

What happens in the tube

Other differences you can test

Using what you know