Answers · Gas exchange and breathing

Air in, air out — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. AQ1 · Practice

Options as numbered on the pupil sheet: 1 — air can pass one way only; 2 — expired air leaves the apparatus; 3 — the person breathes in and out; 4 — room air enters the apparatus.

Model answer. W is the open top of tube A's long tube, where room air is drawn in when the person breathes in. X is the mouthpiece. Y is the open top of tube B's short tube, where expired air leaves after bubbling through B. Z is a one-way valve, which lets air pass in one direction only.

A full answer includes

I matched W to room air entering the apparatus.
I matched X to where the person breathes in and out.
I matched Y to expired air leaving the apparatus.
I matched Z to air passing one way only.
I did not match W to expired air leaving the apparatus.

Also accept

W room air in; X mouthpiece; Y expired air out; Z one-way valve

Watch for

“W: expired air leaves the apparatus.” — W is on tube A, which only room air passes through. Expired air bubbles through B and leaves at Y.
“Z: the person breathes in and out.” — The person breathes at the mouthpiece, X. Z is a small part inside a tube: a one-way valve.

2. AQ2 · Practice

Model answer. Carbon dioxide turns limewater from clear to cloudy (milky white). The more carbon dioxide bubbles through, the sooner it turns cloudy. Blue cobalt chloride paper turning pink shows water, not carbon dioxide.

A full answer includes

I chose: limewater turns from clear to cloudy.
I did not choose the cobalt chloride paper: it tests for water.

Also accept

limewater turns from clear to cloudy

Watch for

“Limewater turns from clear to blue.” — Limewater does not change colour. Carbon dioxide makes tiny white specks in it, so it turns cloudy.
“Cobalt chloride paper turns from blue to pink.” — That is the test for water. Carbon dioxide turns limewater cloudy.
“Limewater turns from cloudy to clear.” — Limewater starts clear. Carbon dioxide makes it cloudy, not the other way round.

3. AQ3 · Practice

Model answer. Inspired air is about 21% oxygen and 0.04% carbon dioxide. Expired air is about 16% oxygen and 4% carbon dioxide. The body keeps about a quarter of the oxygen and adds carbon dioxide.

Checked automatically

Oxygen in inspired air: 21 % (within ±0.1 %)
Oxygen in expired air: 16 % (within ±1 %)
- Common wrong answer 0 %: Expired air still has plenty of oxygen: about 16%.
Carbon dioxide in inspired air: 0.04 % (within ±0.01 %)
- Common wrong answer 0 %: Inspired air has a little carbon dioxide: about 0.04%.
Carbon dioxide in expired air: 4 % (within ±0.5 %)
- Common wrong answer 16 %: That is the oxygen in expired air. Carbon dioxide is about 4%.

A full answer includes

I wrote about 21% (20.9% counts) for oxygen in inspired air.
I wrote about 16% (15–17%) for oxygen in expired air.
I wrote about 0.04% (0.03% counts) for carbon dioxide in inspired air.
I wrote about 4% (3.5–4.5%) for carbon dioxide in expired air.
I did not write 0% for oxygen in expired air.

Also accept

oxygen 21% and 16%; carbon dioxide 0.04% and 4%

Watch for

“Expired air contains no oxygen.” — Expired air is still about 16% oxygen. The body takes only part of it.
“Inspired air contains no carbon dioxide.” — Inspired air has a little carbon dioxide, about 0.04%.

4. AQ4 · Practice

Model answer. Expired air contains less oxygen (16% instead of 21%), more carbon dioxide (4% instead of 0.04%) and more water vapour. It still contains a lot of oxygen, and the nitrogen, about 78%, does not change.

A full answer includes

I chose: it contains less oxygen.
I chose: it contains more carbon dioxide.
I chose: it contains more water vapour.
I did not choose: it contains no oxygen.

Also accept

less oxygen; more carbon dioxide; more water vapour

Watch for

“It contains no oxygen.” — It is still about 16% oxygen. The body takes only part of the oxygen in each breath.
“It contains less nitrogen.” — The body does not use nitrogen, so there is about the same amount, 78%, in both.

5. AQ5 · Practice

Options as numbered on the pupil sheet: 1 — The blood carries it to a muscle.; 2 — Oxygen is in the air of an alveolus.; 3 — A muscle cell uses it in aerobic respiration.; 4 — It diffuses through the walls of the alveolus and a capillary into the blood..

Model answer. Oxygen in the air of an alveolus diffuses through the wall of the alveolus and the wall of a capillary into the blood. The blood carries it to the muscle, where the cells use it in aerobic respiration. That is why expired air has less oxygen.

A full answer includes

I put the oxygen in the air of an alveolus first.
I put its diffusion into the blood second.
I put the blood carrying it to a muscle third.
I put its use in aerobic respiration last.
I did not put its use in respiration before the blood carries it.

Also accept

alveolus; diffuses into blood; carried by blood; used in respiration

Watch for

“Oxygen is used up in the lungs.” — Oxygen only crosses into the blood in the lungs. It is used by cells all over the body, in respiration.
“A muscle cell uses the oxygen before the blood carries it there.” — The oxygen has to reach the muscle first: the blood carries it there from the lungs.

6. AQ6 · Practice

Model answer. (a) The count stops when the cross can no longer be seen. A thin grey cross disappears with much less cloudiness than a thick black one, so B could seem to turn cloudy first because of its cross, not its air. The end point must be the same: the same card and cross behind both tubes. (b) The breath now enters B above the limewater and leaves through the short tube without bubbling through the liquid, so very little carbon dioxide reacts with the limewater. B stays clear, or turns cloudy only after many more breaths.

A full answer includes

(a) I wrote that a thin grey cross disappears from view with less cloudiness than a thick black one.
(a) I wrote that B could seem cloudy first because of its cross, not because of its air.
(a) I wrote that both tubes need the same card and cross.
(b) I predicted that B stays clear, or turns cloudy only after many more breaths.
(b) I wrote that the breath would not bubble through the limewater.
I did not write that the crosses change how much carbon dioxide enters the limewater.

Also accept

(a) a faint cross vanishes sooner, so the end point differs; same cross for both; (b) B stays clear because the breath does not bubble through

Watch for

“The crosses do not matter, because the limewater is the same.” — The count stops when the cross can no longer be seen. A faint cross vanishes with less cloudiness, so the end point is not the same.
“Tube B would still turn cloudy quickly, because the breath still enters it.” — The breath must bubble through the limewater to react with it. Above the liquid it leaves through the short tube almost untouched.

7. AQ7 · Practice

Model answer. Expired air bubbled through B and inspired air through A. Both turned cloudy, so both airs contain carbon dioxide. B needed far fewer breaths, so expired air contains much more. Limewater does not react with oxygen, so the results say nothing about oxygen.

A full answer includes

I chose: both airs contain carbon dioxide, but expired air contains much more.
I did not choose "inspired air contains no carbon dioxide": tube A did turn cloudy.

Also accept

both airs contain carbon dioxide, but expired air contains much more

Watch for

“Inspired air contains more carbon dioxide, as tube A took longer.” — Taking longer means less carbon dioxide in each breath. More carbon dioxide turns the limewater cloudy sooner.
“Expired air contains less oxygen, as tube B turned cloudy first.” — Expired air does have less oxygen, but limewater does not react with oxygen. These results cannot show it.
“Inspired air contains no carbon dioxide, so tube A must have leaked.” — Inspired air has a little carbon dioxide, about 0.04%, so tube A turns cloudy after many breaths.

8. AQ8 · Practice

Model answer. (a) 102 cm³ ÷ 600 cm³ × 100 = 17%. (b) 21 ÷ 100 × 600 cm³ = 126 cm³. (c) 126 cm³ − 102 cm³ = 24 cm³ of oxygen stays in the body from each 600 cm³: it diffused into the blood in the alveoli.

Checked automatically

(a) Oxygen in this expired air: 17 % (exact value)
- Common wrong answer 0.17 %: You divided correctly but did not multiply by 100.
(b) Oxygen in 600 cm³ of inspired air: 126 cm³ (exact value)
- Common wrong answer 12600 cm³: You multiplied 21 by 600 but did not divide by 100.
(c) Oxygen kept by the body: 24 cm³ (exact value)
- Common wrong answer 4 cm³: That is the difference in percentages. Subtract the volumes: 126 − 102.

A full answer includes

(a) I divided 102 cm³ by 600 cm³ and multiplied by 100.
(a) I wrote 17%.
(b) I worked out 21 ÷ 100 × 600 cm³.
(b) I wrote 126 cm³.
(c) I subtracted 102 cm³ from 126 cm³.
(c) I wrote 24 cm³.
I did not give 4 cm³, the difference between the two percentages.

Also accept

(a) 17%; (b) 126 cm³; (c) 24 cm³

Watch for

“The body keeps 102 cm³, the oxygen in expired air.” — The 102 cm³ in expired air leaves the body. The body keeps the difference, 24 cm³.
“The body keeps 4 cm³.” — That is 21 − 17, the difference in percentages. Subtract the volumes instead: 126 − 102.

9. AQ9 · Practice

Model answer. (a) Strip 2 turned pink in the end, so room (inspired) air does contain some water vapour. (b) After the same 15 s, the only difference between the strips is the breath, and only strip 1 had turned pink, so expired air contains more water vapour. After 30 minutes both strips are pink, so the comparison shows no difference. The control must be judged after the same time.

A full answer includes

(a) I stated that inspired air contains some water vapour.
(b) I wrote that after the same 15 s, only the breathed-on strip had turned pink.
(b) I wrote that this shows expired air contains more water vapour.
(b) I wrote that after 30 minutes both strips are pink, so no difference shows.
I did not write that inspired air contains no water vapour.

Also accept

(a) inspired air has some water vapour; (b) same time, so only the breath differs; after 30 minutes both are pink

Watch for

“Strip 2 shows that inspired air contains no water vapour.” — Strip 2 did turn pink after 30 minutes, so room air has some water vapour, just less than expired air.
“Waiting longer gives a better result.” — After 30 minutes both strips are pink, so the difference between the two airs no longer shows.

10. AQ10 · Practice

Model answer. Cells all over the body make carbon dioxide in aerobic respiration. The blood carries it to the lungs. The blood arriving at an alveolus has a higher concentration of carbon dioxide than the air in it, so carbon dioxide diffuses out of the blood into the air and is breathed out.

A full answer includes

I wrote that cells make carbon dioxide in respiration.
I wrote that the blood carries it to the lungs.
I wrote that it diffuses from the blood into the air in the alveoli.
I did not write that carbon dioxide is made in the lungs.

Also accept

made in respiration; carried by blood; diffuses into the alveoli

Watch for

“Carbon dioxide is made in the lungs.” — It is made in every respiring cell and carried to the lungs by the blood.

11. AQ11 · Practice

Model answer. A. In the alveoli, oxygen diffuses from the air into the blood, and the blood takes it to cells all over the body. Carbon dioxide made by respiring cells is carried to the lungs and diffuses from the blood into the air. Water evaporates from the moist lining of the alveoli into the warm air, so the air leaves saturated with water vapour.

A full answer includes

I chose row A.
I did not choose row B: cells all over the body use the oxygen, not the lungs.

Also accept

Watch for

“Row B: lung cells use the oxygen in respiration.” — Oxygen only crosses into the blood in the lungs; cells all over the body use it.
“Row C: oxygen turns into carbon dioxide inside the alveoli.” — Carbon dioxide is made by respiring cells and brought by the blood; nothing turns into it in the air sacs.
“Row D: oxygen and carbon dioxide react in the alveoli to make water.” — The gases do not react; water evaporates from the wet lining into the warm air.

12. AT1 · Apply

Model answer. (a) The cloudy limewater shows that expired air contains carbon dioxide. (b) Inspired air was never tested, so nothing can be concluded about it. A second tube with the same volume of limewater, with room air drawn through it, gives a fair comparison. It would show that inspired air contains much less carbon dioxide (about 0.04%), not none.

A full answer includes

(a) I stated that expired air contains carbon dioxide.
(b) I wrote that inspired air was not tested.
(b) I described a second tube with the same volume of limewater, with room air drawn through it.
I did not write that inspired air contains no carbon dioxide.

Also accept

(a) expired air contains carbon dioxide; (b) inspired air not tested; a second tube of limewater with room air

Watch for

“One tube is enough, because limewater only reacts with breath.” — Limewater reacts with carbon dioxide from any air. Room air must be tested the same way to compare.
“The second part is right, because inspired air has no carbon dioxide.” — Inspired air has a little carbon dioxide, about 0.04%. It just takes many more breaths to turn limewater cloudy.

13. AT2 · Apply

Model answer. (a) Sample X is expired air. (b) It has less oxygen, 16.4% instead of 20.9%, because the body took oxygen into the blood, and about a hundred times more carbon dioxide, 4.1% instead of 0.04%, because the body added carbon dioxide made in respiration.

A full answer includes

(a) I identified sample X.
(b) I wrote that X has less oxygen: 16.4% compared with 20.9%.
(b) I wrote that X has more carbon dioxide: 4.1% compared with 0.04%.
I did not choose Y because it has more oxygen.

Also accept

(a) X; (b) less oxygen and more carbon dioxide, with the figures

Watch for

“Y is expired air, because it has more oxygen.” — Breathing out returns less oxygen than was breathed in, so expired air has less.

14. AT3 · Apply

Model answer. Expired air is warm and saturated with water vapour, far more than the cold inspired air holds. When it meets the cold air, the water vapour cools and condenses into tiny droplets of liquid water, which form the white cloud. Inspired air has much less water vapour, so no cloud forms.

A full answer includes

I wrote that expired air contains more water vapour than inspired air.
I wrote that the water vapour cools and condenses in the cold air.
I wrote that the cloud is tiny droplets of liquid water.
I did not write that the cloud is carbon dioxide.

Also accept

more water vapour in expired air; condenses in the cold into droplets

Watch for

“The cloud is carbon dioxide.” — Carbon dioxide is colourless. The cloud is droplets of condensed water.

15. AT4 · Apply

Model answer. While she held her breath, the same air stayed in her alveoli. Blood kept flowing past, so oxygen kept diffusing from the air into the blood and carbon dioxide kept diffusing from the blood into the air. The longer this goes on, the less oxygen and the more carbon dioxide the air contains.

A full answer includes

I wrote that the air stayed in the alveoli for longer.
I wrote that more oxygen diffused from the air into the blood.
I wrote that more carbon dioxide diffused from the blood into the air.
I did not write that she used up the oxygen in her lungs.

Also accept

air stays longer in alveoli; more oxygen diffuses into blood; more carbon dioxide diffuses out

Watch for

“She used up the oxygen by holding her breath.” — Oxygen is not used in the lungs. It diffuses into the blood and is used by cells in respiration.