Investigating breathing in humans

The first investigation shows that the air we breathe out contains more carbon dioxide than the air we breathe in.

The chemical test for carbon dioxide:

• Limewater (calcium hydroxide solution) turns cloudy / milky when carbon dioxide is bubbled through it. The more CO₂, the faster and more strongly it goes milky.
• Hydrogencarbonate indicator is an alternative: it starts red, turns orange in moderate CO₂, and yellow in high CO₂.

The two-tube apparatus:

• Two test tubes (or boiling tubes) each contain limewater, joined by a central T-piece / mouthpiece.
• Tube A is connected so that when you breathe IN, air is drawn in through tube A — this is inhaled (atmospheric) air.
• Tube B is connected so that when you breathe OUT, air is pushed out through tube B — this is exhaled air.
• Breathe gently in and out through the mouthpiece for a fixed time.

Exam tip. The valve/tube arrangement only matters in that one tube samples inhaled air and the other samples exhaled air through the same liquid — so it is a fair comparison.

What you observe:

• The limewater that exhaled air bubbles through goes cloudy/milky much faster (and more strongly).
• The limewater for inhaled air stays clear for far longer (atmospheric air is only ~0.04% CO₂).

	Inhaled (atmospheric) air	Exhaled air
Carbon dioxide	very little (~0.04%)	much more (~4%)
Limewater	stays clear for a long time	goes milky quickly
Hydrogencarbonate indicator	stays red(dish)	turns yellow

Conclusion:

• Exhaled air contains more carbon dioxide than inhaled air.
• This is because the body respires, using oxygen and producing carbon dioxide as a waste product, which is then breathed out.

Skills marks examiners look for:

• A valid comparison (you must compare exhaled with inhaled — not just say "exhaled air has CO₂").
• Stating the result in terms of how quickly the limewater changes, not just that it changed.

The second investigation shows what happens to breathing when you exercise.

What to measure:

• Breathing rate = the number of breaths per minute (count chest rises in 1 minute).
• Breathing depth = how much air moves in/out per breath (harder to measure precisely — note "deeper/shallower" by watching how far the chest moves).

Method:

1. Rest for a few minutes, then count the number of breaths in 1 minute (the "before"/resting reading).
2. Exercise for a fixed time at a fixed intensity (e.g. step-ups or jogging on the spot for 2 minutes).
3. Immediately after exercise, count the breaths in 1 minute again (the "after" reading). Note whether breaths look deeper.
4. Repeat the whole thing several times (resting fully in between) and calculate a mean for each condition.

Controlling variables (fair test):

• Use the same person throughout.
• Keep the type, time and intensity of exercise the same for each repeat.
• Allow the person to return to rest before any repeat.

Exam tip. Examiners reward a before vs after design with repeats and a mean, plus a stated control (same exercise, same person).

Typical results:

Condition	Breaths per minute (example)	Depth
At rest	~12–15	normal
Just after exercise	~25–35	deeper

• After exercise, breathing rate increases and breaths become deeper.
• The values gradually return to resting as you recover.

Conclusion: exercise increases breathing rate and depth.

Explanation (the key marks):

• During exercise the muscles contract more, so they respire faster (more aerobic respiration).
• They need more oxygen to release the energy and they produce more carbon dioxide.
• Breathing faster and deeper delivers more oxygen to the blood (and muscles) and removes the extra carbon dioxide more quickly.

Going deeper (for top marks). Breathing also stays raised for a while after stopping because the muscles must clear an oxygen debt — the extra oxygen needed to break down lactic acid built up during any anaerobic respiration.