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Biology · Respiration

You Are a Human CO₂ Sensor

Testing your breath with limewater — before and after exercise

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Learning objectives

  • Use limewater safely to test for carbon dioxide
  • Measure how much CO₂ is in my breath at rest and after exercise
  • Explain the results using respiration

Success criteria

  • I follow all 3 safety rules with no reminders
  • I record two cloudiness scores: rest and active
  • I can finish the sentence: "More cloudy means…"
🌟 Aspire: Link your result to the respiration word equation — and explain why anyone's reading might not fit the pattern.
Arrival · Recall

Quick recap — limewater & CO₂

Two you already know from before. Tap your answer — we confirm together, then print for your file.

Stuck? Think back to Friday — you made the limewater. What was it for?
Starter · Hook

Your breath is full of clues

The air you breathe IN is about 0.04% carbon dioxide.

The air you breathe OUT is about 4% carbon dioxide.

That's roughly 100× more CO₂ coming out than going in. Your body is making it.

Predict: After 90 seconds of exercise, will your breath have more or less CO₂ than at rest? Turn and tell your partner why.
Exercise makes your muscles work harder, so they respire faster to release energy. Respiration makes CO₂ — so after exercise there is more CO₂ in every breath out. Today we prove it.
📝 Capture predictions (tap each pupil's guess — it prints on their sheet)
I Do · Quick lead-in

You know this test — here's the safe way to do it

Recap: you did the limewater test in gas-tests week, and you made this limewater on Friday. So the test itself is old news. Only two new things today: (1) do it safely with a straw, and (2) turn "cloudy" into a 1–5 score.
The danger: sucking the limewater up the straw. Limewater is an irritant. We only ever blow OUT.

🥽 Rule 1 — Goggles ON

Splash protection. On before you touch a beaker.

💨 Rule 2 — BLOW, never suck

Gentle out-breaths, like blowing out candles. If you feel like breathing in — take the straw out of your mouth first.

🧍 Rule 3 — Your straw, your beaker

No sharing. Wash hands after. Tell me instantly if any gets in your mouth or eyes.

Quick reminder — cloudy is the endpoint

CO₂ + limewater → the liquid goes cloudy / milky. The more CO₂, the cloudier it gets.

How we measure — the fair test: everyone blows the same number of gentle breaths (I'll count us in). Then rate your beaker on the cloudiness scale, 1–5, using the reference strip. Cloudier = higher score = more CO₂.
Same beaker, same amount of limewater each time (about 3–4 cm deep, never more than half full — gentle breaths so it doesn't froth over). View it the same way every time so the comparison is fair.
1 Clear
2
3
4
5 Milky

👀 The trick: rate by how much you can see through the beaker. 1 = read text straight through · 3 = fuzzy · 5 = can't see through. Everyone uses the same rule, so our scores match.

We Do · Together

Reading 1 — at REST (we do this as one team)

  1. Goggles on. Same amount of limewater in your REST beaker (about a third full). Straw in that beaker only.
  2. On my count, everyone blows the same number of gentle breaths together — then straws out, hands still.
  3. Look through your beaker (same amount, same way each time) and rate it 1–5 against the strip. That's your rest score.
1 Clear
2
3
4
5 Milky

👀 Judge it by how much you can see through the beaker: 1 = read text straight through · 3 = text looks fuzzy · 5 = can't see through at all. Use your desk card.

Write your rest score on your card: "At rest my cloudiness was ____ / 5."
Watch for: we all blow the same number of breaths — that's what makes it a fair comparison. Don't sneak extra puffs!
Get Active · Reading 2

Now move — then test straight away 🦉

⏱ Speed matters: the moment you sit back down, blow into your ACTIVE beaker immediately — before your breathing calms down. Don't chat first!
  1. Leave goggles on. Line up calmly at the door.
  2. 90-second burst outside / in the corridor (jog on the spot, shuttle, star jumps).
  3. Walk straight back, sit, and blow the same number of breaths into your ACTIVE beaker. Rate it 1–5 straight away.
1 Clear
2
3
4
5 Milky

👀 Same rule: 1 = see straight through · 5 = can't see through. Rate against your desk card before your breathing settles.

1:30
Add to your card: "After exercise my cloudiness was ____ / 5."
🌟 Finished? Whisper a prediction: will the class average be higher or lower after exercise?
Class Breath Lab · Analyse

Put every score on the wall

Type each person's cloudiness score (1–5) for rest and after exercise. Watch the bars build — then we hunt for the pattern.

1 Clear
2
3
4
5 Milky
Rest cloudinessAfter-exercise cloudiness
Exit Ticket

Show what you know

🎯 Back to our objectives — did we meet them?

  • Use limewater safely — goggles on, blow not suck, our 3 rules
  • Measure CO₂ at rest & after exercise — your two cloudiness scores
  • Explain it with respiration — more movement → more CO₂

👍 Thumbs up if you can now do each one.

Foundation: Ade's limewater was cloudiness 2 / 5 at rest, but 5 / 5 after exercise (same number of breaths each time). Which reading had more CO₂? Tick one: ▢ rest ▢ after exercise.
Middle: Explain, using the word respiration, why Ade's beaker was cloudier after exercise.
Higher: Write the word equation for respiration, then explain why exercise increases the CO₂ in exhaled breath. Why did we keep the number of breaths the same each time?
Exam tip: name the gas (carbon dioxide) and the process (respiration) to secure both marks.
Sentence stem: "A higher cloudiness score means there was ____ carbon dioxide in my breath, because exercise increases ____."
🗣️ Say the conclusion, then close the loop (teacher-led — the class says it aloud at their depth; you confirm the model live)
🟩 BUILD — say it aloud, then tap to confirm
After exercise my limewater was cloudier, so my breath had more carbon dioxide.
🟧 GROW — say it aloud, then tap to confirm
Exercise makes my muscles respire faster. Respiration makes carbon dioxide, so there was more CO₂ in my breath after exercise — the limewater went cloudier.
🟦 LAUNCH — say it aloud, then tap to confirm
glucose + oxygen → carbon dioxide + water. Exercise raises the rate of respiration, releasing more energy and more CO₂, so each breath out carries more carbon dioxide and the limewater turns cloudier. An odd reading can happen if the breaths weren't counted equally, the beaker wasn't viewed the same way, or breathing had already settled.

🟡 Yellow Box — live next step (tap to push a pupil up a level)

🔁 Feedback loop no loops opened yet
One tap per pupil: ◻ → 🟡 VF given → 🟢 R (responded). Reach 🟢 R only once they've acted on your feedback.
📷 Snap each pupil’s beakers / result on their row (📷 button) — it prints on their evidence sheet.
⭐ XP
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🎉 Lesson complete!

You tested your own breath and explained it with science.

Don't forget to print your evidence pack.