Inspired and expired air sim
Breathe in through one tube of limewater and out through the other, watch the cross behind each tube, and find out why only the air you breathe out turns limewater milky.
Gas exchange: comparing inspired and expired air with limewater · covers the IGCSE (investigation), O-Level (context) topic
What the sim shows
Breathing in draws room air through tube A: its long tube dips into the limewater, so the air bubbles through it on the way to your mouth. Breathing out pushes your expired air down tube B's long tube and through its limewater. Carbon dioxide turns limewater milky: calcium hydroxide + carbon dioxide → calcium carbonate (a white solid) + water. Expired air has about 4 % carbon dioxide, about 100 times the 0.04 % in inspired air, because your cells release carbon dioxide in respiration. So in this model tube B goes cloudy after 2 breaths and milky after 8, while tube A would need about 120 breaths just to go cloudy: watch the cross behind tube B disappear, while tube A's stays clear. The model: a gentle breath is 0.5 dm³; each tube holds 10 cm³ of limewater with 0.2 mmol of calcium hydroxide; the bubbles pass quickly, so only 2 % of their carbon dioxide reacts (illustrative); a tube looks cloudy when a tenth of its calcium hydroxide has reacted. With far more carbon dioxide (23 breaths here) the calcium carbonate dissolves again as calcium hydrogencarbonate and the tube clears: so do not keep breathing out for too long. Model limits: the absorption and the thresholds are chosen for the model, and the breath size is a typical value; expired air also has less oxygen and more water vapour, which limewater does not show.
- Tube A: the air you breathe in bubbles through it
- Tube B: the air you breathe out bubbles through it
- Card with a cross: it disappears behind milky limewater
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