Back to Biology
Biology · Respiration · Limewater test
Comparing inhaled vs exhaled air for CO₂
Air is drawn through one boiling tube of limewater when inhaling, and pushed through another when exhaling. The exhaled tube goes milky much faster — confirming that exhaled air contains more CO₂.
0 breaths
Inhaled side
Stays clear
Exhaled side
Clear
Reaction
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
The insoluble calcium carbonate is what makes the solution appear milky.
Exam-style questions
1. Which gas, present in higher concentration in exhaled air, causes limewater to go milky?
2. Why is it important that air must bubble THROUGH the limewater (not just sit above it)?
3. What is the white precipitate that forms in limewater when CO₂ passes through?
4. In the U-tube experiment, why does the inhaled-air side stay almost clear?