Limewater CO₂ Test

Classic test for carbon dioxide in breath

Difficulty: Easy | Time: 5 minutes | Visual Impact: Medium

Historical Context

This simple test helped establish modern chemistry. In 1756, Joseph Black discovered that heating limestone produced a gas he called “fixed air” - the first gas recognized as distinct from ordinary air. When he bubbled this gas through limewater, it turned cloudy white.

Black’s work showed that gases were not all the same “air” but distinct substances with different properties. This discovery was crucial to dismantling phlogiston theory and establishing oxygen chemistry. Antoine Lavoisier later identified “fixed air” as carbon dioxide.

The test remains a standard chemistry demonstration because it beautifully illustrates gas detection, chemical precipitation, and the carbon dioxide we exhale with every breath.

Materials

  • Calcium hydroxide - 2g (1/2 teaspoon)
  • Water - 200mL
  • Straw
  • Clear container or beaker - 250mL
  • Filter paper or coffee filter (optional)

Procedure

  1. Add 2g calcium hydroxide to 200mL water, shake vigorously
  2. Let settle for 5-10 minutes until liquid is clear (or filter)
  3. Pour 100mL clear limewater into container
  4. Blow exhaled breath through straw for 30-60 seconds
  5. Solution turns milky white as calcium carbonate forms!

Reaction

\[\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}\]

Calcium hydroxide + Carbon dioxide → Calcium carbonate (precipitate) + Water

The Science

Limewater is a saturated solution of calcium hydroxide (slightly soluble). When CO₂ dissolves in it, it reacts to form calcium carbonate - the same compound as limestone, marble, and chalk.

Calcium carbonate is insoluble, so it precipitates as tiny white particles that scatter light, making the solution appear “milky.”

Your breath contains ~4% CO₂ (compared to 0.04% in air). This CO₂ comes from cellular respiration - your cells burning glucose and producing CO₂ as waste.

Bonus: If you continue blowing, the milkiness eventually clears! Excess CO₂ converts calcium carbonate to soluble calcium bicarbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

Variations

  • Compare exhaled breath with room air (blow gently over surface)
  • Test gas from baking soda + vinegar reaction
  • Test gas from fermentation
  • Generate CO₂ from calcium carbonate + vinegar (chalk or eggshell) instead of breathing

Resources