Detecting Carbon Dioxide
Turn lime water milky to prove CO2 is in air.
What this lesson covers
Why it matters
Breathe out through a straw into clear lime water and watch it turn milky — the carbon dioxide in your breath is the giveaway.
The idea in plain words
Map the reactants to products: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Note the white solid.
Predict first
Why does clear lime water turn milky when exposed to carbon dioxide?
The milky appearance is a white precipitate of calcium carbonate formed by a chemical reaction, not just mixing.
- Carbon dioxide dissolves to make a cloudy liquid mixture
- A new solid, calcium carbonate, forms and scatters light — correct
- The water boils and creates steam clouds
- Carbon dioxide is a white gas that mixes with water
What you do
Bubble CO₂ through the lime water. Observe the formation of the white precipitate.
Check yourself
What is the chemical identity of the white precipitate?
The reaction produces solid CaCO₃, which appears as a white precipitate.
Which equation correctly represents this test?
Lime water is Ca(OH)₂ solution; reacting with CO₂ yields CaCO₃ and water.
Why is this reaction used to detect carbon dioxide?
The formation of insoluble CaCO₃ makes the solution turn milky, a distinct visual indicator.
- Calcium hydroxide
- Calcium carbonate — correct
- Carbon monoxide
- Water
- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — correct
- CaO + CO₂ → CaCO₃
- Ca(OH)₂ + CO → CaCO₃ + H₂
- CaCO₃ → CaO + CO₂
- It produces a visible white precipitate — correct
- It produces a loud sound
- It turns the solution blue
- It evaporates the water instantly