Limewater CO₂ Test
Summary
Bubbling your breath through clear limewater turns it milky, a classic test that detects carbon dioxide by forming an insoluble precipitate1. You’ll be able to explain why the cloudiness appears, why it can clear again if you keep blowing, and how much more CO₂ your breath carries than ordinary air.
History
This simple test helped establish modern chemistry. In 1756, Joseph Black2 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 Lavoisier3 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.
Hazards & preparation
PPE: safety glasses; calcium hydroxide (limewater) is a mild alkali.
- Blow out only — never inhale through the straw. Rinse any splashes of limewater from skin or eyes with plenty of water.
- Keep it off surfaces you care about; wipe spills promptly.
Disposal: the mixture is a weak base with harmless calcium carbonate — dilute and rinse it down the drain with water. See the Safety page.
Materials
- Calcium hydroxide - 2g (½ teaspoon)
- Water - 200mL
- Straw
- Clear container or glass - 250mL
- Filter paper or a coffee filter (optional, for a clearer solution)
Procedure
- Add 2g calcium hydroxide to 200mL water and shake vigorously.
- Let it settle 5-10 minutes until the liquid above is clear (or filter it).
- Pour 100mL of the clear limewater into the container.
- Blow exhaled breath through the straw for 30-60 seconds.
- Watch the solution turn milky white as calcium carbonate forms.
What you should see
The clear limewater clouds to a milky white within a few breaths as fine white particles form. If you keep blowing well past that point, the milkiness can gradually clear again as the solid redissolves.
| Symptom | Likely cause | Fix |
|---|---|---|
| Stays clear, no cloudiness | Limewater too weak, or not enough breath | Shake in more calcium hydroxide, settle again; blow longer |
| Milky from the start / gritty | Undissolved solid carried over | Let it settle longer or filter before use |
| Clears after going milky | You over-blew (excess CO₂) | Expected — see Question 1; start fresh to re-test |
The reactions
\[\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}\]
calcium hydroxide + carbon dioxide → calcium carbonate (white 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: \(\ce{CaCO3 + CO2 + H2O -> Ca(HCO3)2}\)
Questions to Explore
Why does continued blowing clear the milkiness again? The precipitate (calcium carbonate) dissolves when you keep blowing. What is the additional CO₂ doing chemically that converts the insoluble carbonate into a soluble form — and how does this relate to the chemistry of hard water and limescale?
Hint / answer
Extra CO₂ plus water turns the insoluble carbonate into soluble calcium bicarbonate, so the cloud dissolves. This is exactly how rain rich in CO₂ dissolves limestone into caves — and, in reverse, how that bicarbonate later deposits limescale when the CO₂ escapes.
Why is calcium carbonate insoluble when calcium chloride dissolves freely? Both contain Ca²⁺ ions, yet one dissolves and the other precipitates. What determines the solubility of an ionic compound — and why does the carbonate ion specifically form an insoluble product with calcium?
Hint / answer
Solubility is a tug-of-war between how strongly ions grip each other in the solid and how strongly water pulls them apart. Calcium and carbonate bind tightly into a very stable lattice that water can’t overcome, so it stays solid; calcium and chloride are held loosely enough that water wins.
What does the cloudiness tell you quantitatively? The test is qualitative, but the degree of cloudiness depends on how much CO₂ is present. If you wanted to measure the concentration of CO₂ in exhaled breath versus room air, what would you need to measure or control to make a fair comparison?
Hint / answer
You’d need to pass an equal, measured volume of each gas through identical limewater and compare — for example by timing how long until it clouds, or measuring how much light it blocks. Controls: same limewater strength, temperature, and bubbling rate for both.
Could you test other gases this way? Limewater turns cloudy specifically with acidic gases that react with calcium hydroxide. Would sulfur dioxide or hydrogen chloride gas also produce a precipitate? Would hydrogen gas, or nitrogen? What property do the reactive gases share?
Hint / answer
Acidic gases like SO₂ react with the alkaline limewater (SO₂ gives a similar white cloud of calcium sulfite); HCl reacts too but its calcium salt is soluble, so no cloud. Neutral hydrogen and nitrogen don’t react at all. The test responds to acidic gases.
Why does your breath contain so much more CO₂ than air? Exhaled air has ~4% CO₂ while atmospheric air has ~0.04% — a hundred-fold difference. What metabolic process produces this CO₂, and why can’t cells keep it rather than exhaling it?
Hint / answer
Your cells burn glucose for energy (respiration), and CO₂ is the waste product. It has to leave: dissolved CO₂ makes blood acidic, so the body carries it to the lungs and breathes it out to keep blood chemistry in balance.
Going further
- Compare sources. Test breath against gas from baking soda + vinegar, or from yeast fermentation — all should cloud the limewater, confirming they’re producing CO₂.
- Generate CO₂ chemically from calcium carbonate (chalk or eggshell) + vinegar and pipe it in, instead of using your breath.
- Next in the Acid–Base track: watch a base neutralize an acid with a colour change in the Milk of Magnesia Rainbow.
Footnotes
Precipitate — An insoluble solid that forms and separates out when two solutions are mixed.↩︎
Joseph Black — Scottish chemist (1728–1799) who discovered carbon dioxide (‘fixed air’).↩︎
Antoine Lavoisier — French chemist (1743–1794), the ‘father of modern chemistry’, who named oxygen and hydrogen.↩︎