Limewater – IGCSE Chemistry Definition and the Carbon Dioxide Test

Limewater is one of the most frequently tested gas tests in IGCSE Chemistry. It appears on every paper: as a one-mark recall question, as an observation in a reaction, and as the confirmation step in qualitative analysis. Any candidate who can state the test, the result, and the equation has secured reliable marks.

The 0620 definition

Limewater is a dilute aqueous solution of calcium hydroxide, Ca(OH)₂. Calcium hydroxide is only slightly soluble in water, so limewater is a weak, clear, colourless alkaline solution. It is used almost exclusively as the test for carbon dioxide.

The test for carbon dioxide

To test a gas for carbon dioxide, bubble it through limewater.

• Positive result: the limewater turns milky (cloudy white).
• If CO₂ continues to be bubbled through in excess, the milkiness may disappear again as soluble calcium hydrogencarbonate forms (a Supplement extension, not always required).

This is the standard confirmation that a gas is carbon dioxide — for example, the gas produced when an acid reacts with a carbonate, or the gas released during thermal decomposition of a metal carbonate.

The chemistry behind the milkiness

Carbon dioxide reacts with calcium hydroxide to form calcium carbonate, which is insoluble and forms a white precipitate:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

The fine, suspended CaCO₃ particles scatter light, so the clear solution turns cloudy or milky. The (s) state symbol on the calcium carbonate is the reason for the visible change — a soluble product would leave the solution clear.

The excess carbon dioxide extension (Supplement)

If excess CO₂ is passed through the milky mixture, the insoluble calcium carbonate reacts further to form soluble calcium hydrogencarbonate, and the solution turns clear again:

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

This reversal is the chemistry behind temporary hardness of water and the formation of stalactites and stalagmites. It is not required at Core level but can appear in Supplement questions.

How limewater is made

Limewater is prepared by adding calcium oxide (quicklime) or calcium hydroxide (slaked lime) to water, stirring, and filtering off the excess undissolved solid. The clear filtrate is limewater. Because calcium hydroxide is only slightly soluble, the solution is dilute.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Where limewater appears in exams

Context	What limewater does
Testing gas from acid + carbonate	Turns milky, confirming CO₂
Testing gas from thermal decomposition of carbonates	Turns milky, confirming CO₂
Testing exhaled breath (biology crossover)	Turns milky, showing CO₂ is exhaled
Respiration/combustion product identification	Confirms CO₂ is produced

Common exam mistakes

1. Saying limewater “turns white” instead of “turns milky/cloudy”. The mark scheme wants the words milky or cloudy — the solution does not become a solid white block.
2. Confusing limewater with universal indicator. Limewater is a specific test for CO₂, not a pH indicator.
3. Writing the wrong formula for calcium hydroxide. It is Ca(OH)₂ — the bracket and subscript 2 matter.
4. Forgetting the state symbol on calcium carbonate. CaCO₃ must be (s) — the insolubility is the whole point of the test.
5. Claiming the gas is “carbon dioxide because it is a gas”. The milky limewater result is the evidence; naming the gas without the test does not earn the identification mark.