Calcium Hydroxide

Calcium hydroxide, slaked lime, is the one alkali most students meet as a solution with its own name: limewater. Its dilute solution gives the standard test for carbon dioxide, and as a cheap base it neutralises acidic soil and acidic water on an industrial scale.

Key facts

Property	Detail
Formula	Ca(OH)2
Relative formula mass (Mr)	74 (Ca 40, O 16, H 1)
Bonding / structure	Giant ionic lattice (Ca2+ and OH- ions)
Solubility	Slightly soluble; the solution is limewater
pH of limewater	About 12 (alkaline)
Main exam roles	Limewater test for CO2, neutralisation, limestone cycle

Where this compound appears in 0620

• Tests for gases (Topic 11): limewater is the test for carbon dioxide.
• Acids, bases and indicators (Topic 8): a base that neutralises acids; a slightly soluble alkali.
• Limestone cycle: the intermediate that closes the loop back to calcium carbonate.
• Fertilisers / soil treatment: liming acidic soil.

Production

CaO(s) + H2O(l) -> Ca(OH)2(s) (exothermic)

Formed by adding water to calcium oxide — the “slaking” of quicklime.

Reactions and tests you must know

The limewater test for carbon dioxide:

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

Bubble the gas through limewater; if it turns milky/cloudy, the gas is carbon dioxide. The cloudiness is the insoluble white precipitate of calcium carbonate. This is the defining CO2 test in qualitative analysis.

Neutralising acids:

Ca(OH)2(aq) + 2HCl(aq) -> CaCl2(aq) + 2H2O(l) Ca(OH)2(aq) + H2SO4(aq) -> CaSO4(s) + 2H2O(l)

Completing the limestone cycle: CaCO3 -> CaO -> Ca(OH)2 -> CaCO3, where the last step is the limewater reaction regenerating the carbonate.

Uses

Use	Explanation
Limewater (CO2 test)	Laboratory test for carbon dioxide
Neutralising acidic soil	Cheaper and gentler than quicklime
Water and sewage treatment	Raises the pH of acidic water
Making mortar	Mixed with sand and water for building

Worked exam question

A student bubbles the gas from a fizzing mixture of hydrochloric acid and a carbonate through limewater.

(a) State what the student would observe and name the gas responsible. (2 marks)

(b) Write the equation, with state symbols, for the reaction that causes this observation. (2 marks)

(c) Explain why limewater, rather than pure water, is used for this test. (1 mark)

Mark scheme

• (a) The limewater turns milky / cloudy / a white precipitate forms [1]; the gas is carbon dioxide [1]
• (b) Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l) [1]; correct state symbols [1]
• (c) Carbon dioxide reacts with the dissolved calcium hydroxide to form insoluble calcium carbonate, which pure water cannot do / pure water would show no change [1]

Examiner note: “goes cloudy/milky” is the expected wording — “goes white” is usually accepted but “turns clear” is wrong, since the change is from clear to cloudy. State symbols in (b) are worth their own mark, so the (s) on CaCO3 is essential.

Common exam mistakes

• Saying limewater “turns clear” or “goes colourless”. The change is from a clear, colourless solution to a cloudy/milky one — describe the appearance of the precipitate, not the colour.
• Confusing calcium hydroxide with calcium oxide. Ca(OH)2 is slaked lime (made by adding water to CaO); it is the compound dissolved in limewater.
• Calling limewater a strong, fully soluble alkali. Ca(OH)2 is only slightly soluble, so limewater is dilute — its pH is around 12, not 14.
• Forgetting the water product in the limewater equation. CO2 + Ca(OH)2 gives CaCO3 and H2O; leaving out the water leaves the equation unbalanced.