How to prepare lime water and test for carbon dioxide

Limewater is the clear, saturated aqueous solution of calcium hydroxide, Ca(OH)₂(aq). It is commonly used to test for carbon dioxide, CO₂(g).

1) Preparing Limewater

1. Add slaked lime to water: Place a spoonful of Ca(OH)₂ powder into a bottle of distilled water.
2. Shake vigorously: A cloudy suspension (“milk of lime”) forms since Ca(OH)₂ is only slightly soluble.
3. Let it settle: Stand for several hours or overnight so excess solid settles.
4. Decant or filter: Carefully pour off (or filter) the clear, colorless supernatant; this is the saturated solution, i.e., limewater.
5. Store sealed: Keep tightly closed to avoid reaction with atmospheric CO₂.

2) Testing for Carbon Dioxide

Procedure: Bubble the test gas through a small volume of clear limewater (use a straw or delivery tube).

Positive result: Limewater turns milky due to formation of insoluble calcium carbonate (chalk).

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

Note (excess CO₂): With prolonged bubbling, the precipitate can redissolve to form soluble calcium bicarbonate:

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

Industrial Uses of Slaked Lime & Limewater

• Water treatment: softening and pH adjustment.
• Construction: mortars, plasters, whitewash.
• Flue-gas treatment: scrubbing acidic gases (SO₂, etc.).
• Soil stabilization: civil engineering and road base.
• Food & sugar refining: clarification steps (under regulated conditions).