What is the colour of the precipitate obtained by passing CO₂ gas through lime water?
- (a)Green
- (b)Blue
- (c)White
- (d)Brown
Answer
Why
Correct — C, white. Lime water is a clear solution of calcium hydroxide, Ca(OH)2. When carbon dioxide is passed through it, the two react to form insoluble calcium carbonate, CaCO3, which appears as a white precipitate and turns the lime water milky. This 'lime water turns milky' change is the standard laboratory test for carbon dioxide gas.
Why the others are wrong
- (a)Green — Calcium carbonate is white; a green precipitate would suggest iron or copper compounds, which are not formed here.
- (b)Blue — A blue precipitate points to copper hydroxide, not to the calcium carbonate that forms in this reaction.
- (d)Brown — A brown precipitate is typical of iron compounds; the product of carbon dioxide with lime water is white calcium carbonate.
Concept
Lime water is a dilute, clear solution of calcium hydroxide. Carbon dioxide is a mildly acidic gas, and passing it through lime water gives calcium carbonate and water: Ca(OH)2 + CO2 gives CaCO3 and H2O. Calcium carbonate is only slightly soluble, so it drops out as a fine white solid that clouds the liquid.
The reasoning is to identify the product, not the reactant colours. The solid formed is chalk, calcium carbonate, which is white. If excess carbon dioxide is bubbled through, the milkiness even disappears as soluble calcium bicarbonate forms, but the first precipitate is white.
Key facts
- Lime water is a clear solution of calcium hydroxide, Ca(OH)2.
- Carbon dioxide reacts with it to form calcium carbonate, CaCO3, a white precipitate.
- The milkiness of lime water is the standard test for carbon dioxide gas.
- Excess carbon dioxide redissolves the solid as soluble calcium bicarbonate, clearing the milkiness.
Study next
Common traps
- Guessing a colour from the reactants instead of identifying the white calcium carbonate product.
- Forgetting that passing excess carbon dioxide clears the milkiness by forming soluble calcium bicarbonate.
Asked as the colour of the precipitate or as the gas test; the milky white solid is calcium carbonate, confirming carbon dioxide.
Related PYQs
Carbon dioxide gas, when passed through lime water, brings about which of the following reactions?
- (a) CaCO3 + H2O + CO2 → Ca(HCO3)2
- (b) Ca(OH)2 + CO2 → CaCO3 + H2O
- (c) CaO + CO2 → CaCO3
- (d) CaCl2 + H2O + CO2 → CaCO3 + 2HCl
Answer(b) Ca(OH)2 + CO2 → CaCO3 + H2O
The same reaction written as an equation — carbon dioxide plus calcium hydroxide gives calcium carbonate. This NDA (II) 2021 item asks the colour of that calcium carbonate, which is white.
Practice
- practice — not a real PYQ
Lime water turns milky on passing a gas through it. The gas is
- (a)Oxygen
- (b)Carbon dioxide
- (c)Hydrogen
- (d)Nitrogen
Answer(b) Carbon dioxide — it forms white calcium carbonate, the milkiness being the standard test. - practice — not a real PYQ
The white precipitate formed when carbon dioxide is passed through lime water is
- (a)Calcium hydroxide
- (b)Calcium carbonate
- (c)Calcium bicarbonate
- (d)Calcium oxide
Answer(b) Calcium carbonate — CaCO3, insoluble and white.