The reaction that occurs on passing carbon dioxide gas through lime water is
- (a)CaCO3 + H2O + CO2 → Ca(HCO3)2
- (b)Ca(OH)2 + CO2 → CaCO3 + H2O
- (c)CaO + CO2 → CaCO3
- (d)CaCl2 + H2O + CO2 → CaCO3 + 2HCl
Correct — B, Ca(OH)2 + CO2 → CaCO3 + H2O. Lime water is a dilute solution of calcium hydroxide, Ca(OH)2. When carbon dioxide is bubbled through it, insoluble calcium carbonate (CaCO3) forms and makes the solution turn milky — this milkiness is the standard laboratory test for CO2. Water is the other product.
- (a)CaCO3 + H2O + CO2 → Ca(HCO3)2 — This is the follow-up reaction, not the first one. If CO2 is passed in EXCESS, the milky calcium carbonate redissolves to soluble calcium bicarbonate, so the milkiness disappears — but it is not the reaction lime water undergoes on first contact with CO2.
- (c)CaO + CO2 → CaCO3 — This is the reaction of solid quicklime (calcium oxide) with CO2, not of lime water. Lime water contains dissolved calcium HYDROXIDE, not calcium oxide.
- (d)CaCl2 + H2O + CO2 → CaCO3 + 2HCl — This involves calcium chloride and would produce hydrochloric acid; it is not the lime-water reaction and is not a feasible route from CO2 to CaCO3.
Lime water is a clear, saturated solution of calcium hydroxide, Ca(OH)2. Carbon dioxide is a mildly acidic oxide; it reacts with the alkaline lime water to precipitate calcium carbonate, turning the solution milky. Passing further CO2 converts the carbonate to soluble calcium bicarbonate, clearing the milkiness — this two-step behaviour is a classic demonstration.
The distractors are all genuine calcium reactions, so the key is to match the exact reactant. 'Lime water' fixes the reactant as Ca(OH)2 in solution (not CaO, not CaCl2), and 'brings about' asks for the first reaction, which yields CaCO3 — option (b).
- Lime water is a solution of calcium hydroxide, Ca(OH)2.
- CO2 + Ca(OH)2 → CaCO3 (white, insoluble) + H2O; the CaCO3 makes lime water turn milky.
- Turning lime water milky is the standard confirmatory test for carbon dioxide gas.
- Excess CO2 dissolves the precipitate as soluble calcium bicarbonate, Ca(HCO3)2, so the milkiness disappears.
- Confusing lime water (a Ca(OH)2 solution) with quicklime (CaO).
- Forgetting that excess CO2 redissolves the carbonate, so the milkiness is not permanent.
Asked as the reaction or test for CO2 with lime water, or why lime water first turns milky and then clears.
No directly related past PYQ was found.
- practice — not a real PYQ
Lime water turns milky when carbon dioxide is passed through it because of the formation of
- (a)calcium bicarbonate
- (b)calcium carbonate
- (c)calcium oxide
- (d)calcium chloride
Answer(b) calcium carbonate (CaCO3), which is insoluble and makes the solution milky.
- practice — not a real PYQ
The milkiness of lime water disappears when excess carbon dioxide is passed because
- (a)the CaCO3 evaporates
- (b)soluble calcium bicarbonate, Ca(HCO3)2, is formed
- (c)the water boils off
- (d)calcium hydroxide reforms
Answer(b) soluble calcium bicarbonate, Ca(HCO3)2, is formed.