The reaction that occurs on passing carbon dioxide gas through lime water
- 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 answer
Explanation
The correct answer is B, Ca(OH)2 + CO2 → CaCO3 + H2O. Lime water is a clear solution of calcium hydroxide, and carbon dioxide reacts with it to form insoluble calcium carbonate, which turns the solution milky. This milkiness is the standard laboratory test for carbon dioxide, and it is also why lime water turns milky when we blow into it through a straw. Calcium carbonate is a white solid that stays suspended in the water as a fine precipitate. If carbon dioxide is passed for a long time, the milkiness disappears, because calcium carbonate turns into soluble calcium hydrogencarbonate. Option A is wrong because it shows that second step, which happens only with excess carbon dioxide, not the reaction with lime water itself. Option C is wrong because it shows quicklime, calcium oxide, reacting directly with the gas; lime water contains calcium hydroxide, not calcium oxide. Option D is wrong because calcium chloride solution does not give calcium carbonate with carbon dioxide; hydrochloric acid would dissolve the carbonate at once. Exam tip: CO2 + lime water = milky (CaCO3); excess CO2 = clear again (Ca(HCO3)2).
Practice Questions
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- C.Ammonium phosphate
- D.Ammonium sulphate
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- A.12.5 cm
- B.5 cm
- C.2 cm
- D.10 cm
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For an astronomical telescope in normal adjustment the magnifying power equals the focal length of the objective divided by that of the eyepiece, so the eyepiece focal length is 50 ÷ 25 = 2 cm.
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- A.black gram and wheat.
- B.rice and mustard.
- C.vegetables and fruits.
- D.soya bean and milk.
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Read the full explanation →Which one of the following compounds is used in black and white photography?
- A.AgF
- B.AgBr
- C.AgCl
- D.Ag2SO4
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Silver bromide is the light-sensitive compound coated on black and white photographic film and paper, because light breaks it down into tiny grains of silver metal.
Read the full explanation →Which one of the following statements about speed and velocity is correct?
- A.Speed and velocity both are vector quantities.
- B.Speed and velocity both are scalar quantities.
- C.Speed is vector quantity and velocity is scalar quantity.
- D.Speed is scalar quantity and velocity is vector quantity.
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