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Question
What do you observe when CO2 is passed through lime water first and then a little in excess.
Solution
When the gas is passed through lime water, it turns milky due to the formation of a white precipitate of calcium carbonate.
\[\ce{Ca(OH)2 + CO2 -> CaCO3↓ + H2O}\]
The milkiness disappears when CO2 is passed in excess.
\[\ce{\underset{\text{Black}}{CaCO3} + CO2 -> H2O + \underset{\text{(Soluble)}}{Ca(HCO3)2}}\]
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