Question

What do you observe when CO₂ is passed through lime water first and then a little in excess.

Answer 1

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.

 \[\ce{\underset{\text{Black}}{CaCO3} + CO2 -> H2O + \underset{\text{(Soluble)}}{Ca(HCO3)2}}\]