Question 7
Why does the milkiness disappear if more carbon dioxide is passed into limewater that has already turned milky?
The right answer is the one marked above.
Because water-soluble calcium bicarbonate forms. The CO2 reacts with the CaCO3 to give {Ca(HCO3)2}, which dissolves — so the suspension disappears.
The limewater test must therefore be read promptly — pass the gas for too long and the positive result itself vanishes.