Why milky color disappears when the excess of carbon dioxide is passed through lime water?

When you first start to bubble carbon dioxide through limewater (calcium hydroxide solution), they react and form calcium carbonate. This isn't very soluble, so you see it as a mist of fine particles of chalk, which we describe as 'the limewater has gone cloudy/ milky'. As you continue to bubble, carbon dioxide dissolves in the water to form carbonic acid, which dissolves the calcium carbonate, so the milkiness disappears. Here are the reactions: Ca(OH)2 + CO2 --> CaCO3 + H2O forming the milkiness 2H2CO3 + CaCO3 --> Ca(HCO3)2 + H2O +CO2 removing the milkiness again This second reaction is the same one as for rainwater flowing over limestone rocks.