This is a wonderful example of Chemistry. Let’s see why this happens.

When we blow in a test tube filled with lime water, it color turns milky. It happens due to the reaction of carbon dioxide (CO2) gas which comes along with our breath to lime water. This reaction forms insoluble calcium carbonate (CaCO3). Due to presence of white color Calcium Carbonate the color of lime water becomes milky.

But if we keep blowing into test tube, due to excess amount of CO2, further reaction occurs in which the CaCO3 formed further gets converted into Calcium-bi-Carbonate. This product is soluble in water and hence, the milky color vanishes.

Soluble substances– Substances which dissolve in water or any other liquid. For example, sugar, salt etc. get dissolved in water. Note that not all compounds get dissolved in water.

Insoluble substances– Substances which do not dissolve in water or any other liquid. For example, oil and water do not mix.