When carbon dioxide is passed through lime water, the solution turns milky, but, on prolonged passage, the solution turns clear. This is because

When carbon dioxide (CO2) is passed through lime water (calcium hydroxide), it initially reacts to form calcium carbonate (CaCO3), which is an insoluble white precipitate that makes the solution appear milky [1]. However, if the passage of carbon dioxide is prolonged, the excess CO2 reacts with the water and the existing calcium carbonate to form calcium bicarbonate (also known as calcium hydrogencarbonate). Unlike the carbonate, calcium bicarbonate is highly soluble in water, which causes the milky precipitate to dissolve and the solution to become clear and colorless again [1]. This chemical process is a standard test for identifying carbon dioxide and is also fundamental in understanding natural phenomena like the formation of hard water and limestone caves [1].

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > Fig. 5.2: Blowing air in (a) tap water; (b) lime water > p. 61