Here's why:

* Lime water is a solution of calcium hydroxide (Ca(OH)2) in water.

* Carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO3), which is a white, insoluble solid.

* This reaction is represented by the following equation:

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

The formation of the insoluble calcium carbonate causes the lime water to turn milky white. This is a common test used to detect the presence of carbon dioxide.