Here's why:

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

* When carbon dioxide is bubbled through limewater, it reacts with the calcium hydroxide to form calcium carbonate (CaCO3).

* Calcium carbonate is a white, insoluble solid.

* The formation of calcium carbonate makes the limewater appear milky.

The reaction:

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