Lime water turns milky when carbon dioxide is passed through it. It is due to the formation of:

Calcium carbonate is a chemical compound with the formula CaCO3. Limestone, chalk, and marble are different forms of calcium carbonate. It is commonly found in rocks as the minerals calcite and aragonite, notably in limestone, a sedimentary rock composed mainly of calcite. Calcium carbonate is a major component of eggshells, snail shells, sea shells, and pearls.