Carbon dioxide turns limewater milky. Limewater is an alkaline solution of calcium hydroxide. It reacts with carbon dioxide to form calcium carbonate, which is the reason limewater becomes milky.
Carbon dioxide, an odorless, non-flammable and colorless gas, is not reactive chemically. However, if the concentration of carbon dioxide is high, it can be an asphyxiant and a toxic gas that can cause irritation of the nose, throat and eyes.
When calcium carbonate or limestone is heated at temperatures of around 1,100 to 1,200 degrees Celsius, limestone can be quickly converted into carbon dioxide and lime or calcium oxide. When lime dissolves in water, it becomes slaked lime. Both slaked lime and lime are used for lessening sulfur emissions in gas-fired and coal power stations.