When lime water is exposed to air for a long time, very thin white coating is found floating on the surface. Give reasons.

Explanation

Air contains a low percentage of `CO_(2)`. Though `CO_(2)` turns lime water milky, since the amount of `CO_(2)` is less in the tmospheric air, it does not turn lime water milky. Only over a long period of exposure, a thin white film of `CaCO_(3)` is formed due to the minor amounts of `CO_(2)` in the atmospheric air.