Alkali metals become opaque when they are kept open in air-Why?

Explanation

As the alkali metals are highly reactive, they readily react with oxygen to form oxides. These oxides undergo reaction with the water vapour present in air to produce hydroxides. The formed hydroxides immediately react with `CO_2` of air to produce carbonate compounds. These carbonate compounds form a layer on the surface of alkali metals. Consequently they become opaque.