Carbon dioxide is heavier than oxygen and nitrogen but is does not form the lower layer of the atmosphere. Explain.

The diffusion of the gases is quite independent of the gravitational pull by the earth. The molecules of carbon dioxide remain distributed throughout the atmosphere. Therefore, carbon dioxide does not form the lower layer of the atmosphere.

Out of dry air and wet air, which is heavier?

Dry air mainly contains gases like nitrogen and oxygen along with small proportions of the other gases. The wet air on the other hand, also contains a small amount of water vapours. Now, vapour density of water vapours is less than that of nitrogen and oxygen because molar mass of H2O (18) is less than that of N2 (28) and O2 (32). This means that heavier molecules or nitrogen and oxygen have been replaced by lighter molecules of water vapours when the dry air changes to we air. Therefore, dry air is heavier than wet air.