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Air is a mixture of gases surrounding our planet. The most abundant components are nitrogen and oxygen. To be specific, about 78% of air is nitrogen and 21% is oxygen. The remaining 1% includes various gases like argon, carbon dioxide, and traces of others.
However, another significant component of air is water vapor. The amount of water vapor in the air can vary greatly, from almost zero up to about 4% by volume. This variation in water vapor changes the overall composition of air, which impacts its properties such as density.
In dry air, the composition is fixed with negligible water vapor. In contrast, moist air includes a higher percentage of water vapor, which alters its overall makeup.

Molecular mass is a concept that plays a crucial role in understanding the density of air. It is the mass of a molecule, usually measured in atomic mass units (u).
Dry air is primarily made up of nitrogen (Nâ‚‚) and oxygen (Oâ‚‚), with molecular masses of around 28 u for nitrogen and 32 u for oxygen. When comparing these with water vapor (Hâ‚‚O), which has a molecular mass of approximately 18 u, the difference becomes evident.
Because water vapor has a lower molecular mass than both nitrogen and oxygen, any increase in water vapor content in the air lowers the overall average molecular mass of the air mixture.

Density is a measure of how much mass is packed into a given volume. For air, density depends directly on its molecular mass in a specific volume.
When water vapor, which has a lower molecular mass, replaces some of the nitrogen and oxygen in the air, the overall molecular mass of the air decreases. This decrease leads to a lower air density because the same volume of air now contains lighter molecules.
Hence, moist air, which contains a higher percentage of water vapor, is less dense than dry air. This can be illustrated by the proportional relationship:density = (mass/volume)Therefore, if the mass is reduced due to lighter water vapor replacing heavier nitrogen and oxygen, the density of air decreases.