Question

Why is air dense at sea level

Answer 1

when water molecules are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the density decreases

Answer 2

When water molecules are added to a given volume of air, the dry air molecules
must decrease by the same number, to keep the pressure or temperature from increasing .Hence,the mass per unit volume of the gas decreases