Question

Why are units used that are to express the densities of gases different from those used to express the densities of solids or liquids

Answer 1

If the units for gases were the same as the units for numbers would be too small to be convenient.

For example, the density of iron is 7.87 g/cm³.

Using the same units, the density of hydrogen is 0.000 0899 g/cm³.

It is much more convenient to express the density in numbers that are easier to visualize: 0.0899 g/L, and that is still a small number.