What units do chemists normally use for density of liquids and solids? for gas density? explain the differences?

We can define density as per unit volume.

The SI unit to measure the density of solid, liquid and gas is kilogram per cubic metre (kg/m3) and in the centimetre–gram–second system of units (cgs unit) is gram per cubic centimetre (g/cm3). Gas density is very dependent of pressure and temperature whereas the density of solids and liquids is not so dependent, that is why the gas density is given at a standard temperature and pressure.

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