Why is 1 g not equal to roughly 1 liter while 1 g equals 1000 mg and 1 liter equals 1000 ml ie why is 1 kg which equals 1000 g equal to roughly 1 liter should 1 gram not be equal to roughly 1 liter?

Answer 1

One millilitre is a volume. One gram is a unit of mass. The two are NOT interchangeable. Except in one special case. That of water. As this is part of what was initially used to create the system.

The SI or metric system, though much simpler than the Imperial system, has certain inconsistencies

The base unit of mass is the KILOGRAM, a gram is a 1/1000 of a kilogram

The base unit of volume is the CUBIC METRE, a litre is 1/1000 of a cubic metre.

A cubic metre of water, and only water, has a mass of approximately 1000 kilograms (which is one metric ton).

One millilitre of WATER has a mass of approximately one gram.

One litre of WATER has a mass of approximately one kilogram.

Incidentally the Kilogram is the only base unit that has a physical object to define it.

Answer 2

The discrepancy arises from the density of the substance being measured. A kilogram of water occupies roughly 1 liter because water has a density close to 1 g/ml. However, for substances with different densities (like most solids), 1 gram does not necessarily equal 1 liter in volume. So, the equivalence between mass and volume depends on the substance's density.