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DENSITY :-
Density of a substance is defined as mass per unit volume.
Mathematically , it is written as :

 Density = Mass / Volume

SI unit – kgm-3
CGS unit- gcm-3

RELATIVE DENSITY :-
The relative density of a substance is defined as the ratio of the density of the substance to the density of water at 4°C. Another term used for relative density is specific gravity.

Relative Density (RD) = Density of the substance / Density of water at 4°C
Relative density is a ratio of the same physical quantities, it has no unit.

The relative density of a substance can also be defined as the ratio of the mass of a certain volume of a substance to the mass of the same volume of water at 4°C.

Relative Density (RD) = Density of the substance / Density of water at 4°C

Density = Mass / Volume   for equal volumes of a substance and water,
⸫ RD = mass of the substance / volume of the substance   /      mass of waterat 4°C / volume of water

⸫ RD = mass of the substance / mass of water at 4°C

RELATION BETWEEN DENSITY AND RELATIVE DENSITY :-
Density = Relative density x 1 gcm-3
Density = Relative density x 1000 kgm-3