Relative Density (or Specific gravity)

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Relative density (RD) of a substance is defined as the ratio of the density of the substance to density of water at 4^oC.

R.D

This RD of a substance is also defined as the ratio of the mass of the substance to mass of equal volume of water.

R.D has no unit as it is a mere ratio.

In C.g.s system density of water at 4^oC is 1 g cm^-3

= numerical value of density of the substance.

For example density of mercury is 13.6 g cm^-3, it R.D is 13.6

In S.I system density of water is 1000 kg m^-3 and that of mercury is 13.6 x 10³ kg m^-3.

R.D of mercury =

R.D of a substance has the same value in any system.

R.D of a substance indicates how much heavier a certain volume of the substance is as compared with water of same volume.