Fluids : They are materials which slip of no definite shape, they take the shape of the container , e.g. Liquids of constant volumes and gases of changeable volumes.

Density of a Substance : The mass of unit volume of this substance at certain temperature.
it's a property of a homogeneous substance, measured in kg/m^3.

Density = mass/volume    r = m/v     kg/m^3

Relative Density, Specific Density, Specific Gravity of a substance (RD)

R.D. =  r Substance/1000 = Mass of any volume of substance/Mass of equal volume of water = weight of any volume of substance/weight of equal volume of water . at the same temperature . it's a ratio ( unit less )

NB : Rising the temperature of solid or liquid increases the volume, and since the mass remains unchanged, the density decreases

On mixing two non-reacting liquids :
M = M1+M2    or    V=  r1V1+ r2V2   or  r = r1 V1/V + r2 V2/V

On mixing two equal masses of non-reacting immiscible liquids :
V = V1+V2    or     2m/r =  m/r1 +   m/r2    or   2 [ r1.r2/r1+r2]

On mixing two equal volumes of non-reacting immiscible liquids :
M = M1+M2    or   2rV = r1V  + r2V    or    r = 1/2 (r1+r2)