Density is important next to the size of the particle. Density of the particle can be mass density or weight density. Mass density is the mass of the particle per unit volume whereas Weight density is the weight of the particle per unit volume. Specific gravity is defined as the ratio of the density of the particle to the density of water at 4◦C. In C.G.S system of units, numerical value of density and specific gravity is same. Hence, it is normal practice to indicate specific gravity for density.