Density and Specific Gravity

Density

Density is a measure of mass per unit volume. It is defined as:

Density = \frac{Mass}{Volume}

The SI unit of density is kilograms per cubic meter (kg/m³). Other common units include grams per cubic centimeter (g/cm³) and pounds per cubic foot (lb/ft³).

Specific Gravity

Specific Gravity (SG) is the ratio of the density of a substance to the density of a reference substance, usually water at 4°C (where the density of water is 1 g/cm³).

Specific Gravity = \frac{Density of Substance}{Density of Water}

Key Points:

Dimensionless: Specific Gravity is a dimensionless quantity.
Water at 4°C: The reference point for water is chosen at 4°C because it has the highest density.
Approximation: For many practical purposes, the density of water is taken as 1 g/cm³, making specific gravity and density nearly equivalent for many substances.

Example:

The density of water at 4°C is 1 g/cm³.
The density of a substance is 2.7 g/cm³.
The specific gravity of the substance is:

Specific Gravity = \frac{2.7 {g/cm}^{3}}{1 {g/cm}^{3}} = 2.7