Difference Between Relative Density and Specific Gravity

Relative Density vs Specific Gravity

Relative density and specific gravity are two concepts used in comparing densities of solids, liquids and gas. Both these concepts hold almost the same idea. These concepts are very useful in the food industry, rubber industry and whole of the material science. In this article, we are going to discuss relative density and specific gravity in depth and about their applications, definitions, similarities and differences.

Relative Density

To understand the concept of relative density one must first understand the concept of density. The density of a material tells you how close the molecules are packed and how heavy the molecules are. The density is defined as the mass of a substance per unit volume. This is mathematically written as density = mass / volume. Relative density actually, relates the densities of two compounds. It tells you, how dense a material is compared to another. Relative density of a given material is defined as density of the given material / density of the reference material. The term “relative density” is dimensionless. In other words, it has no units. It’s a measurement of how many times the given material is denser than the reference material. Water is generally taken as the reference material for liquids. If the relative density of a given substance is greater than a unit the substance sinks in water. If the relative density is lesser than a unit the substance floats on water. The relative density of any material cannot be zero. If the relative density is zero the mass must be zero. For any given substance the mass in a given volume cannot be zero. Therefore the relative density cannot be zero for any given material. The relative density is pressure and temperature dependent. The same material can have different relative densities in different pressures and temperatures.

Specific Gravity

Specific gravity is defined as the mass of a unit volume of the given material divided by the mass of the unit volume of the reference material. The reference material is air for gases and water for liquids most of the times. Specific gravity is also pressure and temperature dependent. Specific gravity is used in simple industries, such as milk and rubber industries, to determine the quality of raw materials. Pycnometer is one of the many different instruments used to determine the specific gravity. It is also known as a specific gravity bottle. Specific gravity also is a dimensionless quantity, which varies between zero and infinity. But, it cannot have the value zero itself. There is also another definition for specific gravity called apparent specific gravity.