Specific gravity

Relative density is a ratio of densities. So, it is a dimensionless quantity and has no unit. Relative density is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water.

Relative density, also known as specific gravity, is defined as the ratio of the density of a substance to the density of a reference substance. It is a dimensionless quantity that does not have any units. Therefore, the correct answer is option 'D' - No unit.

Explanation:

Relative density is a concept used to compare the density of a substance to the density of another substance. It is a dimensionless quantity because it is the ratio of two similar quantities, and the units cancel out. The reference substance is usually water, which has a density of 1 gram per cubic centimeter (g/cm3) or 1000 kilograms per cubic meter (kg/m3).

To calculate the relative density of a substance, you divide its density by the density of the reference substance. For example, if the density of a substance is 2.5 g/cm3 and the density of water is 1 g/cm3, the relative density would be 2.5/1 = 2.5.

Relative density is often used to determine the purity or composition of a substance. For example, if the relative density of a liquid is greater than 1, it indicates that the liquid is denser than water. If the relative density is less than 1, it indicates that the liquid is less dense than water. This information can be used to identify substances or determine their concentrations in mixtures.

In conclusion, the unit of relative density is dimensionless and does not have any units. It is represented by the symbol "No unit" and is used to compare the density of a substance to the density of a reference substance.