Relative Density vs Density
Density and relative density are two closely related physical properties of matter. Both parameters describe the amount of matter in a unit volume. These terms are mostly used in fluid statistics/dynamics and chemistry.
Density is a measure of the amount of matter available in a unit volume. Density of an object doesn’t change with the size of the sample, and hence, called an intensive property. Density is the ratio between mass to volume, and therefore, has the physical dimensions of ML-3. Measuring unit for density may be kilograms per cubic meter (kgm-3) or grams per millilitre (g/ml).
When a solid object is put in to liquid, it will float, if the solid has a lesser density than liquid. This is the reason for ice floating on water. If two liquids (which do not mix with each other) with different densities are put together, the liquid with a lesser density floats on the liquid with the higher density.
In some specific applications, density is defined as weight/volume. This is known as specific weight, and in this case, measuring units should be Newtons per cubic meter.
Relative density is density of an object in terms of the density of another reference object. Relative density is defined as the ratio between density and density of the reference object. Hence, it is a dimensionless quantity, and has no measuring unit. In most cases, water is used as the standard material, and in this case, relative density is also called as ‘specific gravity’.
Also, relative density does not depend on the measured amount, and therefore, an intensive property. For instance, relative density of steel is 7.82 when the standard material is water at 4 Celsius degrees and atmospheric pressure. Since, the density is depended on temperature and pressure, these two parameters should be given to make the measurement meaningful. If the relative density of a material is less than one (with respect to water), it floats on water.
Difference between Relative Density and Density
1. Both density and relative density measures the amount of matter available in a unit volume.
2. Density measures a direct physical property, though relative density tells a density of a material in terms of another material.
3. Density has dimensions and measuring units, whereas relative density is dimensionless and doesn’t have a measuring unit.
4. An object at particular conditions can have only one density, though it may have many relative densities with respect to different reference materials.