What Is Relative Density?

Density is a measure of how much mass is present in a given volume of matter. Relative density is the comparison of the density of one material to the density of a particular standard, such as water, at a given temperature or pressure. Relative density is also known as specific gravity.

Most commonly, density is calculated in metric units. A widely used option is to state density in grams per cubic centimeter (g/cm³.) A gram is a measure of mass, and a cubic centimeter is a measure of volume.

Mass is the amount of matter in a substance and the inertia of that substance. It is different from weight because weight can change with variations in gravity on the object. An object is heavier on Earth than on the moon, for example, because there is less gravity on the moon. Mass remains the same in both cases, however.

In calculating the density of a substance, the mass and the volume must be known. The equation necessary is density = mass / volume. As an example, 1 gram of water takes up 1 cubic centimeter of volume. A cubic centimeter is also known as a milliliter when referring to liquids, so the equation for the density of water would be 1g / 1ml. The density is therefore equal to 1 g/ml, which also can be expressed as 1 g/cc or 1 g/cm³.

Measurements of density require units of mass and volume to be included in the answer. Relative density, on the other hand, is merely a comparison of densities between two substances as a ratio, so units are not necessary. For example, the relative density of one sample of water to another sample of water is 1.0 g/ml : 1.0 g/ml, and because the units on both sides cancel each other out, the final answer simply stands at 1.0.

Water typically is used as the standard for relative density calculations for solids and liquids. This is useful because the substance is compared with a substance that has a simple density of 1 g/ml. For gases, a standard of a specific gas might be used. The density of materials can vary with environmental pressure and temperature, so the standard density is restricted to a particular temperature or, for gases, a particular pressure.

Using water as a standard, substances that are heavier than water, such as gold, result in relative density values that are higher than 1.0. Gold has a relative density of 19.30, and common table salt a relative density of 2.16. Less dense materials have a value of less than 1.0, such as ammonia at 0.8974 or sticks of pine at about 0.50.