Measurement: Derived Units

In 1960, the General Conference of Weights and Measures proposed the International System of Units, or the SI system, for measurement units.

This system defines seven base units for length, mass, time, temperature, amount of substance, electric current, and luminous intensity. These are called the Standard Units.

Units of measurements which are a combination of one or more base units are known as the derived units. Two of the most commonly used derived units in chemistry are volume and density.

Volume is a measure of the space occupied by an object. The SI unit for volume is defined by the base unit of length, the meter. Thus, any unit of length, when cubed or raised to a power of three, becomes a unit of volume. For example, m³, cm³, and mm³, are all units of volume.

Density, on the other hand, is the ratio of the mass of a substance to its volume. Thus the units for density are defined by the base units of mass and length. The SI unit for density is kg/m³. Often, g/m³ is used for the densities of solids and liquids, and g/L, for gases.

Standard and derived measurements are very helpful in defining various properties of matter. These properties can be broadly divided into two groups — intensive and extensive.

When the property is independent of the amount of matter present, it is known as an intensive property. Temperature, density, boiling and melting point, odor, and hardness are examples of intensive properties. The density of tin, for example, is 7.3 g/cm³ — and this remains the same whether we have a gram or a kilogram of tin. Hence, a density of 7.3 g/cm³ of a substance can be used to identify it as tin. In fact, all intensive properties can be used to identify a substance, as these characteristics do not change with the amount of, or conditions of, the sample.

An extensive property, on the other hand, depends on the amount of matter present. Mass, volume, and length are all extensive properties.

Extensive properties change with the sample size or conditions. This makes them a poor tool for identifying substances. The mass of a sample, for example, will not be helpful in identifying it as tin.