Chemists measure the properties of matter and express these measurements as quantities. A quantity is an amount of something and consists of a number and a unit. The number indicates how many (or how much), and the unit describes the scale of measurement. For example, when a distance is reported as “5 miles,” the quantity has been expressed in units of "miles" and the numerical value is "5." The magnitude of this distance is conveyed by its unit. Changing the unit that is associated with a number alters the interpretation of the quantity that is being expressed. For example, "5 inches," "5 feet," and "5 miles" are all very different distances, because of the units that are associated with each numerical quantity. Furthermore, reporting a numerical value without a unit renders that quantity meaningless. Both a number and a unit must be included in order to express a quantity properly.
The remaining content in this section relates to the systems in which scientists classify units of measurement. The following sections of this chapter will present and apply the rules that are used for expressing numbers.