Together, elements and compounds are called pure substances . A physical property of a pure substance is a characteristic of that substance alone. It can be determined and described without changing the substance into other substances. A chemical property of a substance can be observed and described only in terms of at least one other substance. For example, water and sodium react violently.
Mixtures occur more commonly in nature than do pure substances.
A mixture is composed of two or more pure substances, and is characterized by variable composition. A mixture may be separated into components using physical methods, which do not alter the chemical composition of any of the pure substances composing it. A solution , though homogeneous, may nonetheless have variable composition. Any amount of salt, up to a maximum limit, can be dissolved in a given amount of water. Solutions are called homogeneous mixtures .