Substances can be classified into three categories: elements, compounds and mixtures. An element is a pure form of matter and is composed of atoms of only one type and cannot be further divided. A compound is made up of two or more elements in a specific ratio and can be further divided. A mixture is a combination of different elements and/or compounds.
A compound can be divided into elements by using chemical means. The properties of a compound are different from those of its individual constituent elements. An example of a compound is water (`H_2O` ), in which two elements, hydrogen and oxygen are mixed in a 2:1 ratio. Irrespective of the amount of water, all its molecules will always have this fixed ratio. Also, hydrogen is combustible, whereas oxygen is necessary for combustion. In comparison, water is used for extinguishing fires (and hence has different properties than hydrogen or water).
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