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gsenviro eNotes educator| Certified Educator

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).

Hope this helps. 

elzbaker | Student

Most substances are not pure, but are mixtures of two or more different substances (elements or compounds) which are not chemi­cally combined. Air is a good example of a mixture.

It contains many different elements and compounds whose proportions are not always the same. Scientists discovered that pure substances such as carbon cannot be split into anything simpler by chemical reactions.

These substances are the elements, and they cannot be split because they contain only one sort of atom. Using chemical reactions scientists are also able to make many new compounds—substances made up of two or more different elements combined together.

For example, when carbon is heated it reacts with the oxygen in the air to form the compound carbon dioxide.