A sample of matter, either an element or a compound, that consists of only one component with definite physical and chemical properties and a definite composition. To put it more simply, a pure substance is one is predictably arranged and can not be broken off into separate substances. For example. water, or H2O. The formation of water is a combination of the elements hydrogen and oxygen in a predictable pattern. If you break off parts of that pattern, you remain with only elements - hydrogen and oxygen. You can't break the two elements off and form a new substance. Think of pure substances in the same way that you think of lowest common denominator in math.
All pure substances can be further separated by asking this question "Can the sample be further decomposed by chemical means?" Like with the water example, can you break it off into parts. With water, you can - it is broken into hydrogen and oxygen. Therefore, this pure substance is a compound. If the substance can not be broken into parts - like hydrogen by itself - than the pure substance is an element.
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