Chemistry may be classified in many different ways, such as state of matter (gas, liquid or solid), chemical form (element, mixture or compound), chemical structure (atoms or molecules etc.) and so on.
In addition to "element", "mixture" and "compound", another useful term is the word "substance", which can be used to refer to either an element or a compound - but not to a mixture because a "substance" always has a definite composition.
The relationship between these terms may be summarized in the form of a diagram:
"A form of matter that has both definite composition and distinct properties. A pure substance cannot be separated into simpler components without chemical change. Physical changes can alter the state of matter but not the chemical identity of a pure substance."
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