An element is a pure substance which cannot be broken down by further chemical techniques. These include heating, cooling, electrolysis and reacting with other chemicals. (By the way, it is correct that an atom can be destroyed, but NOT by chemical means. You must use a more powerful reaction, called a nuclear reaction, to destroy or change atoms.
Pure substances are defined as substances that are made of only one type of atom or molecule. The purity of a substance is the measure of the extent to which a given substance is pure. The properties of pure substances are known (e.g. the boiling point of oxygen is exactly -183°C and the melting point is exactly -218.8°C). So if we do tests on a substance and the boiling and melting points aren’t what we’re looking for in a pure substance, then it must mean that it is contaminated with another substance that has different boiling/melting points.
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