Pure substances are either elements or compounds and have a definite set of properties. Mixtures, two or more components mixed in any ratio, undergo a ho chemical change and retain their individual properties, can be separated by simple physical methods. A pure substance, any compound or element, can't be separated into different atoms by physical methods. Remember that pure substances are compounds and elements (made up of the same atom or same molecule respectively), while mixtures are an assortment of different substances put together. Mixtures can always be separated again into pure component substances because bonding among the atoms of the constituent substances does not occur in a mixture. Whereas a compound may have very different properties from the elements that compose it, in mixtures, the substances keep their individual properties.