Elements, compounds and mixtures
Chemical substances consist of either elements in their pure form or combinations of elements. An element cannot be physically broken down into simpler substances. Different elements have different properties. Elements comprise metals and non-metals. Iron is an example of metal. Non-metals include hydrogen and oxygen. Elements combine to form compounds or mixtures. Compounds are combinations of elements joined chemically, in fixed proportion. Compounds can not be physically separated into their elements. A mixture can be separated into their elements. Mixtures are separated by processes such as filtration, sieving and evaporation. All of these involve a physical change. We can determine whether a substance is pure or not by trying to separate it physically.