Based on the chemical composition of a substance, it can be classed as an element, a compound or a mixture. All of these are made of atoms, the basic building blocks of all matter. Elements, such as copper, silver and gold, cannot be reduced to simpler materials either by physical or chemical changes. Both compounds, such as water, carbon dioxide and sodium chloride, and mixtures, such as air and sea water, are composed of atoms, but that is the only similarity.

A mixture is composed of two or more elements or compounds in a non-fixed ratio, which means you can vary the amount of substance in a mixture. A compound is composed of two or more elements in a fixed ratio, so you cannot vary the amount of each element in a compound. For example, a mixture of iron and sulphur can consist of 1 gram of sulphur with 1 gram of iron or 2 grams of iron (and so on), but the compound is consistently comprised of identical quantities of iron and sulphur.

The different substances in a mixture are not chemically combined, whereas the different elements in a compound are. The atoms do not combine in the mixture, but they combine when they form a compound. The properties of a mixture are the sum of the properties of its components, but a compound has properties peculiar to itself, and they are often very different than the properties of the elements it contains. For example, iron and sulphur act like iron and sulphur as part of a mixture, but iron sulphide has distinct properties from both iron and sulphur.

These differences can be highlighted using by comparing the compound iron sulphide to the mixture of iron and sulphur. If you want to create the compound in science class, fill a test tube with equal amounts of powdered iron and powdered sulphur and heat it over a flame. When the mixture becomes a compound, it turns black.

Each substance in a mixture is easily separated from the mixture because they are not combined (i.e., joined as a result of a chemical reaction), but separating a compound requires a chemical reaction. For example, when iron and sulphur in powder form are mixed together, you can separate the iron from the mixture using a magnet because iron is attracted to a magnet and sulphur is not. However, holding a magnet to iron sulphide will not separate the iron, and other separation methods like distillation and filtration will not work either.