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Everything around us - animal, vegetable and mineral - is made up of chemicals. Some are simple substances such as the water molecule, which is made up of hydrogen and oxygen atoms, and others are very complicated compounds made up of many different chemical elements. As well as naturally occurring chemical compounds, scientists have created millions more in the past 100 years that do not exist in nature. Chemicals come as powders, pellets, dusts, liquids, vapours, gases, etc.

Chemists tend to divide the world into inorganic and organic compounds, largely depending on whether they contain the element carbon. Carbon-containing compounds were originally described as organic because many of them came from nature. Since then, chemists have synthesised many organic compounds, many of which are important in industrial products and processes. The organic solvents and organophosphate pesticides involved in the case studies (see later) are just a few examples of these organic compounds. Inorganic materials, comprising metals, minerals and salts, among other types, are much less likely to contain carbon.

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