Oils are liquids that do not dissolve in water and burn easily.
Oils are usually made from long chains of carbon and hydrogen atoms.
There are three main kinds of oil: essential, fixed and mineral oils.
Essential oils are thin, perfumed oils from plants. They are used in flavoring and aromatherapy.
Fixed oils are made by plants and animals from fatty acids. They include fish oils and nut and seed oils.
Mineral oils come from petroleum formed underground over millions of years from the remains of micro-organisms.
Petroleum, or crude oil, is made mainly of hydrocarbons. These are compounds made only of hydrogen and carbon, such as methane (see oil compounds).
Hydrocarbons in petroleum are mixed with oxygen, sulphur, nitrogen and other elements.
Petroleum is separated by distillation into various substances such as aviation fuel, petrol or gasoline and paraffin. As oil is heated in a distillation column, a mixture of gases evaporates. Each gas cools and condenses at different heights to a liquid, or fraction, which is then drawn off.
Hydrocarbons are compounds made only of carbon and hydrogen atoms. Most oil products are hydrocarbons.
The simplest hydrocarbon is methane, the main gas in natural gas (and flatulence from cows!). Methane molecules are one carbon atom and four hydrogen atoms.
Alkanes or paraffins are a family of hydrocarbons in which the number of hydrogen atoms is two more than twice the number of carbon atoms.
Lighter alkanes are gases such as methane and propane which make good fuels.
Candles contain a mixture of alkanes.
Alkenes or olefins are a family of hydrocarbons in which there are twice as many hydrogen atoms as carbon atoms.
The simplest alkene is ethene, also called ethylene (C2 1-1 4) which is used to make polythene and other plastics such as PVC.
Green bananas and tomatoes are often ripened rapidly in ripening rooms filled with ethene.
Ethene is the basis of many paint strippers.
Ethene can be used to make ethanol, the alcohol in alcoholic drinks.