Every substance you will meet in chemistry is one of three things: an element, a compound or a mixture. The difference is not what the atoms are, but whether they are chemically joined, and in what ratio.
An element is a substance made of only one type of atom. A compound is a substance made of two or more elements chemically combined in a fixed ratio, and it has new properties of its own. A mixture contains two or more substances that are not chemically combined: its composition can vary and its parts can be separated by physical methods such as a magnet, filtering or distillation.
Fig. 1: particle diagrams for an element, a compound and a mixture made from iron and sulfur
Section 01
One type of atom.
Look at Fig. 1 and ask two questions of any box: how many types of atom are there, and are the atoms joined to each other? Those two answers classify every substance.
Iron filings and yellow sulfur powder can be stirred together or heated together. The two results look similar in a test tube but are completely different substances.
The mixture: grey specks of iron and yellow specks of sulfur are both visible. A magnet pulls the iron out, so the mixture is separated by a physical method. Any ratio of iron to sulfur can be used.
The compound: heat the mixture and it glows as a reaction spreads through it: Fe + S → FeS. The black solid, iron(II) sulfide, is not attracted to a magnet and the iron cannot be picked out.
Fixed ratio: 56 g of iron always combines with 32 g of sulfur. Any extra iron or sulfur is simply left over.
New properties: sodium is a soft, very reactive metal and chlorine is a toxic green gas, yet the compound they form, sodium chloride, is the white solid we put on food.
Examiner trap
A molecule is not the same thing as a compound. Oxygen, O2, and nitrogen, N2, are made of molecules but contain only one type of atom, so they are elements. A compound needs at least two different elements, as in carbon dioxide, CO2.
Stage 1 · Learn
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Which is the best definition of an element?
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Which of these is a compound?
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Air is best described as:
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In a compound, the elements are:
Section 03
Everyday mixtures.
Most materials around you are mixtures. The test is always the same: are the parts chemically joined in a fixed ratio, or not?
Air: about 78% nitrogen, 21% oxygen, about 0.9% argon and small amounts of carbon dioxide and water vapour. It contains elements and compounds that are not joined, and its composition varies slightly from place to place.
Sea water: water with dissolved salts, mainly sodium chloride. Evaporating the water leaves the salt behind, a physical separation.
Alloys: brass is a mixture of copper and zinc; the proportions can be changed to give harder or softer brass.
Pure water, by contrast, is a single compound, H2O, with a fixed ratio of two hydrogen atoms to one oxygen atom.
Stage 2 · Exam
Exam-style questions
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A student stirs iron filings into powdered sulfur without heating. Which method separates the iron from this mixture?
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Which statement about a mixture is correct?
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Sea water is best classified as:
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Which pair shows an element followed by a compound?
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