Elements, Compounds and Mixtures
The most basic classification in Chemistry — and one that questions return to constantly, because getting it wrong makes bonding, formulae and separation impossible.
1. The three definitions
An ELEMENT is a substance made of ONE TYPE OF ATOM ONLY, which cannot be broken down into simpler substances by chemical means.
A COMPOUND is a substance in which TWO OR MORE ELEMENTS are CHEMICALLY BONDED together in a FIXED RATIO.
A MIXTURE contains two or more substances NOT chemically bonded, which can be present in any proportion.
Examples:
| Substance | Type |
|---|---|
| Oxygen (O₂), copper, carbon | element |
| Water (H₂O), carbon dioxide, sodium chloride | compound |
| Air, sea water, crude oil, brass, sand and salt | mixture |
An element can still be a molecule. O₂ and Cl₂ are elements — they contain only one type of atom. A substance is only a compound if it contains different elements bonded together.
Graphite and diamond are ELEMENTS — both are pure carbon. Calling graphite a compound was a recorded error.
2. Compound vs mixture — the key comparison
This is the most examined distinction on the page.
| Compound | Mixture | |
|---|---|---|
| Bonded? | YES — chemically bonded | NO |
| Proportion | FIXED ratio | any proportion |
| Separation | only by a CHEMICAL reaction | by PHYSICAL means (filtration, distillation) |
| Properties | totally DIFFERENT from its elements | keeps the properties of its components |
| Energy change on forming | yes — heat given out or taken in | no significant change |
| Melting point | sharp and fixed | melts over a range |
The classic example — iron and sulfur:
A MIXTURE of iron filings and sulfur powder: the iron is still magnetic and can be separated with a magnet; the two can be mixed in any ratio. Heating them forms IRON SULFIDE, a COMPOUND: it is not magnetic, it cannot be separated physically, and it has a fixed formula, FeS.
A compound has completely different properties from the elements that made it. Sodium is a violently reactive metal and chlorine is a poisonous green gas — but sodium chloride is table salt. This is the strongest illustration of the difference, and it is worth quoting.
3. Pure substances
A PURE substance contains only ONE substance — a single element or a single compound.
A pure substance has a SHARP, FIXED melting and boiling point. An impure substance (a mixture) melts and boils over a RANGE, with a lower melting point and a higher boiling point.
This is why melting point is used as a purity test — and why chromatography giving one spot indicates purity.
Note that in everyday language “pure orange juice” means no additives; in chemistry, orange juice is a mixture and therefore not pure.
4. Recognising the type from a formula or diagram
From a formula:
One symbol only (with or without a subscript) → element — O₂, Fe, S₈ Two or more different symbols joined → compound — H₂O, NaCl, CuSO₄ Substances separated by commas or shown side by side → mixture
From a particle diagram:
All circles the same → element Different circles JOINED together, all identical groups → compound Different particles NOT joined, or a variety of groups → mixture
Look at whether the different atoms are BONDED. Two different atoms drawn side by side but not joined is a mixture; the same two joined is a compound.
5. Why it matters
- Compounds have formulae; mixtures do not
- Mixtures can be separated physically — which is the whole basis of filtration, distillation and chromatography
- Air and crude oil are mixtures, which is why fractional distillation works on them
- Alloys are mixtures, which is why their composition can be varied to change properties
6. Mistakes that cost marks
Calling O₂ or S₈ a compound — they are elements.
Calling graphite or diamond a compound.
Saying a mixture has a fixed ratio.
Saying a compound can be separated physically.
Saying a compound keeps its elements’ properties.
Saying a mixture has a sharp melting point.
Not mentioning “chemically bonded” in the compound definition.
Omitting “fixed ratio”.
Frequently asked questions
What is an element? A substance made of one type of atom only.
Is O₂ an element or a compound? An element — it contains only oxygen atoms.
What is a compound? Two or more elements chemically bonded in a fixed ratio.
What is a mixture? Two or more substances not chemically bonded, in any proportion.
How do I separate a mixture? By physical means — filtration, distillation, chromatography.
How do I separate a compound? Only by a chemical reaction, such as electrolysis.
Is air a compound or a mixture? A mixture.
Do compounds have the properties of their elements? No — they are completely different. Sodium + chlorine gives table salt.
How can I tell a pure substance? It has a sharp, fixed melting point.
What happens to the melting point of an impure substance? It is lowered and occurs over a range.
Quick revision checklist
- I can define element, compound and mixture precisely
- I know a compound needs chemically bonded and fixed ratio
- I know elements can be molecules (O₂, S₈)
- I can classify common substances
- I can compare compounds and mixtures across all six rows
- I can explain the iron and sulfur example
- I know compounds have different properties from their elements
- I know pure substances have sharp melting points
- I can classify from a formula or a particle diagram
- I check whether different atoms are bonded
These notes cover elements, compounds and mixtures in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
Finished this topic?
Saved on this device — no account needed.