Elements, Compounds and Mixtures Explained

By Thread Academy · 13 September 2026 · Chemistry

A fizzy drink lists carbonated water, sugar, flavourings, caffeine and acids. Which of those are elements, compounds or mixtures — and what is the drink itself? Students often blur these three ideas together, but the distinction is one of the most tested concepts in IGCSE chemistry. This article gives you crisp definitions, clear examples and a method for telling them apart in any question.

What is an element?

An element is a pure substance made of only one type of atom. You cannot break an element down into anything simpler by chemical means.

The periodic table lists every known element — around 118 of them. Some are familiar from everyday life:

Oxygen (O) — the gas we breathe.
Iron (Fe) — the metal in steel and in your blood.
Carbon (C) — found in charcoal, diamond and all living things.
Sodium (Na) — a soft, reactive metal.

A sample of pure iron contains only iron atoms. A sample of pure oxygen contains only oxygen atoms (joined in pairs as O2 molecules, but still only oxygen atoms). Nothing simpler hides inside.

What is a compound?

A compound is a pure substance made of two or more different elements chemically joined together. The atoms are bonded to each other, and a compound can only be split back into its elements by a chemical reaction — never by physical methods like filtering.

Everyday compounds include:

Water (H2O) — hydrogen and oxygen chemically joined.
Carbon dioxide (CO2) — carbon and oxygen chemically joined.
Sodium chloride (NaCl) — table salt, made of sodium and chlorine.
Calcium carbonate (CaCO3) — found in limestone and chalk.

Compounds have completely different properties from the elements that make them. Sodium is a dangerously reactive metal; chlorine is a poisonous green gas. Yet combined chemically, they form sodium chloride — ordinary table salt, which you eat every day. The compound is not a blend of its ingredients; it is something new.

Compounds always contain their elements in a fixed ratio. Water is always H2O — two hydrogen atoms for every oxygen atom. You cannot have water with a slightly different ratio; that would be a different substance entirely.

What is a mixture?

A mixture is made of two or more substances that are not chemically joined. The substances are simply jumbled together, each keeping its own properties. Mixtures can be separated by physical methods, and they can be combined in any proportions.

Examples are everywhere:

Air — a mixture of nitrogen, oxygen, carbon dioxide and other gases.
Sea water — a mixture of water and dissolved salts.
Sand and iron filings — a mixture of two solids.
Orange juice with pulp — a mixture of juice, pulp and water.

In a mixture of iron filings and sulfur powder, you can still see the separate yellow and grey grains. Each substance behaves exactly as it would on its own.

Compounds versus mixtures: the crucial difference

Examiners love this comparison, so learn it as a paired list of opposites.

How they form: A compound forms through a chemical reaction; a mixture forms by simply mixing substances together physically.
Separation: A compound can only be split by chemical methods (for example, electrolysis of water). A mixture can be separated by physical methods (for example, filtration or evaporation).
Proportions: A compound always has its elements in a fixed ratio. A mixture can have its components in any ratio.
Properties: A compound has new properties, different from its elements. In a mixture, each substance keeps its own properties.
Energy: Forming a compound involves an energy change (heat is given out or taken in); making a mixture does not.

A classic comparison: iron and sulfur

Heating iron filings and sulfur powder together produces iron sulfide, and the contrast between the before and after is the perfect illustration.

Before heating, it is a mixture: yellow and grey grains sit side by side, and a magnet pulls out the iron. Each substance keeps its own properties.

After strong heating, a chemical reaction forms iron sulfide, a dark grey solid. The magnet now picks up nothing, and no physical method will separate it. The sulfur's yellowness is gone too. A new substance with new properties has formed: a compound.

Separation techniques for mixtures

Because mixtures are only physically combined, we can separate them with physical techniques. Choose the method based on the properties of the substances involved.

Filtration — an insoluble solid from a liquid, such as sand from water.
Evaporation — a dissolved solid from a liquid, such as salt from sea water.
Crystallisation — slow evaporation to grow pure crystals, such as copper sulfate crystals.
Simple distillation — boiling a liquid and condensing the vapour, giving pure distilled water from sea water.
Fractional distillation — liquids with close boiling points, such as the fractions of crude oil, using a fractionating column.
Chromatography — dissolved substances that travel at different speeds, such as the dyes in a felt-tip ink.
Magnetism — magnetic from non-magnetic materials, such as iron filings from sand.
Separating funnel — immiscible liquids that do not mix, such as oil and water.

Worked example: separating sand, salt and water

Imagine you are given a mixture of sand, salt and water, and asked to recover each substance pure.

Step 1 — Filtration. Pour the mixture through filter paper. The sand is insoluble, so it stays on the paper. The salt solution (salt dissolved in water) passes through as the filtrate. You have recovered the sand.

Step 2 — Evaporation or crystallisation. Gently heat the salt solution. The water evaporates, leaving solid salt crystals behind. You have recovered the salt.

The water could be recovered too, by distillation instead of open evaporation: boil the solution and condense the steam.

Notice the logic: each step exploits one difference in properties — first solubility (filtration), then boiling point (evaporation).

Particle diagrams described in words

Exam questions often show boxes of circles and ask you to identify elements, compounds and mixtures. Here is how to read them.

An element — a box of identical single circles: every particle is the same unjoined atom, like pure iron.
A compound — a box of identical joined groups: every particle is the same molecule, but each molecule combines different atoms, like carbon monoxide.
A mixture — a box containing a jumble of different particles. More than one kind of particle means it is not a pure substance.

The decision rule is simple: more than one kind of particle in the box means a mixture. One kind of joined particle means a compound; one kind of single atom means an element.

Exam-style comparison questions

Practise explaining the difference out loud, because exam marks reward precise wording.

Question 1: Explain why air is a mixture and not a compound. (2 marks)

Model answer: Air's gases, such as nitrogen and oxygen, are not chemically joined. Each keeps its own properties, and the proportions can vary — whereas a compound has a fixed composition.

Question 2: Sodium is a reactive metal and chlorine is a poisonous gas, yet sodium chloride is safe to eat. Explain this observation. (2 marks)

Model answer: Sodium chloride is a compound — sodium and chlorine are chemically bonded into a new substance, and compounds have completely different properties from their elements.

Question 3: Describe how you would obtain pure water from a mixture of water, sand and salt. (3 marks)

Model answer: Filter out the insoluble sand, then distil the salt solution — boil it, condense the steam, and pure water collects while the salt stays behind.

Key takeaways

An element is a pure substance made of only one type of atom, and cannot be broken down chemically.
A compound is a pure substance of two or more elements chemically joined; it has a fixed ratio of elements and brand-new properties.
A mixture is two or more substances physically jumbled together, each keeping its own properties, in any proportions.
Compounds are separated only by chemical methods; mixtures are separated by physical methods such as filtration, evaporation, distillation, chromatography or magnetism.
Forming a compound involves a chemical reaction and an energy change; making a mixture does not.
To read a particle diagram: one kind of single atom means an element; one kind of joined particle means a compound; a jumble of different particles means a mixture.
In exams, always justify with the key ideas: chemically joined versus physically mixed, fixed ratio versus variable, new properties versus unchanged properties.