2.1Atoms, Elements and Compounds

Elements, compounds and mixtures — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

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📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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Lesson summary

2.1 Elements, compounds and mixtures — Notes — IGCSE Chemistry notes: Distinguish elements, compounds and mixtures and describe how mixtures are separated by physical methods.Element = a substance made of atoms that all contain the same number of protons, which cannot be split in…

Molecules of one element
Composition
Other methods
Filtration
Evaporation or crystallisation
Simple distillation

Part 1 of 7Elements

Element

Element a substance made of atoms that all contain the same number of protons, which cannot be split into anything simpler by chemical means.

About 118 elements are in the Periodic Table.

Examples — iron, carbon, oxygen.

Molecules of one element

O₂ is an element because every molecule contains only oxygen atoms.

Part 2 of 7Compounds

1Compound

Compound a pure substance of two or more elements chemically combined.

An unlimited number of compounds exist.

The elements are always in a fixed ratio, so water is always H₂O.

The properties differ from those of the elements that made it.

It cannot be separated into its elements by physical means, only by a chemical reaction.

Examples — copper(II) sulfate (CuSO₄), calcium carbonate (CaCO₃), carbon dioxide (CO₂), water.

7Formula

Formula symbols showing which atoms are in a compound and how many of each, such as H₂O.

Sodium is a reactive metal and chlorine is a poisonous gas, but sodium chloride is a safe ionic solid.

Part 3 of 7Mixtures

Mixture

Mixture two or more substances (elements and/or compounds) that are not chemically combined.

The parts keep their own properties, and can be in any proportions.

Mixtures can be separated by physical methods such as filtration or evaporation.

Examples — sand and water, oil and water, sulfur powder and iron filings, and air (a mixture of gases).

Composition

Composition it can vary, unlike a compound.

Part 4 of 7Comparing elements, compounds and mixtures

Feature	Element	Compound	Mixture
Made of	One type of atom	Two or more elements	Two or more substances
Chemically joined	Not applicable	Yes	No
Proportions	Not applicable	Fixed	Any amounts
Properties	Its own	Different from its elements	Each part keeps its own
Separated by	Cannot be split	A chemical reaction	Physical methods
Example	Iron, oxygen	Water, iron sulfide	Air, salty water

Joining iron and sulfur by heating makes the compound iron sulfide, whereas iron filings mixed with sulfur form a mixture that can be separated.

Part 5 of 7Separating a mixture: salt from rock salt

Crush the rock salt.

Stir it into warm water so the salt dissolves.

Filter

Filter sand stays on the filter paper and salt solution passes through.

Heat the salt solution in an evaporating basin.

Salt crystals are left as the water evaporates.

Other methods

Other methods a magnet separates iron filings, filtration separates sand from water and distillation recovers water from salty water.

Part 6 of 7Physical separation methods

1Filtration

Filtration separates an insoluble solid, such as sand from water.

2Evaporation or crystallisation

Evaporation or crystallisation obtains a dissolved solid, such as salt, from a solution.

3Simple distillation

Simple distillation obtains the solvent, such as water, from a solution.

4Chromatography

Chromatography separates soluble substances such as dyes.

Part 7 of 7Reading particle diagrams

Reading particle diagrams

Element

Element every particle contains the same type of atom, either single atoms or pairs of identical atoms.

Compound

Compound every particle contains different atoms chemically bonded in the same ratio.

Mixture

Mixture more than one type of substance is present, and it may contain elements, compounds or both.

Key terms

Term	Meaning
Element	a substance made of only one type of atom.
Compound	two or more elements chemically combined in a fixed ratio.
Mixture	two or more substances mixed but not chemically combined.
Filtration	separating an insoluble solid from a liquid using filter paper.
Distillation	separating a liquid from a solution by boiling and condensing the vapour.
Evaporation	heating a solution to leave the solid behind.
Crystallisation	obtaining a dissolved solid from a solution by evaporating some of the solvent and cooling.

Avoid these mistakes

Air is a compound — air is a mixture of gases, water is a compound and oxygen is an element.

A mixture is chemically joined — its parts are not joined and keep their own properties.

Compounds can be separated by filtering — they need a chemical reaction.

A molecule always means a compound — O₂ is a molecule of an element

A mixture has a fixed composition — its composition can vary

Last updated: 18 October 2026