Elements, Compounds and Mixtures

An element contains only one type of atom. A compound contains two or more elements chemically bonded in a fixed ratio, with properties different from its elements. A mixture contains two or more substances not chemically joined; they can be separated by physical methods. A pure substance melts and boils at a sharp, fixed temperature; impurities make it melt over a range and at a lower temperature.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 25 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Define element, compound and mixture.
Classify substances from formulae and particle diagrams.
Use melting and boiling points to judge purity.

Pause and think

Sodium is a soft, explosive metal and chlorine is a poisonous green gas. Why can you safely eat sodium chloride?

Section 2 of 14

Core idea

Start hereDangerous ingredients, safe meal

Sodium explodes in water and chlorine was used as a poison gas in World War I. Yet you sprinkle sodium chloride on your food. Joining elements chemically creates a brand-new substance.

The big idea

Elements contain one type of atom. Compounds are elements chemically bonded in a fixed ratio, with new properties. Mixtures are substances that are not bonded and can be separated physically. Pure substances melt and boil at sharp, fixed temperatures.

1. Telling them apart

Classifying matter

Drawing diagram…

Ask: one type of atom? chemically bonded?

Worked exampleIron and sulfur

Iron filings and sulfur powder are mixed, then the mixture is heated strongly.

Goal: Compare the substance before and after heating.
Strategy: Check whether a chemical bond forms.

$\text{before: mixture}$
Iron can be pulled out with a magnet; sulfur stays yellow.
$\mathrm{Fe}+\mathrm{S}\rightarrow\mathrm{FeS}$
Heating makes them react.
$\text{after: compound}$
Iron(II) sulfide is not magnetic and has new properties.

AnswerA mixture becomes a compound.

✓ CheckA magnet no longer separates the black product ✓.

2. Purity

In chemistry, pure means a single substance with nothing else mixed in. A pure substance melts at one sharp temperature. An impure one melts lower and over a range, and boils higher than the pure substance.

Common misconceptionMolecules of an element

\mathrm{O_2}

\mathrm{N_2}

and

\mathrm{Cl_2}

contain two atoms but only one type of atom, so they are elements, not compounds.

Edge case"Pure" orange juice Advertisers call juice "pure" when nothing has been added. A chemist would call it a mixture of water, sugars, acids and much more.

Pause and think

Why does salted water take longer to boil when cooking pasta?

Summary

Element: one type of atom.
Compound: fixed ratio, chemically bonded.
Mixture: physically separable.
Impurities lower the melting point and raise the boiling point.

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 not chemically combined.

HistoryLavoisier. In 1789 Antoine Lavoisier published the first modern list of elements, defining an element as a substance that can't be broken down further. He was executed in the French Revolution five years later.

Fun factSalt on icy roads. Spreading salt on icy roads works because an impurity lowers the melting point of ice, so it melts at temperatures below

0\,^\circ\mathrm{C}

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