Element, Compound or Mixture? KS3 Chemistry Explained

An element contains only one type of atom; a compound contains two or more elements chemically joined in fixed proportions; a mixture contains two or more substances not chemically joined, so they keep their own properties and can be separated by physical methods. Knowing how to tell these three apart is a core KS3 chemistry skill tested in Year 7, Year 8, and beyond.

What is an element?

An element is a pure substance made of only one type of atom. It cannot be broken down into a simpler substance by chemical reactions. There are 118 known elements, all listed in the periodic table.

Examples:

Oxygen (O) — the gas in the air we breathe for respiration

Iron (Fe) — used to make steel

Carbon (C) — found as graphite (in pencils) and diamond

Hydrogen (H) — the most abundant element in the universe

Copper (Cu) — used in electrical wiring

Some elements exist as molecules of two atoms joined together rather than as single atoms. These are called diatomic molecules: hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂). Even so, they are still elements because they contain only one type of atom.

What is a compound?

A compound is a pure substance formed when two or more different elements are chemically combined in fixed proportions. The atoms are held together by chemical bonds. The properties of a compound are usually very different from the properties of its constituent elements.

Examples:

Water (H₂O) — two hydrogen atoms chemically bonded to one oxygen atom. Hydrogen is a flammable gas; oxygen supports combustion. Water, formed when they react together, puts fires out.

Salt (sodium chloride, NaCl) — sodium is a reactive metal that reacts violently with water; chlorine is a toxic green gas. Together they form white table salt that we eat safely.

Carbon dioxide (CO₂) — one carbon atom bonded to two oxygen atoms.

Iron sulfide (FeS) — iron and sulfur chemically combined.

The key distinction: in a compound, the elements are chemically joined and cannot be separated by physical means. You need a chemical reaction to separate them.

What is a mixture?

A mixture contains two or more substances (elements or compounds) that are not chemically bonded. The substances in a mixture:

Keep their own chemical properties

Are present in variable proportions (unlike a compound, which always has the same ratio of atoms)

Can be separated by physical methods

Examples:

Air — a mixture of nitrogen (~78%), oxygen (~21%), argon (~1%), carbon dioxide, and other gases

Seawater — water with dissolved salts, gases, and other substances

Sand and water — a mixture easily separated by filtration

Iron filings and sulfur powder — can be separated with a magnet

A famous KS3 demonstration compares iron filings and sulfur as a mixture versus iron sulfide (FeS) as a compound. In the mixture, the iron and sulfur keep their own properties — you can separate them with a magnet because iron is magnetic, but sulfur is not. When you heat the mixture strongly, the iron and sulfur react chemically to form iron sulfide, a grey-black compound. You can no longer use a magnet to separate them because the iron has been chemically transformed.

A worked example: iron and sulfur

The mixture (iron filings + sulfur powder):

Iron filings are grey and magnetic.

Sulfur powder is yellow and not magnetic.

Mix them together in a test tube — you can see both colours.

Hold a magnet to the outside of the test tube — the iron filings move towards the magnet and the sulfur does not. The substances can be separated physically.

The compound (iron sulfide, FeS):

Heat the mixture strongly until it glows. A chemical reaction occurs and the two elements combine.

The product is iron sulfide — a dark grey-black solid.

A magnet no longer attracts it because the iron atoms are now chemically bonded to sulfur atoms and the magnetic property is lost.

The properties of iron sulfide are completely different from those of iron or sulfur individually.

This single demonstration illustrates all three concepts: you start with elements (iron, sulfur), create a mixture by combining them physically, and then form a compound by heating them.

How to separate mixtures

Because the substances in a mixture keep their own properties and are not chemically bonded, they can be separated by physical methods:

Method | What it separates | Example
Filtration | Insoluble solid from a liquid | Sand from water
Evaporation | Dissolved solid from a liquid | Salt from salt water
Distillation | Liquids with different boiling points | Ethanol from water
Chromatography | Different coloured dyes | Ink colours in a felt-tip pen
Magnetism | Magnetic material from non-magnetic | Iron filings from sand
Crystallisation | Dissolved solid from a solution | Large salt crystals from solution

Key rule: all these methods are physical — they do not involve chemical reactions, and no new substances are formed.

How to tell an element, compound or mixture apart

Use these three questions in sequence:

Does it contain only one type of atom? If yes → element. If no → go to question 2.

Are the different types of atoms chemically bonded in fixed proportions? If yes → compound. If no → go to question 3.

Are different substances present but not chemically joined? If yes → mixture.

Quick classification exercise:

Substance | Classification | Reason
Nitrogen gas (N₂) | Element | Only nitrogen atoms
Pure water (H₂O) | Compound | Hydrogen and oxygen bonded in fixed ratio 2:1
Air | Mixture | Several gases, variable proportions, not chemically bonded
Copper sulfate (CuSO₄) | Compound | Copper, sulfur, oxygen chemically bonded
Bronze (copper + tin) | Mixture | Metals mixed but not chemically bonded (an alloy)

What does the national curriculum say?

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to understand "the differences between atoms, elements and compounds" and "the properties, synthesis and separation of mixtures." BBC Bitesize KS3 chemistry covers elements, compounds, and mixtures directly and includes the iron + sulfur demonstration as a core example.

Frequently asked questions

What is the difference between a compound and a mixture?

In a compound, elements are chemically bonded together in fixed proportions — the compound has different properties from its elements and cannot be separated by physical means. In a mixture, substances are present together but not chemically joined — they keep their own properties and can be separated physically.

Is water a compound or a mixture?

Pure water (H₂O) is a compound. It consists of hydrogen and oxygen chemically bonded in a fixed 2:1 ratio. Its properties are completely different from those of hydrogen gas or oxygen gas. You cannot separate it into hydrogen and oxygen by filtering or evaporating — you need a chemical process such as electrolysis.

How do you separate iron filings from sulfur powder?

Use a magnet. Iron is magnetic; sulfur is not. Passing a magnet through the mixture attracts the iron filings and leaves the sulfur behind. This works because iron and sulfur are only mixed, not chemically combined — each substance keeps its own properties in a mixture.

Can a mixture contain compounds?

Yes. A mixture can contain elements, compounds, or both, as long as the substances are not chemically bonded to each other. Seawater, for example, is a mixture containing the compound water (H₂O) along with dissolved salts such as sodium chloride (NaCl).

Why do elements in a compound behave differently from the pure elements?

When elements form a compound, their atoms bond chemically, which changes the electronic structure and therefore the properties. Sodium is a soft, silvery metal that reacts explosively with water. Chlorine is a toxic yellow-green gas. When they combine to form sodium chloride (table salt, NaCl), the resulting compound is a safe, white, edible crystalline solid with entirely different properties.

KS3 chemistry tutoring with Professor Curie is available at aitutors.me.