Compounds & Mixtures

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• Chemistry
• Lower Secondary (Key Stage 3)

Compounds & Mixtures

Open with the big idea that chemists classify substances by what particles are present and how they are combined.

Learning goal

I can explain the difference between elements, compounds and mixtures.

What words come to mind when you hear elements, compounds and mixtures?

Use this to surface prior ideas and misconceptions such as 'mixed', 'bonded', 'same atoms' or everyday examples like air and water.

Why classification matters

Chemists group substances by their particles.

• An element contains one type of atom only

• A compound contains atoms of different elements chemically joined

• A mixture contains substances together, but not chemically joined

Link this to the sorting activity students may do with substance cards: they are not sorting by appearance alone, but by particle-level structure.

Elements

An element is a pure substance made from only one type of atom.

• All particles contain the same atom

• Some elements exist as single atoms

• Others exist as molecules of the same element, such as oxygen, O₂

• They cannot be broken down into simpler substances by chemical means

Students often think an element must be a single atom. Emphasise that oxygen gas is still an element even though it is made of O₂ molecules.

Examples of elements

Examples include:

• Gold (Au)

• Copper (Cu)

• Oxygen (O₂)

• Carbon (C)

Each example contains only one kind of atom.

Use familiar examples and keep returning to the test: one type of atom only.

Compounds

A compound is a pure substance made when atoms of different elements are chemically joined.

• The atoms are bonded in fixed proportions

• A compound has its own properties

• It is not the same as the elements that formed it

• It can only be separated by chemical reactions, not simple physical methods

Stress 'chemically joined' and 'fixed proportions' as the key features that separate compounds from mixtures.

Examples of compounds

Common compounds include:

• Water (H₂O)

• Carbon dioxide (CO₂)

• Sodium chloride (NaCl)

Each particle contains more than one element joined together.

Avoid going deeply into bonding types here; the main goal is recognising that different atoms are joined in each compound.

Which substance is an element?

A

Water

B

Oxygen

C

Salt water

D

Carbon dioxide

Checks the key idea from slides 2 to 6 that an element contains only one type of atom. Oxygen is a useful distractor-resister because students may wrongly think O₂ cannot be an element.

Pure substances and compounds

An element is a pure substance made from only one type of atom. A compound is also a pure substance, but it contains atoms of different elements chemically joined. Compounds can be separated only by chemical reactions, not simple physical methods.

pure substance

chemically joined

chemical reactions

mixture

physically joined

physical methods

same atoms

Echoes the wording of slides 3 and 5 so students secure the contrast between element and compound before moving on to mixtures.

Mixtures

A mixture contains two or more substances together, but the substances are not chemically joined.

• The substances can be elements, compounds, or both

• The amounts can vary

• Each substance keeps its own properties

• The substances can often be separated by physical methods

Examples the teacher can mention aloud: air, salt water, and a bowl of sand and iron filings.

Examples of mixtures

Examples include:

• Air - a mixture of gases

• Salt water - salt mixed with water

• Fruit salad - different substances together

In a mixture, the particles are close together but not chemically bonded to each other.

Use simple everyday examples first, then connect back to particle diagrams so students do not rely only on what they can see.

Compounds and mixtures compared

Both contain more than one element, but they are different.

• Compound: elements are chemically joined

• Mixture: substances are not chemically joined

• Compound: fixed ratio of elements

• Mixture: variable amounts of substances

• Compound: separated chemically

• Mixture: separated physically

This is the key contrast slide. Students often confuse 'contains different elements' with 'is a compound'.

Which feature shows a substance is a mixture?

A

Fixed ratio

B

One atom type

C

Not bonded

D

Pure substance

Targets the main distinction in slides 7 to 9: substances in a mixture are together but not chemically joined.

Why is salt water a mixture, not a compound?

Expected elements: a compound has substances chemically joined in fixed proportions and needs chemical separation; a mixture is not chemically joined, can vary in amount, and can often be separated physically.

Reading particle diagrams

Particle diagrams help show the difference.

• Element: all particles are the same type of atom

• Compound: each particle contains different atoms joined together

• Mixture: more than one type of particle is present together

• Look for whether particles are bonded and whether all particles are the same

Use the provided diagrams here. Encourage students to classify by asking two questions: Are the particles all the same? Are different atoms bonded together?

A simple way to classify

You can sort a substance by following this thinking:

One type of particle only → pure substance

Different types of particles present → mixture

One atom type only in each particle → element

Different atom types joined in each particle → compound

This mirrors the classification worksheet and substance-card task. Read the chain slowly and apply it to one or two examples aloud.

A particle diagram shows two different unbonded particles. What is it?

A

Element

B

Compound

C

Mixture

D

Molecule

Uses the decision rules from slides 10 and 11. 'Molecule' is a plausible distractor because students may focus on particles rather than classification.

Classifying with particle diagrams

Particle diagrams help us classify substances. If all particles are the same, the substance is pure. If different types of particles are present together, it is a mixture. If each particle has different atoms joined together, it is a compound.

same

different

joined

separated

single

variable

physical

Reinforces the two diagnostic questions from slide 10: are all particles the same, and are different atoms joined together?

What is one thing you learned that you will remember?

Which idea helped you classify substances best?

Glossary

atom - the tiny particle that makes up an element

bonded - chemically joined together

compound - a pure substance made from different elements chemically joined

element - a pure substance containing only one type of atom

mixture - two or more substances together without chemical bonding

particle diagram - a model showing how particles are arranged

pure substance - a substance made of only one kind of particle