Elements, mixtures and compounds Revision Notes for AQA GCSE Chemistry

Elements, mixtures and compounds

All matter around us can be grouped into three main types: elements, compounds, and mixtures. Understanding the differences between these is essential for GCSE chemistry.

What are atoms?

Atoms are the tiny building blocks that make up everything around you. They are so small you cannot see them, even with a powerful microscope.

An atom is the smallest part of an element that can exist
All substances are made of atoms
You contain millions and millions of atoms in your body

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Think of atoms like tiny LEGO bricks - everything is built from these basic pieces.

Elements

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

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Key facts about elements:

There are about 100 different elements
Each element has its own chemical symbol
Chemical symbols always start with a capital letter
Examples: O for oxygen, N for nitrogen, Na for sodium

Oxygen is a good example of an element. It contains only oxygen atoms - nothing else.

All elements are shown in the periodic table, which organises them in a special way.

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Chemical symbols always start with a capital letter - this is a fundamental rule you must remember!

Compounds

Compounds form when atoms of different elements join together during a chemical reaction.

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Key facts about compounds:

Made from two or more different elements
The elements are chemically bonded together
They have fixed proportions shown in the chemical formula
You need a chemical reaction to separate the elements

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Worked Example: Water as a Compound

Water has the formula $H_2O$ . This means:

Each water molecule contains 2 hydrogen atoms and 1 oxygen atom
The hydrogen and oxygen are chemically bonded together
Every water molecule has exactly the same proportions

Mixtures

A mixture contains two or more elements or compounds that are not chemically bonded.

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Key facts about mixtures:

The components keep their original properties
No chemical reaction has occurred
You can separate mixtures using physical methods (like using a magnet)

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Worked Example: Iron and Sulphur Mixture

When iron powder and sulphur powder are mixed together, you get a mixture
The iron stays magnetic - a magnet will attract it
If you heat this mixture, iron sulphide compound forms
The new compound is not magnetic because its properties are different from the original elements

Telling substances apart

You can identify whether something is an element, compound, or mixture by looking at its properties:

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How to identify substances:

Elements: Contains only one type of atom
Compounds: Different elements chemically joined in fixed proportions
Mixtures: Components not chemically bonded, keep original properties

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Key Points to Remember:

Atoms are the smallest building blocks of matter
Elements contain only one type of atom and have chemical symbols
Compounds are different elements chemically bonded in fixed proportions
Mixtures contain substances that are not chemically bonded together
Chemical symbols always start with a capital letter (like O for oxygen, Na for sodium)

Elements, mixtures and compounds (AQA GCSE Chemistry): Revision Notes