Pure substances

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Key points

  • Most materials that we use are , and just a few are pure elements or pure compounds.
  • In chemistry, a pure substance is a single substance made of only one type of particle.
  • change the temperature at which a melts and boils.

Cartons of fruit juice often say they contain ‘pure’ juice. Does the word ‘pure’ have the same meaning for a scientist as it does in everyday life?

In everyday life, for example on food packaging, the word ‘pure’ usually means that something is in its natural state without anything added to it, like sweeteners or preservatives. Natural fruit juice is made up of sugar, water and many other naturally occurring chemicals. In chemistry this is not considered 'pure' and is a mixture.


Watch this video to find out about the difference between pure and impure substances.

Is the air that we breathe a pure or impure substance?

The air that we breathe is impure. It is a mixture of oxygen, nitrogen, carbon dioxide, carbon monoxide and many impurities such as methane.

Pure substances

substances are made from only one chemical or one .

For example, salt is a pure substance made only of sodium chloride.

A bowl of salt
  • Elements are listed on the periodic table
  • Elements are made from one type of
  • A compound is made from two or more elements bonded together

Can you name two examples of compounds?

Examples include pure water, which is H₂O, and carbon dioxide, CO₂.

Impure substances


A substance made from more than one element or one compound is impure, meaning it is a .

A label for a bottle of water will often include a list of small amounts of other . These are called impurities.

For example, it is challenging to make pure water. This mineral water contains small amounts of such as sodium and nitrate.

Mineral water is a mixture of water and other substances

Which of the following, if any, is a pure substance?

  • Sea water
  • Water in a mountain stream
  • Distilled water in the school science laboratory
  • None of the above

None of these examples is a chemically pure substance.

  • Sea water is not pure because it is a mixture of water, salt, and other dissolved substances.
  • The water in a mountain stream contains much less salt than sea water. It still contains some impurities, such as dissolved oxygen, carbon dioxide, minerals and microorganisms.
  • Distilled water contains very small amounts of impurities.

Particle diagrams

Particle diagrams are used to show how chemicals are arranged.


A particle diagram for an element will have only one type of atom.
For example, pure oxygen contains only oxygen atoms. Oxygen is shown here as a red circle.

Pure element - oxygen


  • A particle diagram for a compound will have two or more types of atom bonded together in a fixed ratio.
  • The compound of carbon dioxide is made up of two types of atom, shown here as one red and one black circle. Carbon is shown as black and oxygen is red.
  • The compound has a fixed ratio of one black atom joined to two red atoms.
Pure compound - carbon dioxide


A particle diagram for a mixture has more than one type of atom which are not chemically bonded together, or more than one type of molecule.

For example, this particle diagram shows helium in white and oxygen in red.

The two red atoms are joined together but not to the white atom, so this is a mixture of two elements.

A mixture of two elements - helium and oxygen
A mixture of two compounds - water mixed with alcohol
A mixture of elements and compounds - air

Fixed boiling points

A pure substance has a fixed and , which means they will always melt or boil at exactly the same temperature. For example, the melting point of pure water is 0 °C and its boiling point is 100 °C.

Impurities change the melting and boiling point

When a substance contains impurities, its melting and boiling points change. When salt is added to water, the mixture freezes below 0 °C, and boils at a temperature over 100°C. However, It is possible to find out if a substance is pure or not by measuring its melting or boiling point.

Working scientifically

Using a thermometer

Here are some top tips for using a thermometer to measure the temperature of a substance.

Top tips

  1. Make sure that the bulb of the thermometer is covered by the liquid but not touching the bottom of the container.
  2. Give the thermometer enough time to reach the same temperature as the liquid you’ve dipped it into. Stir the liquid with the thermometer to make sure all the liquid has the same temperature.
  3. Watch the coloured liquid in the thermometer move. When it stops moving, it's ready to read.
  4. Hold the thermometer in the liquid and get down to read the temperature at .

Using a thermometer is a useful measuring tool for collecting data.

Find out more about collecting data in science.

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