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Compounds and mixtures

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Mixtures

A mixture is made from different substances that are not chemically joined.

For example powdered iron and powdered sulphur mixed together makes a mixture of iron and sulphur. They can be separated from each other without a chemical reaction, in the way that different coloured sweets can be picked out from a mixed packet and put into separate piles.

A mixed pile of sweets is separated into 4 piles of different colours - red, green, yellow and purple

Mixture and compounds

Mixtures have different properties from compounds. The table summarises these differences.

  Mixture Compound

Composition

Variable composition – you can vary the amount of each substance in a mixture. Definite composition – you cannot vary the amount of each element in a compound.

Joined or not

The different substances are not chemically joined together. The different elements are chemically joined together.

Properties

Each substance in the mixture keeps its own properties. The compound has properties different from the elements it contains.

Separation

Each substance is easily separated from the mixture. It can only be separated into its elements using chemical reactions.

Examples

Air, sea water, most rocks. Water, carbon dioxide, magnesium oxide, sodium chloride.

An example - iron, sulphur and iron sulphide

Remember that iron and sulphur react together when they are heated to make a compound called iron sulphide. What are the differences between a mixture of iron and sulphur, and iron sulphide? Here are some of them:

  • The mixture can contain more or less iron, but iron sulphide always contains equal amounts of iron and sulphur.

  • The iron and sulphur atoms are not joined together in the mixture, but they are joined together in iron sulphide.

  • The iron and sulphur still behave like iron and sulphur in the mixture, but iron sulphide has different properties from both iron and sulphur.

  • You can separate the iron from the mixture using a magnet but this does not work for iron sulphide.

Activity

Compounds and mixtures activity

How will you react to this activity?

Play

Elemental

Can you conquer the elements?

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