Understand Some Differences Between Pure and Impure Substances

Have you heard the word 'pure' before?

In chemistry, we refer to a pure substance as being a substance made from only one element or one compound. And nothing else.

The opposite of a pure substance is an impure substance. An impure substance is one made from more than one element or one compound, meaning it is a mixture.

A substance being impure doesn't mean it's necessarily bad. Water taken straight from a mountain stream would be classed as impure as it would contain more than just water molecules - it would also probably contain some impurities such as salt, dissolved gases and microorganisms. 

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

If you added some impurities to this water, say in the form of some salt or sugar, it would change the melting and boiling point, showing us that the water is now impure.

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