Mixtures

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

For example, powdered iron and powdered sulfur mixed together makes a mixture of iron and sulfur. 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 four piles of different colours - red, green, yellow and purple.

There are times when the purity of a substance is very important. For example, a medicine must not contain any harmful chemicals.

Mixtures and compounds

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

MixtureCompound
CompositionVariable composition – you can vary the amount of each substance in a mixtureDefinite composition – you cannot vary the amount of each element in a compound
Joined or notThe different substances are not chemically joined togetherThe different elements are chemically joined together
PropertiesEach substance in the mixture keeps its own propertiesThe compound has properties which are different from the elements it contains
SeparationEach substance is easily separated from the mixtureIt can only be separated into its elements using chemical reactions
ExamplesAir, sea water, most rocksWater, carbon dioxide, magnesium oxide, sodium chloride

An example - iron, sulfur and iron sulfide

Iron and sulfur react together when they are heated to make a compound called iron sulfide. What are the differences between a mixture of iron and sulfur, and iron sulfide? Here are some of them:

  • the mixture can contain more or less iron, but iron sulfide always contains equal amounts of iron and sulfur
  • the iron and sulfur atoms are not joined together in the mixture, but they are joined together in iron sulfide
  • the iron and sulfur still behave like iron and sulfur in the mixture, but iron sulfide has different properties from both iron and sulfur
  • you can separate the iron from the mixture using a magnet, but this does not work for iron sulfide