Something is a MIXTURE if it contains two (or more) substances that are not chemically joined together.
The substances in a mixture can often be easily separated from one another.
A PURE substance only contains one material and so cannot be separated in any way (unless a chemical reaction takes place)
Different ways of separating mixtures
1 A magnet can be used to separate IRON from SAND.
How it works: The magnet sticks to the iron but not to the sand
3. Filtering (filtration) can be used to separate a solid (or suspension) from a liquid.
How it works: The liquid (and anything dissolved in the liquid) passes through holes in the filter paper but the solid particles are too big and get stuck.
Example: Filtration would be used to separate the dirt from some salty water.
4. Evaporation can be used to separate a dissolved SOLUTE from a SOLUTION
Example: Evaporation would be used to obtain some pure salt from salty water.
How it works: When salty water is warmed the water evaporates leaving behind crystals of salt.
Rock salt is a MIXTURE of several substances. 
In order to make pure salt it is necessary to remove the INSOLUBLE dirt from the SOLUBLE salt.
[Note: The method described below could be used to separate any two substances when one of them can dissolve in water and the other cannot]
1. CRUSH the rock salt using a mortar and pestle. This makes the salt dissolve more easily.
2. STIR the crushed salt into a beaker of warm water. The salt will DISSOLVE into the water but the dirt, which is insoluble in water, will not.
3. FILTER the mixture. The dirt will get caught in the filter paper as a residue and the clear SALTY WATER will drip through and form the filtrate.
4. Warm the filtrate in an evaporating basin. The water will EVAPORATE leaving behind pure salt crystals. The heat is stopped BEFORE all of the water has evaporated. This makes the salt more pure and helps prevent the evaporating basin from cracking.
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