Separating mixtures

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Making rock salt pure    Methods to separate a mixture (summary)    Making water pure (Distillation)

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..

The last little bit of salty water is filtered out

Summary of methods used to separate a mixture
PROCESS What it is used for Examples
Filtration Used for separating a solid or suspension from a liquid eg separating SAND from water.
Evaporation Used to obtain the solute from a solution eg for obtaining SALT from salty water
Simple distillation Used to obtain the solvent from a solution eg for obtaining pure water from sea water
Fractional distillation Used to separate one liquid from a mixture of different liquids that have different boiling points. eg i. obtaining ALCOHOL (bp78C) from wine.

ii. Oxygen or nitrogen is obtained from air (a mixture of different gasses) by the fractional distillation of liquid air.

Chromatography Used to separate out one colour from a mixture of colours eg for separating out the colours in black ink


Making water pure

Water can be purified by a process called DISTILLATION.

eg: If we wanted to make some PURE WATER from SEA WATER we would distill the sea water.

Apparatus for obtaining pure water from sea water.
This apparatus would be used by a pupil but would not produce very much pure water.

The liquid that condenses in the test tube is known as the distillate

The beaker of cold water helps keep the test tube cold so more steam condenses

This method to make pure water shown above is ok  but a more efficient method is shown below:

The water in the flask EVAPORATES and turns into steam.

Anything dissolved in the water cannot evaporate and so remains in the flask.

The steam CONDENSES in the delivery tube and turns back into water which collects in the test tube.


The Liebig condenser helps cool down the steam (notice which way the cooling water flows through the Liebig condenser)
The apparatus above can be improved by using a Liebig condenser.
This helps the steam cool down.
The flask also hold more water than the boiling tube

How to tell if a liquid is pure or contains a solute:

  • Place one or two drops of the filtrate on to a clean watch glass (or microscope slide)
  • Let the water evaporate and see if there is any residue.
  • If there is it means that some of the solid did dissolve.

Words to know:

Mixture: Something that can be SEPARATED into two (or more) different substances. eg air, sea water, earth, orange juice.

Pure: A material is pure if it contains only one chemical substance and so cannot be separated further Filtrate: The clear liquid that has been filtered.

Residue: The solid left behind after an experiment (eg the solid left in the filter paper).

Filtration: The process used to separate a solid (or suspension) from a liquid.

Suspension: Very fine particles of solid mixed with a liquid. If the solid is in suspension the water will often look cloudy. eg flour and water shaken together.

NOTE: in a suspension the solid has not dissolved but in a solution it has.

Soluble: Can dissolve

Insoluble: Cannot dissolve

Solvent: The liquid that is used to make a solution

Solute: A substance that has been dissolved in a solution.

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