Difference Between a Mixture and a Pure Substance

A mixture is made up of two or more substances that are not chemically combined.

Filtration

Filtration is used to separate insoluble solid particles from a liquid. 
For example, sand can be separated from a mixture of sand and water by filtration. 

Evaporation to Dryness

Evaporation to dryness is used to obtain a soluble solid from a solution by heating the solution until all the water has boiled off. 

The apparatus in the figure below can be used to recover solid salt from a salt solution by evaporation to dryness. 

Evaporation to Dryness (Video)

The video below shows how evaporation to dryness is carried out in the laboratory.

Crystallisation

Crystallisation is used for obtaining a pure solid sample from its solution.

Many substances decompose (break down to form simpler substances) when they are heated strongly. Most crystals, such as copper (II) sulfate crystals, give off water to become powders when heated. For such substances, evaporation to dryness is not a good method of separation and purification. 

The figure below shows how pure copper (II) sulfate crystals are obtained by crystallisation.

Sublimation

Sublimation is used to separate a solid that sublimes from one that does not. 

Some common substances that undergo sublimation upon heating are:

- dry ice (solid carbon dioxide)
- iodine crystals
- naphthalene (mothballs)
- camphor
- *ammonium chloride
Note: ammonium chloride does not actually undergo sublimation upon heating. It decomposes and forms ammonia gas and hydrogen chloride gas.

The video below shows the sublimation of iodine.

Dissolving and Filtration

To separate a mixture of two solids, we use a solvent in which only one solid is soluble. A solvent is a liquid that dissolves a substance (which is the solute). Different solids dissolve in different solvents. Some commonly used solvents are water and ethanol. An example of separating solids by dissolving in a suitable solvent followed by filtration is shown below.

Using a Magnet

A magnet can be used to separate a magnetic substance from a non-magnetic substance. 

Examples of magnetic metals:
- iron
- nickel
- cobalt

The video below shows how a magnet can be used to separate iron from sand.

Simple Distillation

Simple distillation is used to separate a pure solvent (liquid) from a solution.

Distillation is the process of boiling a liquid and condensing the vapour. The figure below shows the apparatus used for simple distillation .

Procedures to Note

There are several procedures that have to be noted when setting up the distillation apparatus.

Fractional Distillation

Fractional distillation is used to separate a mixture of miscible liquids with different boiling points.

During fractional distillation,
- the liquid with the lowest boiling point distills over first
- the vapours of liquids with higher boiling point condense along the fractionating column and fall back into the round-bottomed flask

The figure below shows how ethanol can be separated from water via fractional distillation.

Separating Funnel

A separating funnel can be used to separate immiscible liquids.
The procedure below shows how a separating funnel is used to separate oil and water.

Paper Chromatography

Chromatography is the method of separating two or more components that dissolve in the same solvent.

For example, chromatography can be used to separate the dyes in food colouring. In the example below, paper chromatography will be used to separate the dyes in green food colouring. 

Interpreting the Result of Paper Chromatography

The chromatography paper with the separated components is called a chromatogram.

The resulting chromatogram of the experiment above shows that the green food colouring is a mixture of two dyes. On the other hand, a pure substance is made up of only one dye and gives only one spot on a chromatogram. 

Identical dyes produce spots at the same height and in the same colour on the chromatogram when the same solvent is used.

Chromatography of Colourless Substances

When running a chromatogram of colourless substances such as amino acids, a locating agent is sprayed on the chromatogram. This causes the colourless substances to show up as coloured spots. 

The video below summarises the key points of paper chromatography and explains the use of a locating agent (from 2:25).

Uses of Chromatography

- separate components in a sample
- identify components in a sample (e.g. banned substances in food)
- determine if a sample is pure