Separation Techniques

Filtration

Separates an insoluble solid from a liquid.

Pour the mixture through filter paper in a funnel:

• The solid stays on the paper as the residue.
• The liquid passes through as the filtrate.

Example: sand from water.

Crystallisation

Recovers a soluble solid from a solution.

1. Gently heat the solution in an evaporating basin until concentrated.
2. Leave to cool so crystals form.
3. Filter and dry.

Example: salt from salt water.

Simple Distillation

Separates a solvent from a dissolved solute (for example pure water from salt water).

• Heat the solution – the solvent evaporates.
• Vapour condenses in the condenser and is collected.
• Only works when boiling points are very different.

Example: pure water from salt water.

Fractional Distillation

Separates two or more miscible liquids with different boiling points (for example ethanol from water).

• A fractionating column packed with glass beads gives a temperature gradient.
• The liquid with the lowest boiling point reaches the top first and is collected.

Example: ethanol from water.

Paper Chromatography

Separates dissolved substances by how they distribute between the stationary phase (paper) and the mobile phase (solvent).

1. Draw a pencil line on the paper.
2. Spot the sample on the line.
3. Stand the paper in solvent (below the line so spots are not washed away).

Pure substances give one spot in every solvent.

$R_f=\frac{\text{distance moved by spot}}{\text{distance moved by solvent}}$

Example: $\frac{3.0\text{ cm}}{7.5\text{ cm}}=0.40$. $R_f$ is always between 0 and 1.