Separating mixtures

This lesson covers: 

  1. Filtration and evaporation
  2. Chromatography
  3. Distillation
  4. Using melting and boiling points to check purity

Filtration and evaporation

The sand and the salt in the mixture rock salt have different solubilities

If we've got a mixture of sand and salt, we can use filtration and evaporation to separate them.

Separating rock salt - step 1

  1. First grind up the rock salt into smaller grains. This increases the surface area.

Separating rock salt - step 2

Illustration of stirring rock salt in water to dissolve the salt.
  1. Next, add the rock salt to water and stir. The salt is soluble and dissolves readily in water, but sand is insoluble and does not.

Separating rock salt - step 3

Diagram showing the filtration process with a funnel, filter paper, insoluble sand particles, and salt solution.
  1. Then pour the salt solution through a filter, like filter paper. This removes large insoluble sand particles, leaving just the dissolved salt in the water.

Separating rock salt - step 4

Diagram showing the evaporation process with an evaporating basin containing salt crystals being heated.
  1. Finally, gently heat the solution until all the water evaporates away. This leaves behind pure salt crystals, completely separated from the sand.

Chromatography

Chromatography is a great technique for separating out the different dyes in an ink mixture.

It works because of the dyes' different solubilities.

Diagram showing the process of chromatography with ink spots on paper moving up as solvent travels, separating dyes based on solubility.
  1. Put ink spots on paper and place the bottom of the paper in the solvent.
  2. The solvent moves up the chromatography paper.
  3. Dyes that are more soluble in the solvent move further up the paper.
  4. This results in visible separation into bands at different distances up the paper.

Distillation

Distillation uses differences in boiling points to separate liquid mixtures.

Simple distillation

Simple distillation is used to separate a water from a solution.

Diagram showing the process of simple distillation with a flask of salty water being heated, water vapour rising and condensing into pure water.
  1. The impure liquid mixture is heated in a flask until it boils.
  2. The water vapour rises, enters the condenser and cools, condensing back into a liquid.
  3. Pure water drips out of the condenser and can be collected in a separate container.
  4. Impurities are left behind in the original flask as they require a higher temperature to boil.

Fractional distillation

Fractional distillation is used to separate mixtures of liquids with different boiling points.

Diagram of a fractional distillation apparatus showing a flask with liquid mixture, fractioning column, condenser, and collection container.
  1. The liquid mixture is heated in a flask.
  2. As the temperature rises, the fraction with the lowest boiling point boils first.
  3. When the gas reaches the condenser it is cooled and returns to a liquid.
  4. The condensed liquid is collected in a separate container.
  5. As the temperature rises further, the next fraction starts to boil and condense.
  6. This process results in separation of the mixture into fractions based on boiling point.

Melting and boiling points

Pure substances have defined melting and boiling points that do not change.

For example: pure water melts at 0°C and boils at 100°C

If a substance contains impurities, the melting and boiling points will be different from the pure substance.

Scientists can test the melting and boiling points of a separated substance to check it’s purity.