Separation Methods

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Paper Chromatography

Paper Chromatography

Used to separate mixtures made up of liquids of different colours. Carrying it out:

1) Draw a line near the bottom of a sheet of filter paper (use a pencil because pencil marks are insoluble and won't dissolve in the solvent)

2) Spot (technical term for placing a dot of ink on the pencil line) the ink.

3) Put the sheet upright in a breaker of solvent (eg. Water). The solvent used depends on what's being tested. Ethanol is sometimes used.

4) Make sure the ink isn't touching the solvent to start with - you don't want to dissolve the liquid

5) Place a lid on top of the container to stop the solvent from evaporating

6) The solvent will seep up the paper, carrying the ink with it

7) The different dyes will move up the paper at different rates so the dyes will separated and form spots in different places.

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...Paper Chromatography

8) If any dyes are insoluble, they will stay on the baseline.

9) The point the solvent has reaches as it moves up the paper is called the solvent front.

10) When the solvent front has nearly reached the top of the paper, take the paper out of the beaker, draw a line with a pencil along the solvent from and leave to dry.

11) The end result is a pattern of spots called a chromatogram

 

On some chromatograms, the number of spots tells you the number of substances there are in the mixture. However, two different dyes could travel the same distance and would only show one spot. So we can say there are a least as many substances in the mixture as there are spots on the chromatogram.

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Evaporation

If a solid can be dissolved it is soluble. There are two methods to remove a soluble salt product from a solution - evaporation and crystallisation.

Evaporation

1) Pour the solution into an evaporating dish

2) Place the evaporating dish on top of a tripod and gauze and place a Bunsen burner underneath

3) Slowly heat the solution. The solvent (something in which you dissolve another substance - eg water).will evaporate and the solution get more concentrated. Eventually, the solid will start to form

4) Keep heating the evaporating dish until all you have left is dry solid

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Crystallisation

Crystallisation

Takes longer than evaporation and like evaporation, it used to remove a soluble product from a solution.

Method

1) Place an evaporating dish with the solution on top of a tripod with a gauze mat. Place a Bunsen burner underneath the tripod

2) Gently heat the solution. Some solvent will evaporate and the solution will get more concentrated

3) Once some of the solvent has evaporated, or when you see crystals start to form (point of crystallisation) remove the dish from the heat and leave to cool

4) The salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution

5) Filter the crystals out of the solution and leave them to dry in a warm place

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Simple Distillation

Simple Distillation

Used to separate out a liquid from a mixture. Can be used to get pure water from sea water.

Method

1) Heat mixture. The component of the mixture that has the lowest boiling point will soon evaporate.

2) As the vapour rises, it passes into the condenser, where is it cooled, condensed and is collected in a container below the condenser.

3) Components of the mixture with higher boiling points are left being in the flask.

 

Note - Simple distillation can only be used to separate substances with boiling points significantly apart from one another. If the boiling points are close to each other, the temperature may rise above the boiling point of more than one substance and they'll end up mixing again.

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Fractional Distillation

Fractional Distillation

Used to separate a mixture of different liquids and is especially useful when the boiling points of the liquids are close together.

Method

1)Place mixture in a flask, attach a fractionating column on top and heat it

2) The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, its vapour has reached the top of the column and passed into the condenser. It will then cool and condense and the pure liquid can be collected.

3) Liquids with higher boiling points might also start to evaporate but the column is cooler towards the top. So they will only get part of the way up before condensing and running back down the flask

4) When the first liquid has been collected, you raise the temperature to the next lowest boiling point of the liquids in the mixture.

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Filtration

Filtration

Filtration is used when the product is an insoluble solid that needs to be separated from a liquid. It can be used in purification as well. It is done by:

  1. Fold a piece of filter into a cone.
  2. Place the filter paper point down into a filter funnel that's sitting in the neck of a container such as a conical flask
  3. Pour the mixture into the funnel lined by the filter paper. Make sure none of the mixture goes over the top or side the side of the filter paper
  4. The liquid will pass through the filter paper but the solid won't it will be left behind in the funnel.
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