1.7 Describe experimental techniques for the separation of mixtures, including simple distillation, fractional distillation, filtration, crystallisation and paper chromatography

Simple distillation

This is used to separate out a liquid from a solution.

Method

1- The solution is heated until the part of the solution that has the lowest boiling point evaporates
2- The vapour is cooled, condensed and collected
3- The rest of the solution is just left in the flask.

You can use simple distillation to get pure water from seawater, for example.

Problems: you can only use it to separate out solutions that have very different boiling points, if you have a mixture with similar boiling points, you need to use fractional distillation to separate them.






Fractional distillation

This is used to separate out a mixture of liquids.

Method

1- Put the mixture in a flask and put a fractionating column on top.

2- Heat the flask

3- The different liquids will all have different boiling points, so they will all evaporate at different temperatures. The liquid with the lowest boiling point will evaporate first, when the temperature on the thermometer (attached to the fractionating column) reaches the boiling point of this liquid, the liquid will evaporate and reach the top of the column.
4- This liquid will condense down the tube and run into an empty test tube waiting at the bottom.
5- Once the first liquid has been collected, raise the temperature until the next one reaches the top.

Problems: Liquids with higher boiling points high may also start to evaporate. However, the fractionating column is cooler towards the top, so these will only get part way up before condensing and running back down into the flask.



Filtration

Filtration is used to separate an insoluble solid and a liquid. 

Crystallisation

This can be used to separate a soluble solid from  a solution.

1- Pour the solution into an evaporating dish (also known as a crucible)
2- Heat the solution, this will cause the solution to start to evaporate, stop heating when crystals start to form
3- Remove the dish from the heat and leave it in a warm place for the rest of the solvent to evaporate - this will result in big crystals
4- Dry the product using a drying oven or a desiccator.

Paper chromatography

Method

1- Draw a line near to the bottom of a sheet of filter paper (use a pencil as they are insoluble and won't react with the solvent)
2- Add dots of different dyes to the line at regular intervals.
3- loosely roll the sheet up and put it in a beaker of solvent. The solvent used will depend on what is being separated - some compounds dissolve well in water while others dissolve well in ethanol. Make sure the dyes are not touching the solvent (you dont want them to dissolve)
4- Place a lid on the container to stop the solvent evaporating - NOTE: this step is kind of optional, if your only doing chromatography for a few minutes rather that hours or days, it can be skipped out.
5- Should all go well, the solvent will seep up the paper, carrying the dyes with it. Each dye should move up the paper at a different rate, forming sport at different places on the paper.

The end result is a pattern of spots, this is known as a chromatogram.