1.12- understand how to use the calculation of Rf values to identify the components of a mixture

For each dye, the Rf Value is calculated. To do this, two distances are measured: -        The distance between the baseline and dye (centre) -        The distance between the baseline and the solvent front (how far the solvent travelled from baseline) Rf value is always between 0 and 1 The more soluble a dye is, the higher its Rf value will be. An insoluble dye Rf value is equal to 0

Use the equation:

R_f= distance of dye from baseline/distance of solvent front from baseline

1.11- understand how a chromatogram provides information about the composition of a mixture

Paper chromatography can be used to investigate the composition of a mixture. The mixture is spotted on the baseline and different dyes will travel different heights up the paper. This is compared to some standard reference materials, to find out what type of dyes the mixture contains.

1.10- describe these experimental techniques for the separation of mixtures: • simple distillation • fractional distillation • filtration • crystallisation • paper chromatography.

Simple distillation

Used to separate the components of a solution (usually the liquid). For example, water from saltwater.

1.     The saltwater is boiled, and the water turns to water vapour.

2.     The water vapour is then condensed back into a liquid by the condenser, and the salt is left behind in the flask.

3.     Cold water is passed through the condenser at the lower end to make sure the condenser remains full of water.

Fractional Distillation

Used to separate a mixture of liquids such as ethanol and water, which are miscible with one another (can be mixed together in any proportion, still form single liquid). Water and ethanol have different boiling points, 100⁰C and 78⁰C By heating the mixture, and making sure it remains at 78⁰C in the column, only the ethanol vapour makes it to the top and into the condenser, whilst any water vapour condenses on the way up, and trickles back downwards into the flask.

Filtration

Used to separate an insoluble solid from a liquid. For example, sand being separated from water by filtration. The substance left in the filter paper is known as the residue and the liquid that comes through is called the filtrate. Can also be used to separate two solids, where one of them is soluble in liquid, e.g. rock salt.

Crystallisation

Used to separate a solute from a solution, or a salt which obtains water of crystallisation from a salt solution. (sodium chloride from sodium chloride solution). The solution is heated in an evaporating basin to boil off some of the water until a saturated solution is formed- this is tested by dipping a glass rod in the solution and seeing if crystals form quickly on its surface. The solution is then left in warm place, to allow the crystals to form. Once they have formed, they can be removed by filtration.

Paper Chromatography

Used to separate a variety of mixtures, and at IGCSE usually coloured inks or food colourings- these usually contain a mixture of dyes. The coloured mixture to be separated is dissolved in a solvent and is carefully blotted onto the chromatography paper at the baseline. (drawn in pencil to avoid smudging or running)

The paper is then dipped into the solvent and suspended so that the baseline is above the liquid solvent, in order to stop the spots immediately dissolving. The solvent is absorbed into the paper, and it rises up. As the solvent rises up it will carry the dyes with it, each different dye moving up the paper at different rates depending on how strongly they stick to the paper, and how soluble they are in the solvent.