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- Demonstrate knowledge and understanding of paper chromatography
Coloured chemicals are made up of mixtures of different dyes. The method of separating pigments (coloured substances) using filter paper is called paper chromatography.
It is used to identify which coloured compounds are present in a mixture, and to purify a coloured compound.
This is how it works:
On a piece of chromatography paper, a horizontal line is drawn on the bottom. This is called the base line. Then a drop of the dye mixture (ink, for example) is placed on the base line and a pencil is used to mark its name underneath. A few drops of pure dyes that you think the mixture might contain may also be placed on the base line, next to the dye mixture. These pure dyes should also be labelled. The chromatography paper is then put in a beaker/jar with solvent (e.g. water) in it. The solvent level should be below the spots of dye, otherwise the dye will wash off into the solvent. As the solvent moves up the chromatography paper, the dyes in the mixture will separate. The point where the solvent stops is called the solvent front. The chromatography paper now, with all the different separated pigments is called a chromatogram.
The colours in a dye mixture will separate if the pigments have different solubilities in the solvent and/or if the pigments have different degrees of attraction for the filter paper.
Note: the solvent is sometimes called the mobile phase and the filter paper is sometimes called the stationary phase. Technically, it is the water droplets trapped in the filter pores that make the stationary phase, but for simplicity, many people refer to the filter as the stationary phase.
- Interpret simple chromatograms
Refer to the method described above.
How far a pigment travels depends on its solubility in the mobile phase relative to its solubility in the stationary phase.
Pigments carried further along the paper tend to be much more soluble in the mobile phase than in the stationary phase.
Pigments carried short distances tend to have a smaller difference between their solubility in the mobile phase and stationary phase. This is why these pigments are deposited into the stationary phase (the filter paper) after a relatively short distance.
In the diagram, two of the pure dyes (1 and 3) have risen to the same height as the dyes in the mixture, so the mixture contains only dye 1 and dye 3, not dye 2.
To conduct chromatography on colourless mixtures, the same method as above can be used. However, after performing the chromatography experiment, a chemical called a locating agent is sprayed onto the chromatogram to colour the separated compounds. Now, you can compare the components of the chemical mixture to the pure samples you used on the chromatogram.
- Interpret simple chromatograms, including the use of Rf values
First, prepare a chromatogram.
Mark the location of the solvent front, then measure and record the distance between the solvent front and the baseline.
Now measure and record the distance between each of the pigments and the baseline.
The compounds can then be identified by calculating the Rf (retention factor) value.
Each compound has a unique Rf value, so if you know this value you can just look up which chemical has that Rf value to find out the different compounds in the mixture.
The Rf value of a specific solute is the ratio of the distance travelled by the solute to the distance travelled by the solvent.
- Understand the importance of purity in substances for use in everyday life, e.g. in the manufacture of compounds to use in drugs and food additives
Purity, in chemistry, means that there is only one substance present in anything. Any unwanted substances are called impurities. It is important to ensure that there are no impurities in medical drugs and food additives as it will have harmful effects on our body and health. Even impurities in the silicon used to make computer chips will cause the chip to malfunction.
- Recognise that mixtures melt and boil over a range of temperatures
Melting point is the temperature at which a solid changes into a liquid.
Boiling point is the temperature at which a liquid changes into a gas.
Most pure substances have very distinct melting and boiling points. E.g. the melting point of pure water is 0oC and its boiling point is 100oC.
The presence of impurities, however, can decrease the melting point and increase the boiling point.
This means that mixtures can melt and boil over a range of temperatures.
- Identify substances and assess their purity from melting point and boiling point information
Pure substances have distinct melting and boiling points, so if you know the melting and boiling point of the substance in question, you can identify exactly what it is.
For example, if you were told that a pure mystery substance has a melting point of -39.8oC and a boiling point of 356.7oC, all you would have to do is look up which substance has this specific combination of melting and boiling points, and you would find that the answer is mercury.
Assessing the purity of a substance using melting and boiling points is also very simple.
As mentioned above, the presence of impurities can decrease the melting point and increase the boiling point.
Therefore, if the melting and boiling point of the test sample doesn’t match the melting and boiling point of the pure substance, we can tell that the test sample is not pure.
Notes submitted by Sarah
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