Practical 3
Paper Chromatography
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You will carry out a paper chromatography on a sample of marker ink.
Theory
Chromatography is used to separate a mixture of substances which have different solubilities in a given solvent. If a substance is more soluble in the solvent, it will move with the solvent more easily.
It will travel more quickly towards the solvent front. A less soluble substance will move at
a slower rate.
Read all the instructions carefully before starting the experiment.
1. Cut a strip of filter paper such that it can fit neatly into the plastic cup. The strip should be slightly taller than the plastic cup.
2. On the strip of filter paper, draw a start line with a pencil, 3 cm away from any one of the short sides.
3. Use the marker and mark a small dot in the centre of the pencil line. Allow the drop to dry.
4. Pour about 1cm3 of water into the boiling tube.
5. Place the strip of filter paper into the boiling tube, ensuring that the pencil line is just about the water level.
6. Seal mouth of the boiling tube tightly with the stopper.
7. Leave the apparatus to stand for a short while.
8. Observe the solvent front as the water travels up the paper.
9. Stop when the solvent front reaches just below the top of the paper. DO NOT allow the solvent front to reach the top of the paper.
10. Allow the paper to dry, and glue/tape your filter paper to this worksheet.
“Flower Chromatography”
1. Place a 20-cent coin at the centre of the filter paper.
2. Use a water-based coloured marker to draw a simple design around the coin. (Tip: Keep the design simple and let the chromatography do the magic)
3. Place the filter paper on top of an empty beaker.
4. Soak a cotton bud with water and leave it in the centre of the filter paper for 10 minutes.
5. Repeat step 3 a few times to allow for better separation of the colours.
State the principle behind paper chromatography?
Why is the starting line drawn in pencil instead of a pen?
Why should the starting line and ink dots be above the solvent level?
In some set-ups, the boiling tube would be sealed with a stopper. Suggest why.
Alice wanted to do the Flower Chromatography experiment at home but realised that she did not have any water-soluble markers. She decides to use Zebra permanent markers to draw her designs on the filter paper instead. She carried out Steps 2 to 4, but the spots remained at the starting line.
a) Explain why the spots remained at the starting line after some time.
b) Suggest what Alice could do to make the marker ink separate.
1. The figure shows two methods of paper chromatography.
In the descending method, the solvent flows down the paper and in the ascending method, the solvent travels up the paper.
a) In which method will the solvent travel the faster? Give a reason for your answer.
b) Longer sheets of paper can be used in the descending method than in the ascending method. Why is a longer piece of paper often preferred?
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