Paper chromatography
5 marks · HardA student wants to find out whether the green colour in a piece of candy is due to a single dye or a mixture of dyes. Describe how you would carry out a paper chromatography experiment to investigate this. Include details about:
the preparation of the chromatography paper and application of the sample
how to set up and run the chromatography
how to mark the solvent front and why this is important
what you would expect to observe if the green dye is a mixture of dyes
one step you should take to make sure your results are reliable
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Draw a pencil line about 1 cm from the bottom of the chromatography paper. Place a small spot of the green dye on the line and let it dry.
(b) Suspend the paper in a beaker with the bottom just dipping into the solvent, making sure the spot is above the solvent.
(c) When the solvent has nearly reached the top, remove the paper and mark the furthest point reached by the solvent (solvent front) with a pencil. This is important for measuring distances and calculating R_f values.
(d) If the green dye is a mixture, two or more differently coloured spots will be seen at different heights above the starting line.
(e) Use the same solvent and type of paper for repeats, or include a reference dye for comparison.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly describes drawing a pencil line and applying the sample as a spot on the line.
M2 (b) · 1 mark · Correctly describes setting up the paper in solvent with spot above solvent.
M3 (c) · 1 mark · Accurately explains marking the solvent front with a pencil and states its importance.
M4 (d) · 1 mark · Describes observing multiple spots if the green dye is a mixture.
M5 (e) · 1 mark · Mentions using same solvent/type of paper, or using a reference dye, or repeating for reliability.
Worked solution
a) Draw a straight line with a pencil about 1 cm from the bottom of the chromatography paper. Use a capillary tube to place a small concentrated spot of the candy extract or green dye solution onto the line. Let the spot dry.
b) Place the bottom of the paper in a beaker with a small amount of solvent, making sure the spot remains above the level of the solvent, and cover the beaker with a watch glass.
c) When the solvent is about 1 cm from the top, remove the paper and immediately mark the position of the solvent front with a pencil. This allows you to measure distances accurately for calculating R_f values.
d) If the green dye is a mixture, you will see two or more coloured spots at different heights, rather than just one spot.
e) To ensure reliable results, use the same type of paper and solvent for repeats, or run a known reference dye alongside the sample.