Practical: Investigating Paper Chromatography (Edexcel IGCSE Chemistry (Modular))
Revision Note
Practical: Investigate Paper Chromatography Using Inks & Food Colourings
Aim:
Investigate how paper chromatography can be used to separate and identify a mixture of food colourings
Apparatus:
A 250 cm3 beaker
A wooden spill
A rectangle of chromatography paper
Four known food colourings labelled A–D
An unknown mixture of food colourings labelled U
Five glass capillary tubes
Paper clip
Ruler & pencil
Diagram of the apparatus needed for paper chromatography
Method:
Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper
Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line
Use the fifth tube to put a small spot of the unknown mixture U on the line
Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil
Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots
Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper
should dip into the solvent
Allow the solvent to travel undisturbed at least three quarters of the way up the paper
Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line
Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent
For each of food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot
Practical Tip:
The pencil line must never be below the level of the solvent as the samples will be washed away
Results:
Record your results in a suitable table
Food colouring | Distance moved by spot (mm) | Distance Moved by solvent (mm) | Rf value |
---|---|---|---|
A |
|
|
|
B |
|
|
|
C |
|
|
|
D |
|
|
|
The Rf values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made
Substances with matching Rf values are the same substance and will move the same distance up the paper
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