Paper chromatography
4 marks · NormalThe following steps outline a method to separate a mixture of food colourings using paper chromatography.
Name the purpose of drawing the line in pencil at the bottom of the chromatography paper.
Explain why it is important that the solvent level in the beaker is below the pencil line.
Describe what you would observe on the paper after the experiment if the mixture contained three different dyes.
State one safety precaution you should take when carrying out this experiment.
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.
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(a) So that the dyes do not dissolve and move up the paper with the line; pencil does not dissolve in the solvent.
(b) If the solvent is above the pencil line, the dyes would dissolve directly into the solvent instead of travelling up the paper.
(c) Three separate spots at different heights above the pencil line.
(d) Avoid contact of solvent or dyes with skin or eyes (e.g. wear gloves, use in a well-ventilated area).
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States pencil is used so the line does not dissolve in the solvent.
B2 (b) · 1 mark · Explains solvent must be below the line to prevent dye from dissolving directly into the solvent and not moving up the paper.
B3 (c) · 1 mark · Describes three distinct spots visible, each at a different height above the start line.
B4 (d) · 1 mark · States a valid safety precaution (e.g., avoid skin/eye contact, wear gloves/goggles).
Worked solution
a) The pencil line does not dissolve in the solvent, so it does not interfere with the separation of dyes.
b) The solvent level is kept below the pencil line so that the dyes travel up the paper with the solvent, not directly dissolve into the liquid in the beaker.
c) Three separated spots at different distances above the pencil line will appear, showing separation of the three dyes.
d) A safety precaution: avoid contact of solvent and dyes with skin by wearing gloves or using in a well-ventilated place.