RP6: Chromotography

Question 1

What is the aim of this experiment?

Answer 1

• Investigate how paper chromatography can be used to separate and tell the difference between coloured substances.
• Students should calculate Rf values.

Question 2

Describe how you would investigate how paper chromatography can be used to separate coloured substances

Answer 2

1. Draw a horizontal pencil line 2 cm from the short edge of the chromatography paper.
   Mark pencil spots at equal intervals across the line but not too close to the end of the
   paper.
2. Use a glass capillary tube to put a small spot of each colouring on the pencil spots. A
   small spot ensures that the colouring separates clearly. Label each spot in pencil.
3. Pour about 1 cm3 of water into the beaker.
4. Suspend the paper in the beaker so that bottom edge of the paper dips into the water.
5. Wait for the water solvent to travel at least three quarters of the way up the paper.
   Remove the paper and draw another pencil line at on the dry part of the paper right
   next to the wet edge. This is the solvent front, the distance travelled by the solvent.
6. Hang the paper up to dry thoroughly.
7. Calculate the Rf values for each spot. Rf value is calculated by distance travelled by
   substance/ solvent front.

Question 3

How does paper chromatography work to separate a mixture?

Answer 3

• The mobile phase moves through the stationary phase so anything dissolved in it will move with it
• Compounds interact differently with each phase so will move different amounts through the stationary phase and so will separate

Question 4

How can you use chromatography to see if a certain substance is present in a mixture?

Answer 4

• Run a pure sample of this substance alongside the unknown mixture
• If the Rf value of the pure substance matches the value of one of the spots from the mixture, it is likely to be present

Question 5

Why should pencil be used to draw the line along the bottom of the chromatography paper?

Answer 5

• It will not affect the experiment as it is insoluble in the solvent

Question 6

What are some key things to consider when doing this experiment?

Answer 6

• Make sure the pencil line is above the water surface, otherwise the colouring spots will
  dissolve into the water rather than rise up the paper
• Also make sure that the sides of the paper do not touch the beaker wall

Question 7

Why should you use a lid when carrying out paper chromatography?

Answer 7

• To prevent the solvent evaporating

Question 8

List a safety precaution

Answer 8

Don’t put food colouring in eyes, it may cause irritation

Question 9

How does paper chromatography work to separate a mixture?

Answer 9

• The mobile phase moves through the stationary phase so anything dissolved in it will move with it
• Compounds interact differently with each phase so will move different amounts through the stationary phase and so will separate