Chromatography of sweets

Chromatography of sweets

Class practical

This type of experiment goes down well with students since it uses well known material normally used as confectionery. The coloured dye coating the surface is removed from M&M’S® of various colours. A spot of each is put on to a piece of chromatography paper and water is allowed to soak up the paper separating out the component dyes. The results show which dye mixtures are used to produce particular colours for the sweets.
 

Lesson organisation

Students should have a good basic understanding of chromatography theory and this practical can be a useful introduction to the method of separation. The experiment can be carried out by groups of two or three and takes about 30-40 minutes. Students must be told that the M&M’S® are not to be eaten under any circumstances.

Chemicals

None. 

Apparatus

Beaker (250 cm³)

Small soft paint brush

Paper clips (preferably plastic coated), 2

Chromatography paper, approximately 20 cm x 10 cm (Note 1)

Pencil

Ruler

A communal hairdryer (optional) (Note 2)

A supply of M&M’S® of various colours (Note 3 and 4)

Health & Safety and Technical notes

Students must not attempt to eat the M&M’S® or even lick them. They are for laboratory use only. 

1 Whatman chromatography paper works best for this experiment, but, if unavailable, large sheets of ordinary filter paper can be cut up instead.

2 Ensure that the hairdyer has had an electrical safety check.

3 M&M’S® with a variety of about 6 or 7 different colours are required for each group.

4 If M&M’S® are unavailable this experiment can be carried out with liquid food colouring which is readily available from supermarkets. Chromatography of Smarties® is less successful as they use natural food colourings. Peanut M&M’S® should not be used if there are students with peanut allergies.

Procedure

a Place the piece of chromatography paper on a clean flat surface, with the longer side horizontal and draw a horizontal line in pencil (not biro) about 1.5 cm from the base of the paper.

b Use the dampened paint brush to remove the colour from one of the M&M’S® and paint this colour on the line about 2 cm from one end. Small spots are best.

c Clean the brush in fresh running water and paint the colour of another M&M® on the line about 2 cm from the first spot.

d Repeat this until all the colours are on the paper or until you have reached the other end.

e Use a pencil (not a biro) to write the name of the colour next to the corresponding spot.

f Roll the paper into a cylinder and hold this in place with the paper clips. Try to avoid any overlapping of the paper when you make the cylinder.

g Put water into the beaker up to depth of about 1 cm.

h Lower the paper cylinder into the beaker of water thus allowing the water to rise up the paper. Ensure that the water is below the level of the spots. Try to avoid moving the paper cylinder about once it is in position.

i When the water approaches the top of the paper cylinder remove it from the water. Mark with a pencil the level of the water at the top of the filter paper.

j Allow the paper cylinder to dry, perhaps by using a hairdryer if available or by clamping it and leaving it to dry overnight.

k Unravel the paper cylinder and examine it carefully.

Teaching notes

Encourage the students to make small intense spots on the paper and to avoid smudging.

Some dyes will be found to produce only one spot further up the paper, whilst others will have spread into two or more areas of colour.

If appropriate students should be told that the relative distance travelled by each “spot” depends not only on its solubility in water but also on its attraction for the cellulose components of the paper.

It should be emphasised that each “spot” may well still be a mixture of dyes, and that a more effective separation might occur:

• if the distance travelled by the spots is increased, e.g. by using a taller cylinder in a taller beaker.
• with a different solvent, other than water
• with a different stationary phase (e.g. silica plates).
   

Student questions

Here are some questions for students.

a Why do you think some dyes separate out into different colours whilst others do not ?

b Why do you think some colours move further up the paper than others ?

c Can you think of any way of improving the separation between the different spots ?

d Look on the side of a M&M’S® packet for a list of the coloured dyes used. Try to identify which dyes correspond to the spots on the chromatogram