- To use chromatography to compare different colour coatings on candies.
- Chromatography is the scientific study of
colours. In this experiment, you will use paper chromatography to analyze
the different colour coatings of M&M's, Skittles, Smarties, and Reese's
Paper chromatography is a method of
identifying the components of a mixture by treating them with a solvent and
observing how they "travel" different distances on absorbent paper.
A solvent is a liquid that can dissolve other
substances. The substance that is dissolved in the solvent is called the
solute. The solvent must be chosen according
to the nature of the mixture's components. This lab project uses only water-soluble
The solvent moves through the filter paper because of capillary action. Capillary action is a force that
pushes liquid through small tubes, such as blood vessels. Each dye (solute)
travels different distances according to the attraction it has for the paper
or the solvent.
In other words, when you put dye on the paper and dip it in the salt
solution, the liquid will spread; the solvent will travel farther than the
dye, and you will see a dark area and a lighter area. If you divide
the distance travelled by the dye by the total distance travelled by the
solvent, you get a ratio, called the Rf or Retardation
factor. This value is different for different compounds, which means
you can identify a compound by the value of its Retardation factor.
In this activity the solutes are the various dyes and the the solvent is
- 4 different colours of M&M's
- 4 different colours of Smarties
- 4 different colours of Skittles
- 4 different colours of Reese's Pieces
- 0.1% table salt solution
- 4 sheets of filter paper
- 16 plastic cups
- 4 large beakers (600 ml)
- stirring rods
- mm ruler
- Extract the colour from the M&M's candies
Note: Repeat the steps for each kind of candy. Be sure to include the colour and kind of candy on your label. Also, the colour takes longer to remove from Reese's Pieces.
- Label each of the 4 cups with one colour of the candy.
- Place one sample in each cup.
- Put as few drops of water as
possible (around 5) in each cup.
- Stir carefully with the
toothpick to extract as much colour as possible without disturbing the
white coating or the centre of the candy.
- Remove the sample as
soon as the white coating appears.
- Add each sample in turn to
its appropriate cup until as much colour has been extracted as possible.
- Label the filter paper
- Label your filter paper with a pencil. Do not use ink for labeling the
paper since it may dissolve in the salt solution and interfere with your
- Fold the filter paper in half, and in half again. Unfold it;
now you can see exactly where the centre is.
- Draw a circle around
the centre of each piece of filter paper (one for each colour). The circles
should be approximately 2 cm across. When you add the solvent, make sure the
level stays below the circles you have drawn.
- Paper Chromatography
- Place a spot (using a stirring rod or toothpick) of each coloured
solution in its labelled circle. Repeat this several times, to the same
spot, to concentrate the dye on each spot. Allow drying time between
- Put the salt solution into a clear plastic
cup. Keep the level of salt water below 0.5 cm. (You may want to use a
marker to mark the outside of the cup to make it easier.)
the filter paper, and stand it up in the cup. Only the tip of the paper
must be immersed in the salt solution; the solution must not touch the
circles you drew in pencil.
- Watch as the salt solution
(solvent) moves up the paper, pulling along the components of the dye.
This should take 10 minutes. If the room is very dry or warm, it might
be necessary to put plastic wrap over the top of the cup to slow down
the evaporation of the salt solution.
- Remove the wet filter
paper from the cup when the advancing solution reaches about 1 cm from
the top edge.
- Mark the place on the filter paper where the
solvent finally stopped moving.
- Each sheet of filter paper is
a chromatogram. Allow your chromatograms to
- Compute the Rf values
- Measure the distance from the middle of the filter paper to the edge
of the salt solution marked in step 6 of part C. This gives the total
- Measure the distance from the centre of the
filter paper to the final location of each band of colour in your
chromatogram. This gives the distance travelled by the dye (solute).
- Calculate the retardation factor for the different colours of
each candy by dividing the total distance by the distance travelled by
the dye. You may wish to set up a table to display your results.
For each type of candy, which colour travelled the farthest?
From your table, you can also compare the retardation factors for the same
colour between the different types of candy, for example, red M&M's vs. red
A scientist using sophisticated equipment can tell many things from chromatograms. A scientist can find out exactly what substances each dye is composed of, and how much of each is present.
This technique is used in forensic science: detectives can have a sample of paint analysed to find out information about the car it came from. Not only can they trace the model of car, but they can also pinpoint the year it was made!
Computer Science |
Understanding Science and Technology
Produced by Galactics.
Last updated on 14 August 1998.