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 mixtures.

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 water.

• 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)
• toothpicks
• stirring rods
• mm ruler
• scissors
• pencils

1. Extract the colour from the M&M's candies
   1. Label each of the 4 cups with one colour of the candy.
   2. Place one sample in each cup.
   3. Put as few drops of water as possible (around 5) in each cup.
   4. Stir carefully with the toothpick to extract as much colour as possible without disturbing the white coating or the centre of the candy.
   5. Remove the sample as soon as the white coating appears.
   6. Add each sample in turn to its appropriate cup until as much colour has been extracted as possible.
   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.
2. Label the filter paper
   1. 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 results.
   2. Fold the filter paper in half, and in half again. Unfold it; now you can see exactly where the centre is.
   3. 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.
3. Paper Chromatography
   1. 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 each application.
   2. 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.)
   3. Fold 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.
   4. 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.
   5. Remove the wet filter paper from the cup when the advancing solution reaches about 1 cm from the top edge.
   6. Mark the place on the filter paper where the solvent finally stopped moving.
   7. Each sheet of filter paper is a chromatogram. Allow your chromatograms to dry.
4. Compute the Rf values
   1. 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 distance travelled.
   2. 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).
   3. 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.

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!