Separation of Compounds by Chromatography

Chromatography works due to the fact that molecules which have a property called polarity in common tend to be attracted to each other. A polar molecule is simply a molecule that has a region which is electron rich and a region which is electron poor. These regions are sometimes represented as being negatively charged and positively charged. Polar molecules are held together by forces of attraction between opposite charges on different molecules. Water molecules have electron rich regions on the oxygen atoms and electron poor regions on the hydrogen atoms. Thus, water molecules are polar and consequently they stick together because the positive region of one water molecule is attracted to the negative region of another. These interactions provide an explanation for the high boiling point of water.

Paper is made up of extremely long molecules called cellulose. Cellulose is a polymer which means that it is made up of thousands of smaller molecules strung together like beads. These bead-like molecules that make up the cellulose chains are polar and, as a result, cellulose has many regions of high and low electron density. The "charged" regions on one cellulose chain are attracted to the oppositely charged regions on an adjacent cellulose chains and this helps to hold the fibres together in paper. The fact that the long cellulose chains are aligned in one direction can be demonstrated by tearing a piece of newspaper. You will find that newspaper tears easily and along a fairly straight line if torn in one direction, but when torn at a 90o angle the paper will not tear along a straight line. From what has been said so far, you might be thinking that only identical polar molecules are attracted to each other. But what happens when you dip the end of a strip of paper such as a coffee filter into a cup of water? The water actually climbs the paper! This process occurs because the polar water molecules that are in contact with the strip of coffee filter begin to move up the paper as they find new charged regions on the cellulose to stick to and are replaced from below by other water molecules in the cup. Mixtures of dyes are used to make inks of different colours. Each individual dye is composed of a different compound. Suppose you dabbed the end of a strip of paper with a black felt pen. What do you think would happen if you then dipped this end into the cup of water making sure that the pen mark is above the water level? The water would climb the paper as before, but the ink would also be carried up the paper by the water. In fact, the different dyes in the ink would be carried up the paper to different extents depending on whether they were more attracted to the cellulose in the paper or the water rising up the paper. Eventually all of the different dyes used to make the ink would be separated from one another.

There is an experiment that is fun and easy to do which illustrates this principle.