Chromotography is the un-mixing or separating of chemicals. The easy way to do a chromotography experiment is to have your students add lots of different colors of food coloring to water which, of course, made the water black. Quentin, in 2008, adding food coloring to water.
You can then ask your students if they can think of any way of getting those colors separated again. They should dismiss the things we had tried in the past to seperate mixtures such as filters or evaporation, deciding that they wouldn't work. James, in 2008, having set up his paper chromotography experiment.
Then provide them with sticks ( or pencils), tape and coffee filter strips. Have them tape the strips to the pencils and put them over the cups so that the filters touched the water. Quentin, in 2008, observing the paper strips.
The water will begin to migrate up the paper strips. You should see bands of blue, green, yellow and red on the paper strips.
A more difficult way of doing a chromotography experiment is with colored markers. The markers become the test substances, and are used to draw lines onto strips of paper coffee filter, which are the medium. The strips of paper are then taped to sticks, in this case pencils, so that they can be suspended in water, which is the solvent.
Sam (2008) setting up paper chromotography experiment.
Water is put into the trough so that it can pass through the paper. As in all chromotography, the solvent passes through the test substance, and as it does so some of the test substance may be attracted to the solvant and follow it up the medium. Different types of molecules are transported different distances, causing them to separate. In paper chromotography, when the inks separate, they make little rainbow-like patterns. The green, for example, separated into various shades of yellow and blue.
This year we tested some new materials including brown ink. Katie had some brown ink from art projects that she let us use. You could also break open an ink pen. Black also breaks down into several colors.
Sam also tried a more complicated form of chromotography with plant pigments. First he gathered three leaves. Two were from trees we know (and we know what colors two of them turn into in the autumn) and one we are not familiar with.
Taking each one seperately, he cut the leaves into small pieces.
Then he put a little sand in the mortar and pestle and ground the leaf into a paste.
He then made strips of paper towel.
He put drops of the soaked leaf paste on the paper towel strips.
Then these were suspended in more (plain) isopropyl alcohol by dowels. This was left overnight.
I also put some paper towel strips to soak in the leaf paste jars.
These are the leaf strips that worked the best, which were the ones I soaked in the leaf paste. You can see bands of yellow (carotene), perhaps orange? (xanthophyll) and the green chlorphyll in the first two examples. It is interesting that the different shades of green are called chlorophyll A and chlorophyll B.
The one on the right is from the Sassafras leaf, the middle one is from the Norway Maple and the third is the unknown sample. I know that the Sassafras leaves vary from yellow to orange. The Norway Maple turns yellow.
These are the samples Sam did, which did not break down into bands.
The varied results can be for many reasons. Sometimes there needs to be more of the sample on the paper. Different papers will vary the results, sometimes one type of paper will work better on one type of sample and not on another. The solvent can also make a difference. Alcohol works better on some samples, whereas water or salt water(1/8 teaspoon to 3 cups water) might work better on another. Goo Gone is another good solvant as well.