Chemistry: Making Things Un-Mix: Chromotography

Chromatography is the un-mixing or separating of chemicals. The easy way to do a chromatography experiment is to have your students add lots of different colors of food coloring to water which, of course, made the water black.

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 separate mixtures such as filters or evaporation, deciding that they wouldn't work

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.

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 chromatography 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.

Water is put into the trough so that it can pass through the paper. As in all chromatography, the solvent passes through the test substance, and as it does so some of the test substance may be attracted to the solvent and follow it up the medium. Different types of molecules are transported different distances, causing them to separate. In paper chromatography, 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. My oldest 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.

We also tried soaking the brown coating off M&M's and letting it soak up water.

My middle student also tried a more complicated form of chromatography 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 separately, 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 put each leaf in a separate jar and poured Isopropyl alcohol over the leaf paste. He let this soak for about a half hour.

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.

The Results?

 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 chlorophyll 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 the middle student did, which did not break down into bands.

This is a sample of one of the brown M & M's. You can see that the brown is made of many colors.

The marker didn't seem to break down with just water.

The ink seem to break down into bands of orange.

 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 solvent as well.

sources and resources:

GEMS: Crime Lab Chemistry

Grades 4-8

5 Sessions

Now an updated and enhanced New GEMS edition. Challenged to determine which of several black pens was used to write a ransom note, students learn and use paper chromatography, as they explore the concepts of solubility, pigments, and separation of mixtures. New sessions provide students with multiple opportunities to visualize the molecular nature of matter as they create and revise their own models and consider the limitations of models.