Experiment of the Week - Chromatography

Chromatography is the science of separating chemicals, based on how much they stick to paper. We are going to use this interesting science to look at the differences in inks.

To try this, you will need:

- watercolor paper or similar absorbent paper
- several black ink pens from different manufacturers
- a cup
- isopropyl alcohol, commonly known as rubbing alcohol
- scissors

Cut several strips of the paper, about one inch wide and as long as the cup is deep. On the first strip, use one of the pens to make a black dot about half an inch from one end. That end will be the bottom. At the top of the strip, make a mark that will help you identify which pen made the mark. For example, one my my pens had a blue cap, so I put a “B” on that strip. Do the same thing, using a different pen on each strip.

Put about a quarter of an inch of isopropyl alcohol into the cup. Then place the strips into the cup, with the dot side down. Be sure that the dots are above the level of the alcohol.

Then wait...and wait....and wait. All in all, you need to wait about 20 minutes. You should check on the strips every few minutes, and notice what is happening to the ink.

You should see the alcohol soaking into the paper and slowly rising up the strip. As it passes the ink, it should carry some of the ink with it, but instead of seeing black ink rise up the strip, you will probably see a mixture of several colors. Those colors will separate into bands, with one color at the top, and other colors beneath that. Why does that happen?

The pigments in the ink are supposed to stick to the paper, to keep the ink from rubbing off. Some of them stick better than others. As the alcohol moves across the ink, it carries some of the pigments along with it. Pigments which are very sticky will not move very far or very fast. In fact, if they are sticky enough, they may not move at all.

On the other hand, pigments that are less sticky move farther and faster. You wind up with a band of color (the least sticky) at the top, followed by the pigment that is a little stickier, and so on, until you get to the most sticky at the bottom.

You can try this in other ways too. Try this experiment with several pens, until you find two that are very different. Then make a dot with one pen, and then color the dot again with the other pen. You have mixed the pigments from the two, so now you will get bands for all the colors from both. The more inks you mix, the more complex your chromatography will be. You can also try it with other pigments, besides ink. You can even rub colorful fruits and vegetables on the paper to make the spots, and then use chromatography to see if their color is made up of more than one kind of pigment.

Have a wonder-filled week!
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