Chromatography: The Basics

Did you know the black ink is usually made of a mixture of several different colored inks?  You can use a process called chromatography to separate the mixture. 

Here's the basics...

Cut a piece of filter paper (or a piece of coffee filter, or a piece of paper towel) about an inch wide.

Place a dot of ink about an inch from the bottom.

Place a small amount of water (less than an inch) in a cup.  Place the filter paper into the water, so that the paper below the dot is the only part in the water.

The water will travel up the filter paper.  As it travels, it will carry some of the ink with it.  The ink that is made of larger molecules will not travel very far.  The smaller molecules will travel farther up the paper, resulting in separation. 

Now, here's how I use it in my classroom...
I usually make a sign with a silly threat on it.  Something along the lines of, "8th graders small funny."  Mature, right?  It's enough to get the students attention and I tell them that we're going to figure out who wrote it.

I then go through a story.  Something like... "Mr. Krup always has a black Sharpie.  And I saw Dr. Smith with a black Crayola marker yesterday.  Etc."  I use names of teachers in the school - people who are my friends and are good sports.

The students each get a piece of the threat that contains some ink, so they can use it for chromatography. 

Then, they take a wider strip of filter paper and make a mark with each of the 'suspect' markers.

Then they complete the same process as above with this paper, placing the end in water and letting the water wick up the paper. 

At the end, you compare the chromatography results of the evidence and your four suspect markers.  One of the four markers will match the evidence and you'll have your culprit.