This Week's Experiment - #284 Chromatography
This week's experiment started with a request that I got from a homeschooling
family in Australia. Louise wrote asking for experiments dealing with
forensics. What are forensics? It is the science of crime detection. I
sent her past experiments on fingerprints and footprints. I was writing
instructions for chromatography when I realized that it would make a good
experiment for this week. To learn about chromatography, you will need:
coffee filters
several kinds of black ink pens (not indelible ink)
a small drinking glass
water
scissors
Carefully use the scissors to cut the coffee filter into strips about an inch
wide. Pick one of the strips and make a fold about 1/2 inch from one end, so
that the strip will hang over the side of the glass. About an inch from the
other end, use one of the black ink pens to make a round dot about the size
of this "0".
Hang the strip inside the glass and carefully add water until the bottom 1/2
inch of the strip is in the water. This should put the dot about 1/2 inch
above the water. Quickly the water will begin to soak into the strip, and
you can see the moisture climbing. As it passes the dot, you should start to
see the ink dissolving and running.
Over the next few minutes, something very interesting should happen. As the
ink dissolves in the rising water, it should separate into bands of different
colors.
What you are seeing is the science of chromatography at work. Chromatography
is the science of separating chemicals as they are carried along by a liquid.
The idea is that the black ink is made by mixing several different colors.
Some of the inks stick to the paper more than others. The less they stick to
the paper; the faster the ink travels with the water, so the band of color
that is farthest from the original dot is the ink that is least attracted to
the paper. Density also plays a part, with denser inks traveling slower.
If you try black ink from different brands of pen, you will find that they
use different recipes. A common activity when museums teach a forensics
class for students is to have students use chromatography to find out which
suspect's pen was used to write a mysterious note.
You can try other colors of ink, and you can even write on the same dot with
several colors of pen. This can lead to some very artistic patterns. In a
past experiment we did a similar demonstration with brown M&M candy, which is
a good excuse to get some and eat all the other colors. In the world of
forensics, chromatography is used to separate chemicals to look for and
identify possible poisons and other chemical evidence. Not as much fun as
the M&M's, but still pretty cool stuff.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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