How does Black Magic work? Why do some black inks separate into many
colors on a wet coffee filter?
Most nonpermanent markers use inks that are made of colored pigments
and water. On a coffee filter, the water in the ink carries the pigment
onto the paper. When the ink dries, the pigment remains on the paper.
When you dip the paper in water, the dried pigments dissolve. As the
water travels up the paper, it carries the pigments along with it. Different-colored
pigments are carried along at different rates; some travel farther and faster
than others. How fast each pigment travels depends on the size of the pigment
molecule and on how strongly the pigment is attracted to the paper. Since
the water carries the different pigments at different rates, the black ink
separates to reveal the colors that were mixed to make it.
In this experiment, you're using a technique called chromatography. The
name comes from the Greek words chroma and graph for "color writing."
The technique was developed in 1910 by Russian botanist Mikhail Tsvet. He
used it for separating the pigments that made up plant dyes.
There are many different types of chromatography. In all of them, a gas
or liquid (like the water in
your experiment) flows through a stationary substance (like your
coffee filter). Since different ingredients in a mixture are carried along
at different rates, they end up in different places. By examining where
all the ingredients ended up, scientists can figure out what was combined
to make the mixture.
Chromatography is one of the most valuable techniques biochemists have
for separating mixtures. It can be used to determine the ingredients that
make up a particular flavor or scent, to analyze the components of pollutants,
to find traces of drugs in urine, and to separate blood proteins in various
species of animals (a technique that's used to determine evolutionary relationships).
Why does mixing many colors of ink make black?
Ink and paint get their colors by absorbing some of the colors in white
light and reflecting others. Green ink looks green because it reflects the
green part of white light and absorbs all the other colors. Red ink looks
red because it reflects red light and absorbs all the other colors. When
you mix green, red, blue, and yellow ink, each ink that you add absorbs
more light. That leaves less light to reflect to your eye. Since the mixture
absorbs light of many colors and reflects very little, you end up with black.