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KEY QUESTION
When is black not really black?

POSSIBLE PRECONCEPTIOINS
Black ink is truly black.

KEY CONCEPT
Things aren’t always what they seem.

GENERAL INFORMATION
Grade Level:  3-12
 Group Size:  Whole class (demonstration) or small groups
 Length of Activity: Class period
 Setting:   Classroom
 Method:   Experiment
 Focus:   Chemistry, chromotagraphy
 Skills:   Observation

Materials List
scissors                                         black marker (not permanent) 
water                                           white paper coffee filter 
coffee cup or mug

PROCEDURE
   DOING THE ACTIVITY
Cut a circle out of the coffee filter.  (It doesn't have to be a perfect circle, just a round shape that's about as big as your spread-out hand).  With the black marker, draw a line across the circle, about 1 inch up from the bottom.

Put enough water in the cup to cover the bottom.  Curl the paper circle so it fits inside the cup.  Make sure the bottom of the circle is in the water.  Watch as the water flows up the paper.  When it touches the black line, you'll start to see some different colors.  Leave the paper in the water until the colors go all the way to the top edge. 

CLOSURE
Discuss what happened with your children.

EXTENSIONS
1. Use different brand markers (or even different colors) with fresh coffee filters and water.

2. Use a clean, dry coffee filter circle.  Use a marker to draw a black spot in the center.  Put the circle on a saucer, and put a few drops of water on the spot. In a few minutes you'll see some amazing rings of color that go out from the center of the circle to the edges.

BACKGROUND
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.