Home School Life Journal

Home School Life Journal
"Let us strive to make each moment beautiful."
Saint Francis DeSales
painting by Katie Bergenholtz

Mystery Science; Part III: Ink Chromotography (If you want to do the experiment yourself)

A pigment is a chemical that makes something look a certain color. The brown color of a felt marker, ink pen and brown food coloring are all  mixtures of different colored pigments. These pigments combine in different proportions, but  all appear brown. When a piece of paper is dipped in water, the water gradually moves up the paper as it is absorbed. As the water moves through a water soluble brown stain, it dissolves the pigments in the stain. Depending on the size, shape and weight of pigment, it may be carried along by the water quite a way up the paper or may be carried only a short distance. When an ink is made of a mixture of pigments, as in the case with the ink in this mystery, each pigment will be carried up the paper in a different way, resulting in different bands of color, with each color representing a different pigment. This rainbow streak up the paper is called a chromatogram. We can recognize our mystery ink by comparing its chromatogram to that of the inks from the suspects.

We have experimented with chromotography before. You can play around with them, too, if you would like.

We used a straw cut in half to tape and hang our samples on. We used  brown ink, brown food coloring and a brown washable marker to make our samples. We used a pipette to put our ink on the paper towel strips.
Set up in Advance:
If you would like to set up the samples for your child, then gather together four pencils, straws (you can probably use two straws cut in half) or dowels, some tape, four clear plastic cups, a piece of  paper towel or coffee filter, a brown marker. You will also need two of the following for ink -either brown inks, brown ink pens, or red and green food coloring, which will act as an ink substitute. You will need these two inks and one marker. If you are using the food coloring as your ink substitute, you will need to mix them 2 parts red to 1 part green. Start with 10 drops red and 5 drops green and see if that is enough. Add more in the same ratio as you need it. Put the inks in sealed containers (like pill bottles). Label one "Brown Ink from Mr. Green" and the other  "Brown Ink from Prof. Plum". If you are using an ink pen, you will need to take the pen apart to get to enough ink to use, but don't do this until you are about to start or the ink will dry up. You can do this for the student if they are young. For each student who will be doing the experiment, cut a paper towel into one inch by three inch strips and make a stain across the middle of one strip using a brown marker. This is a piece of your ransom note. I actually wrote a word to make it look more like a piece of ransom note, but you don't need to. You can make just a good size dot if you want. (You might want to make an extra just in case you need it, especially if several children are doing this seperately.) Allow the ink on the strip to dry. Keep the same marker for your child to test. Have whatever you are using for the ink pens of Mr. Green and Prof. Plum ready. 

Now you have set up for your student, one piece of stained paper towel that is the piece of ransom note, some blank paper towel strips, your brown inks, a brown marker and your straws and cups.

Now you can call your kids. Show them the sample from the ransom note. Show them the inks you are using from Mr. Green and Professor Plum. Label two of the white paper towel strips "Mr. Green" or "Prof. Plum." Have your student make a dot in the middle of one of each paper towel strips with each of the inks.

Have your student make a dot in the middle of a blank piece of paper towel with the marker. This is the marker found in the kitchen. Label this "Marker from Kitchen" or just "Kitchen."

Tape each strip to a straw, pencil or dowel.  Set the straws on the rim of the cups.  You will have four: the ransom note, Mr. Green's ink, Prof. Plum's ink and the marker found in the kitchen. The cups need to have just enough water so that the paper touches the water and starts climbing up the paper.
Have your students watch what happens when the water moves through the dots. See if they can see if the marker or one of the inks matches the mystery dot.

Who do they think wrote the ransom note? Have them record this on their sheet. Tell them  not to be discouraged if they cannot tell because more clues will be coming up.
If you would like to duplicate our experiments for next week you will need: Some cola (can be any kind that has phosphoric acid, including flat leftover), 3 glasses and either pH test strips which can be obtained here or at a science supply store OR you can make your own with EITHER red cabbage OR tumeric (found in the spices section of any grocery store) and rubbing alcohol (found at any drug store). If you are making your own test strips you will also need white paper towels OR white coffee filters.

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