Demystifying Mixtures

Kathleen Damonte

The subject of chemistry makes most people think of an elaborate laboratory, but investigating chemistry concepts does not require expensive equipment or chemicals. You can perform some of the same tests scientists do using materials in your own home. One topic chemists study is mixtures and the substances that make them up. In this activity you will use a process called chromatography to find out what makes up a mixture you use often: black ink.

Chemists define a mixture as two or more substances that keep their separate properties when they are mixed together. A bucket of sand and rocks is a mixture. Salt water is a mixture. Just about everything you eat and drink is a mixture. Although the inks used in felt-tip pens and markers appear to be one color, they are often mixtures of several different colored dyes.

Separating Mixtures

One characteristic of all mixtures is that they can be separated into the different parts that make them up. Scientists use many different methods for separating mixtures. A mixture of sand and rocks can be separated by pouring the rocks and sand through a strainer that allows the sand through but not the rocks. Water can be evaporated out of salt water leaving the salt crystals behind. Most types of ink can be separated into the colored substances that make them up using chromatography. This process dissolves the ink and produces a pattern that shows all the colors that were mixed together.

Paper Chromatography

Chromatography can be done in a variety of ways. A common method uses paper as the material on which the mixture separates. The mixture is applied to the paper and then dipped in a solvent, such as water. A solvent is the substance that dissolves the mixture and causes its separation.

In this activity, you will put ink on a strip of paper and dip it in water. The water moves up the paper, eventually reaching the ink and dissolving it. The different colored substances that make up the ink are carried along the paper by the water. The different colors separate out at different rates. Some separate quickly and others are carried farther along the paper by the water. The resulting color pattern, which is called a chromatogram (see illustration), will show all of the colors that make up that particular ink.

Follow the procedure below to make your own chromatogram of a black felt-tip pen or marker.

Tell-Tale Colors

Materials:

• 1 (or more) black felt-tip pens or markers (Flair pens work well. Do not use a permanent marker because it will not dissolve in water)
• 1 clear-plastic cup
• 1 pencil
• Transparent tape
• Water
• 1 paper coffee filter
• 1 ruler
• 1 pair of scissors

Time needed:
30 minutes

Directions:
1. Use the scissors and ruler to cut the coffee filter into strips that are approximately 1 cm wide and 7 cm long.

2. Tape the top of one paper strip to the pencil. Rest the pencil across the top of the plastic cup with the paper hanging down. Trim the length of the strip so that it is just short of touching the bottom of the cup.

3. Draw a line 1 cm from the bottom of the trimmed paper strip with the felt-tip pen. This will be the sample of ink you will be testing. Hang the strip back in the cup.

4. Slowly add just enough water so that it barely touches the bottom of the strip of paper. Do not fill the cup with so much water that it touches the black line drawn on the strip. You will start to see the water moving up the paper.

5. Observe the strip. Leave it in the cup until the water reaches the top of the paper. Remove the paper and let it dry. This is the chromatogram for that ink pen.

6. Test other colored pens or nonpermanent markers and compare the chromatograms you get.

Questions

• What colors were mixed together to make up the samples of black ink you tested?
• What colors moved highest on the test strips? Which ones barely moved?
• Were there similar patterns in the chromatograms of the other pens and markers you tested?

Extension Activity
Test the dyes used in the candy coating of M&M’s. Wet the candy slightly and rub a line across the bottom of a strip of the filter paper. Follow the same procedure to finish the chromatogram. How many different colored dyes are used in M&M’s?

Kathleen Damonte teaches seventh grade science at Julius West Middle School in Rockville, Maryland.

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