BioEd Online

Teacher Background

Small amounts of many different substances can be dissolved in water at the same time. Many of these materials are not visible or distinguishable when they are mixed together in water. In this activity, students will use a simple separation technique to detect the presence of several different food dyes in water.

The technique, called chromatography, takes advantage of the “sticky” qualities of water, which help it travel up a piece of filter paper. When this happens, the water molecules are attracted to charged regions on the paper’s cellulose molecules. As water moves up the paper, it carries other molecules (such as the food coloring used here). Different molecules will move up the paper at different rates, based on their sizes and degrees of attraction to the water molecules. As a result, the different substances (food coloring dyes in this case) will form separate bands or spots on the filter paper.

Objectives and Standards

Concepts

• Many different substances can be dissolved in water at the same time.

Science, Health and Math Skills

• Measuring

• Predicting

• Making observations

• Drawing conclusions

Materials and Setup

Teacher Materials (see Setup)

• 9-oz clear plastic cup

• Red, green and blue food coloring (see Setup)

• Water

Materials per Student Group

• Round basket-type coffee filters (1–2 per group)

• Beaker, 250-mL (or 9-oz clear plastic cup)

• 2 pairs of scissors

• 2 rulers

• Water

Setup

1. Before beginning the activity, prepare a “mystery liquid” by adding 10–15 drops each of red, blue and green food coloring to about one cup of water.

2. Set the materials out in a central area for the Materials Managers to pick up. Have students conduct this activity in groups of four.

Procedure and Extensions

1. Show the mystery liquid to the students. Ask, Can you tell what’s in this liquid? Explain that each student is going to be a detective and investigate the mystery liquid.

2. Students will need to prepare a test strip of filter paper. Give each group 1–2 basket-type coffee filters. Have the students smooth the filters so that they lie as a flat circle. Each student should cut a strip of filter paper 2 cm wide by 10 cm long.

3. Give each group a 250-mL beaker (or 9-oz clear cup) with about 1 cm of the mystery liquid in the bottom. Tell students they will put the tips of the paper strips into the mystery liquid. Ask them to predict what might happen.

4. Have each student write his or her initials in pencil or permanent ink at the top of his or her filter paper strip. Then have students place the strips in the liquid and gently fold the top of the strips over the side of the beaker so that the strips stay upright (see illustration, left sidebar).

5. Have the students observe their strips for 5–10 minutes. As the color begins to rise up the strips, ask, What is happening to the mystery liquid?

6. Once the liquid in the strips has risen to about 2 cm from the top of the beaker, have students carefully remove their strips and lay them on pieces of paper towel to dry. Instruct students to observe the colors. Ask, How many colors are on your strips? Which colors? Let each student report which colors appeared on his or her strip. (Usually three bands will form: blue at the top, followed by yellow or green, followed by red at the base.) Ask, What does this result tell us about the mystery liquid? How many substances were mixed together to make the liquid?

7. The strips may be preserved in a notebook or displayed in class after they are dry. Encourage students to extend their findings to other situations. Ask, Since several different substances were mixed together in the mystery liquid, do you think that other types of liquids can be mixtures of different materials?

Extension

After conducting the activity with filter paper strips, let students fold and cut snowflakes out of coffee filters. You may wish to copy and distribute “How to Make a Paper Snowflake” (see box, above). Set the folded snowflakes in the beakers, with the tips in the mystery liquid, to color the snowflakes in rainbow patterns.

Handouts and Downloads