BioEd Online

Teacher Background

Water that looks clean and clear still may contain many different types of chemical and biological materials. In fact, even water from crystal clear wilderness sources, or “natural spring water” sold in stores contains dissolved minerals and other substances. Most of these are harmless—especially in tiny quantities.

However, some types of water contaminants are harmful to human health, even in very small amounts. The concentration of many of these substances usually is measured in parts per million, or even in parts per billion. The Environmental Protection Agency (EPA) sets limits for the amounts of potentially harmful chemicals in drinking water sources.

In this exercise, students create a solution that contains a concentration of one part per million of commercial food coloring.

Objectives and Standards

Concepts

• Substances dissolved in water can be present in very tiny amounts that are not visible to the eye.

Science, Health and Math Skills

• Using pipettes (droppers) as a measuring tool

• Observing

• Drawing conclusions

Materials and Setup

Materials per Student Group

• 6 cups, 2-oz clear plastic (see Setup)

• 2 cups, 9-oz clear plastic

• 2 pipets (or droppers)

• Small bottle (or container) of blue or red food coloring

• Water

• Copy of “What Does One in a Million Look Like?” page

Setup

1. Prepare 6 small cups for each group of students, numbering each set “1” through “6” with a permanent marker.

2. As an alternative, use commercially available chemistry trays or cut the bottoms of plastic egg cartons in half to create trays with 6 wells.

3. Students should carry out this activity in groups of four. Set up a station in a central area with materials that each group will need.

Procedure and Extensions

1. Make sure that each group has six numbered 2-oz cups (or a tray), one 9-oz cup of clean tap water, one empty cup (for cleaning the pipet) and two pipets (one for use with food coloring and one for use only with water).

2. Following the instructions on the “What Does One in a Million Look Like?” student sheet, have students place 1 drop of food coloring into “Cup 1.” (OR put one drop of food coloring into the cup for each group.) Have students use a clean pipet to add 9 drops of water to the cup. Ask, How many colored drops did you add to the cup? How many drops are in the cup all together?

3. Instruct students to collect 1 drop of the mixture in Cup 1 and place it into Cup 2. Next, have them use a clean pipet to add 9 drops of water to Cup 2. Students may need to rinse their pipets with tap water and squirt the excess into the empty cup.

4. Each group should repeat the procedure, using 1 drop from the previous cup until all 6 cups are filled.

5. When students have made all their solutions, have them observe the color of the solution in each cup. Ask, What happened to the color of the water in the different samples? In which sample does the color seem to disappear? Does this mean that there is no food coloring in the water?

6. Look at the table on the “What Does One in a Million Look Like?” sheet. Be sure students notice that the concentration in Cup 6 is one part in one million. Each cup has a food coloring solution that is 10 times more diluted than the solution in the preceding cup. Ask, Is there another way to make a mixture that has one part in 1 million? (One way is to add 1 drop of food coloring to 999,999 drops of water! Another would be to add one drop of food coloring to a bathtub full of water—this would be an approximation.)

7. Hold up a glass of tap water. Ask, Could this water also contain tiny amounts of other things that we can’t see? What might those tiny things be? Possible answers could include minerals, microorganisms (germs), or chemicals. Ask, Are all of these things necessarily harmful? Help students understand that almost no water, except in a laboratory, is completely pure. On the other hand, point out that some pollutants can be harmful to human beings even in very tiny amounts, often measurable only in parts per million or parts per billion (for example, heavy metals like lead and mercury, pesticides and some industrial chemicals). Mention that certain city, county, state and federal agencies test drinking water for potentially harmful chemicals. Ask, Why might this be important?

Extensions

• Refer students to the “Riff and Rosie Talk to . . .” section on page 7 of the Water unit’s Explorations magazine. It features a microbiologist who tests water for levels of disease-causing organisms.

• The Safe Water Drinking Act of 1974 requires the EPA to set and enforce standards of safety for drinking water in the United States. Have older students check resources in the library or on the Internet to find out which substances currently are considered hazardous by the EPA, and at what concentrations.

• Water treatment plants typically pass water through a complex filtering process to remove suspended particles and to add chlorine to kill disease-causing organisms. Water also may be sprayed into the air to help evaporate some kinds of chemicals and improve its taste and smell. Organize a class visit to your municipal water treatment plant or have a representative from the local water or health department visit your classroom.

Handouts and Downloads