What Is a One Part Per Million Solution?
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 the following exercise, students create a solution that contains a concentration of one part per million of commercial food coloring.
- 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).
- 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?
- 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. Each group should repeat the procedure, using 1 drop from the previous cup until all 6 cups are filled.
- 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?
- 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.)
- 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?
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- Moreno, N., Tharp, B., and Dresden, J. (2011) The Science of Water Teacher’s Guide. Baylor College of Medicine: Houston.
- Illustration © Baylor College of Medicine\M.S. Young.
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Funded by the following grant(s)
My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education