Exercise V. One in a Million
Using Measurements in Real Life
Federal drinking water standards are real life examples of where parts per million measurements are used. You can find standards for some drinking water contaminants within EPA's Safe Drinking Water Web page.
Substances in water are often measured in parts per million (ppm), parts per billion (ppb), or even parts per trillion (ppt). This activity is adapted from Science Demonstration Projects in Drinking Water (U.S. EPA Water Resource Center, Washington, DC, "One in a Million" by Steve Vandas).
Agencies like the U.S. Environmental Protection Agency determine what concentration of a contaminant in water could be considered dangerous. If a substance is highly toxic, it could be dangerous even if it is present only in parts per trillion; if the substance is less toxic, it could be dangerous if present in parts per million.
- 1 eye dropper for each group
- 6 small, clear plastic cups (the smaller the better) for each group
- One 472 mL clear plastic cup filled ¾ full of water for each group
- 1 bottle of food coloring for the teacher/presenter
- Review working with percentages by completing the sheet "Reviewing Percents."
- Food coloring is usually a 10 percent solution. Draw this on your calculations page.
- Using the eye dropper, have one member from each group add nine drops of water to the small cup containing the food coloring. Stir well. Draw this on your calculations page and figure out the concentration of the food coloring. Label the concentration of food coloring on the cup.
- Use the eye dropper to transfer one drop of the 1-part-in-100 solution to a third small plastic cup. Add nine drops of water to this solution. Stir well. The concentration has again been changed by a factor of 10. Draw this concentration on your calculations page, figure out the concentration of food coloring, and label the concentration on the cup.
- Transfer one drop to the 1-part-in-1,000 solution to the next small plastic cup. Add nine drops of water. Stir well. The new concentration is part in 10,000 parts of solution.
- Continue to dilute one drop of each new solution with nine drops of water until you create a solution with a concentration of one part per million. Be sure to label the concentration of food coloring present in each cup.