Water Resources of the United States

ACTIVITY

How Substances are Measured in Water

Introduction

Substances in water often occur in parts per million, parts per billion, or even parts per trillion. Water-quality laboratories can measure concentrations of substances at these levels, but what do these small concentrations mean? Developing an understanding of extremely large and small numbers is difficult. The following activity is designed to help students have an understanding of these extremely small concentrations.

Objective

Students will be able to visualize extremely small concentrations of a material.

Materials-- Each group will need:

1. One eyedropper;
2. Six small, clear plastic cups (the smaller the better); and
3. One 472-mL clear plastic cup filled 3/4 full of water.
   Teacher needs: One bottle of food coloring.

Teacher Preparation

1. This activity is designed for students in groups of three. However, if sufficient materials are not available, this activity can be done as a demonstration.
2. For each group, place one drop of food coloring in one of the small, clear plastic cups and fill the 472-mL cup 3/4 full of water.

Procedure

1. Prior to conducting the activity, solicit responses from the students to the following questions:
   1. What is the largest number of things you can clearly visualize in your mind?
   2. Can you visualize a group of 1,000 people? Are you able to differentiate between 800 or 1,200 people from the 1,000?
2. Hand out one eye dropper; five small, clear plastic cups; one small clear plastic cup containing a drop of food coloring; and one 472-mL clear plastic cup filled 3/4 full of water to each group.
3. Explain that food coloring is usually a 10% solution; that is, 1 part (by volume) of food coloring dye is dissolved in 10 parts (by volume) of solution. For example, 1 mL of dye dissolved in 9 mL of water makes a total of 10 mL of 10% solution.