GETTING READY
Materials

One set of containers per group, ranging in size:
 2 small glasses/test tubes
 One small beaker, glass, or small soda bottle that holds at least 6 ounces (200 mL, or about 3/4 cup)
 Food coloring (try to avoid yellow)
 Tap water
 Eye droppers (one for each group)
 Graduated cylinders or measuring cups and spoons. NOTE: You can use standard scientific equipment and measurement (graduated cylinder, in milliliters (mL)), or, if you think your class will have an easier time with it, you can use standard cookiebaking equipment (measuring cups and spoons) to do this project.
Student Prerequisites
 How to measure volume and square inches
 Parts of a circle
 Proportions
 Averages
 Ranges
 Computer skills
 Library skills
Time Needed
3040 minutes
ACTIVITY PROCEDURE
Have each group set up its work area with a set of supplies. Go through the following steps in which an increasing amount of water is added to a drop of food coloring and students note the change in intensity of the color.
Doing the Activity
Part per ten:
Add one drop of food coloring to the small beaker/glass/bottle. With the eyedropper, add 10 drops of water. Have students note the color (it should be quite dark).
Part per hundred:
In another small beaker/glass/bottle, add a drop of food coloring, and then add 100 drops of water with the dropper. The color should be lighter than in the first beaker.
Part per thousand:
In the 6 oz beaker/glass/bottle, add a drop of food coloring, then 1,000 drops of water. Here, students might count out how many drops of water make up a teaspoon or tablespoon, then add the number of teaspoons of water that would correspond to 1,000 drops. Ask: how many drops of water were in this solution? (1,000). How many drops of food coloring? (1). So – there’s one drop per thousand – or 1 part food coloring per thousand parts water—1 part per thousand.
Students should end up with about 120 mL (1/2 cup) of liquid and it should be about the color intensity of very weak KoolAid. Have students determine how much liquid they have – for example, is it ½ cup? They will use this conversion in the next step.
Part per million:
Now, recall how many water drops were in the previous solution (part per thousand), and how much liquid they had (for example, ½ cup). To get a million drops, how many cups would they need? If they had ½ cup = 1,000 drops, they’d need about 500 cups, which is about 31 gallons. Ask students what kind of container they would need to mix up a solution of 1 part per million – 1 drop of food coloring to 1,000,000 drops of water. Some ideas: 6 fivegallon buckets; a bathtub ¾ full. (A bathtub is typically about 40 gallons).
Part per billion:
We would need 500,000 cups (31,250 gallons or 118,300 liters) of liquid to make a solution that would be one part per billion. Ask students to think of an example of something that has that much liquid in it. A household heating oil tank is typically about 275 gallons. So, you’d need a hundred or so oil tanks – and still just ONE DROP of food coloring – to make a 1 part per billion solution. Ask students how intense the color would be in that solution. Note that this is the unit in which most of our mercury measurements will fall – we’ll end up with things that have tens to hundreds of parts per billion of mercury. So we’re looking for the ‘drop in the oil tank’ amount.
Part per trillion:
To get to parts per trillion we’d need 1,000 parts per billion. That’s 31,250,000 gallons, or 118,300,000 liters. An Olympic swimming pool is about 800,000 gallons (3,000,000 liters). So, we’d need about 40 Olympic pools of water  and still just the ONE DROP of food coloring – to have a part per trillion.
Handy information:
 There are 3 teaspoons in a Tablespoon
 There are 16 Tablespoons in 1 Cup
 1 Teaspoon should be around 40 drops (this will depend on your dropper; just an estimate)
 1 Tablespoon should be around 120 drops
 ½ cup liquid (4 oz) will be about 120 mL
EXTENSION IDEAS
Have the perfect extension for an activity like this? Share it with us in the comments section below.
A useful sheet that talks about the meaning of ppm, ppb, mg/kg, and so on is available from the Center for Hazardous Substance Research.