 optional activity concentrations 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 34 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 1000 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 1000 drops ask how many drops of water were in this solution 1000 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 12 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 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 cup to get a million drops how many cups would they need if they had cup 1000 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 1000000 drops of water some ideas 6 fivegallon buckets a bathtub full a bathtub is typically about 40 gallons  part per billion  we would need 500000 cups 31250 gallons or 118300 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 1000 parts per billion that s 31250000 gallons or 118300000 liters an olympic swimming pool is about 800000 gallons 3000000 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 cup liquid 4 oz will be about 120 ml reflection formative assessment ideas extension ideas have the perfect extension for an activity like this share it with us in the comments section below teacher s cheat sheet handouts resources 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  mercury concentrations mercury concentrations mercury concentrations   mercury in watersheds  