Understanding ppm

Designed by Sharon Hicks, TIC teacher, for the NYC and NYC Watersheds Trout in the Classroom program.

Made possible with funds from the Catskill Watershed Corporation in partnership with New York City Department of Environmental Protection

Objective:  To understand that pollutants are important to monitor and measure in very small quantities     

Background: When aquatic systems are shared for drinking water and habitat it is important to monitor the quality of those systems for all interested parties.  

Introduction:  As part of an ecology unit we can study a local stream and measure substances in  mg/L  or  ppm.  Many environmental pollutants have harmful effects even at very low concentrations.  In this lab students compare different concentrations of a pollutant in water.  

Problem:  Can you detect a pollutant in water at a very low concentration?

Materials:  Chem-plate, Plastic dropper and stirrer, Food coloring, Water, lab worksheet

1.    Read the entire procedure.  Have students write a prediction about the results they expect.  When do they predict you will no longer be able to detect the “pollutant” (food coloring)?

2.  Use a plastic dropper to add 9 drops of water to each cup  #1-9 of the chem.-plate. Try to make all the drops the same size.

3.  Add 1 drop of food coloring to cup #1.  Record the total number of drops (water and dye) now in this cup on the data table.  Swirl gently to mix.

4.  The concentration of dye in cup #1 is 1 in 10 drops or 1 part per 10.  Record this concentration in the data table.

5.   Now rinse the dropper and use it to transfer 1 drop of the mixture from cup #1 into cup #2.  Swirl to mix. Record the total number of drops and the concentration of this cup in the data table. [Hint:  the drop added has a concentration of 1 part (dye) per 10 parts (water).  When you dilute that drop to 1/10 of its strength, the new concentration is 1 part per (10 X 10).]

6.  For cup # 3 through 9, add 1 drop from the previous cup.  Record each new concentration in the data table.

7. Observe the water in each cup.  Record your observations.  If you do not observe any color in a cup, write “colorless.”

Follow-up topics could include:  biomagnifications or bioaccumulation – concentrations of pollutants increases as you move up the food chain or up trophic levels of an energy pyramid.  

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