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Activity I-4: How Small is a PPM?
This youth activity is one in a series of four activities that can be used to introduce water resource concepts.  It is a good introductory activity and can be adapted for all grades.
To demonstrate the concept of parts per million (ppm) and parts per billion (ppb).

To explain how chemicals may be present in very small amounts in water such that they cannot often be detected by sight, taste, or smell even though they may still possibly pose a threat to human health.

Concentrations of chemical pollutants in water are frequently expressed in units of "parts per million" (ppm) which is the same as milligrams per liter (mg/L).  Chemical fertilizers contain nitrate, a chemical that can be dangerous to infants in quantities as small as 10 parts per million.  Trichloroethylene (TCE), a common industrial solvent, is more dangerous than nitrate and when present in drinking water in quantities as small as 5 parts per million can cause a higher than normal incidence of cancer among people who drink the water regularly.
for each group of students:
* 1 stirrer (solid coffee stirrers or tooth picks)
* 2 containers of clean water (one for diluting and one for rinsing)
* 2 dropping pipettes (medicine droppers)
* food coloring (can have different colors for each group)
* set of 10 white plastic spoons, clear containers, or Styrofoam egg carton
* white paper (if clear containers are used)
  1. If clear containers are used, line them up side-by-side, place a piece of white paper under each, and number 1 to 10 (left to right).  If egg cartons are used, number each cup.

  2. Place 10 drops of food coloring into container #1 (food dye is already diluted 1:10).

  3. Either place one drop of food coloring into container #2 or take one drop from #1, transfer it to #2, and rinse dropper.  Transferring the drop of food coloring requires more rinsing but is consistent with the procedure for the remaining containers.  (You may double the drops to obtain more volume, just be certain that you also double the dilution water in step #4).

  4. Add 9 drops of clean water to container #2 and stir the solution.  Rinse the dropper.

  5. Use the medicine dropper to transfer 1 drop of the solution from container #2 into container #3.  Add 9 drops of clean water to container #3 and stir the solution.  Rinse the dropper.

  6. Transfer 1 drop of the solution from container #3 to container #4.  Add 9 drops of clean water to container #4 and stir the solution.  Rinse the dropper.

  7. Continue the same process until no more color is visible in the last spoon, container, or egg carton cup.
  1. The food coloring in container #1 is a food coloring solution which is one part pigment per 10 parts liquid.  What is the concentration for each of the successive dilutions.  Use table below (each dilution decreases by a factor of 10 - 1/10, 1/100, 1/1000, etc.)

  2. What is the concentration of the solution when the diluted solution first appeared colorless?

  3. Do you think there is any of the colored solution present in the diluted solution even though it is colorless?  Explain.  (Yes.  The solution is still present but has been broken down into such small particles that it cannot be seen.)

  4. What would remain in the containers if all the water were removed?  (Residue from the food coloring, i.e., the pigment.)
  1. Allow the water in the containers to evaporate and have students record their observations on what remains in the containers.

  2. Discuss chemical contamination in drinking water.  Use the list of maximum contaminant levels (MCLs) for some toxic or carcinogenic chemicals in drinking water (as regulated by the U.S. Environmental Protection Agency) located at the end of the activity.  These MCLs represent the maximum amount of a chemical that can occur in drinking water without the water being dangerous to human health.  [Note: Some of the MCLs listed are subject to revision by the EPA shortly.]

  3. Explain the relationship between ppm and ppb and the conversion of these units to milligrams and micrograms per liter.  For example:  1 ppm = 1000 ppb; 1 ppm = 1 mg/L; and 1 ppb = 1 ug/L.

  4. Try different colored food colorings.  Does the eye perceive color differently?  Do some colors "disappear" sooner than others?
This activity was adapted from Science Demonstration Projects in Drinking Water (Grades K-12) by the US Environmental Protection Agency, Office of Water, EPA 570/9-90-007, April 1990..

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture, The University of Arizona. The University of Arizona College of Agriculture is an Equal Opportunity employer, authorized to provide research, educational information, and other services only to individuals and institutions that function without regard to sex, race, religion, color, national origin, age, Viet Nam Era Veteran's status, or disability.

For problems or questions regarding this web contact Dr. Kitt Farrell-Poe.
This document was last modified: 31-Aug-2005 .

Container No.

Selected List of Regulated Drinking Water Contaminants
Substance Concentration, ppb
Arsenic 50
Barium 1,000
Cadmium 10
Mercury 2
Nitrate 10,000
Selenium 10
Endrin 0.2
2,4-D (herbicide) 100

One part per Trillion ...

Is A Very Finely-Split Hair

Worker exposure to certain chemicals is limited to a few parts per million.  Regulations reduce an industrial waste discharge to one part per billion.  Pesticide residues are found in drinking water in the low parts per trillion.

Extremely low measurements, and laws and regulations based on them, are growing more commonplace as science perfects increasing sophisticated sensors.

Such infinitesimal figures probably don't register with most people.  Only scientists and plant managers trying to get exposures and effluents down to levels as find as frog hair comprehend and appreciate their meaning.

Shedding light on the subject, Dr. Warren B. Crumell of the Down Chemical Company has made some comparisons that put the figures in perspective (see table below).

UNIT 1 Part Per Million 1 Part Per Billion 1 Part Per Trillion
Length 1 inch/16 miles 1 inch/16,000 miles 1 inch/16,000,000 miles
(a six-inch leap on a journey to the sun)
Time 1 minute/2 years 1 second/32 years 1 second/320 centuries
Money 1 ¢/$10,000,000 1 ¢/$10,000,000,000 1 ¢/$10,000,000,000,000
Weight 1 oz salt/31 tons potato chips 1 pinch salt/10 tons potato chips 1 pinch salt/10,000 tons potato chips
Volume 1 drop vermouth/80 "fifths" gin 1 drop vermouth/500 barrels gin 1 drop vermouth/250,000 hogheads gin
Area 1 sq ft/23 acres 1 sq ft/36 sq miles 1 sq inch/250 sq miles
Action 1 bogey/3,500 golf tournaments
1 lob/1,200 tennis matches
1 bogey/3,500,000 golf tournaments
1 lob/1,200,000 tennis matches
1 bogey/3,500,000,000 golf tournaments
1 lob/1,200,000,000 tennis matches
Quantity 1 bad apple/2,000 barrels 1 bad apple/2,000,000 barrels 1 1 bad apple/2,000,000,000 barrels
Rate 1 dented fender/10 car lifetimes 1 dented fender/10,000 car lifetimes 1 dented fender/10,000,000 car lifetimes