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Chemical Concentrations in Water
Concentrations of chemicals in water are often expressed
as units of "parts per million" (ppm) or "parts per billion
(ppb). For example, chemical fertilizers contain nitrates, chemicals that
have been found to be dangerous to pregnant women even in quantities as
small as 10 ppm.
Trichloroethylene (TCE), a common industrial solvent,
is more dangerous than nitrates and has been found to cause cancer in
humans in quantities as small as 5 ppm. When analyzing the amount of certain
chemicals in water supplies, it is important to determine how much is
too much. Although it is not possible to determine such amounts by sight,
taste or smell, they may still be potentially harmful.
In this experiment you will be able to explain the concept
of parts per million and parts per billion as they are expressed in concentrations
of chemicals of water.
- 9 clear containers (glass or plastic, 50 ml)
- Medicine dropper
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- Solid coffee stirrers or toothpicks
- red food coloring
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- Clean water for rinsing the dropper and for diluting
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- Line up the containers side by side and place a piece of white
paper under each one. Starting left to right, number the containers
1-9.
- Using the dropper, place 10 drops of food coloring into container
#1.
- Using the dropper, place 1 drop of food coloring into container
#2. Rinse the dropper. Add 9 drops of clean water to this container.
Stir the solution. Rinse the dropper.
- Use the medicine dropper to transfer 1 drop of the solution
in container #2 into container #3. Rinse the dropper. Add 9 drops
of clean water to container #3. Stir the solution. Rinse the dropper.
- Use the medicine dropper to transfer 1 drop of the solution
in container #3 to container #4. Rinse the dropper. Add 9 drops
of clean water to container #4. Stir the solution. Rinse the dropper.
- Use the medicine dropper to transfer 1 drop of the solution
in container #4 to container #5. Rinse the dropper. Add 9 drops
of clean water to container #5. Stir the solution. Rinse the dropper.
- Continue this same process until all 9 containers successively
contain more dilute solutions.
- After making the dilutions, answer the questions in the Making
Discoveries section.
- Optional extension: Let the containers sit for a few
days until all of the water in the containers evaporates. Does
anything remain? In which containers?
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- The food coloring in container #1 is a food coloring solution
that is part color per 10 total parts of liquid. What is the concentration
for each of the other successive dilutions?
- What is the concentration of the solution when the diluted solution
first appeared to be colorless?
- Do you think there is any of the colored solution present in
the diluted solution identified in the previous question even
though it is colorless? Explain.
- What would remain in the containers if all the water were to
be removed?
- Explain the relationship between ppm and ppb and other conversion
of these units into milligrams per liter or microgram per liter
(Example: 1 ppm = 1000 ppb).
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