- Using the eye dropper, have one member from each group add
9 drops of water to the small cup containing the food coloring.
Stir well. What is the concentration of the food coloring? This
is 1 drop of the 10% food coloring in 10 drops of the new solution.
Thus, the concentration is one-tenth of the original or 1 part
in 100 (10 x 10) parts of solution.
- Use the eye dropper to transfer one drop of the 1 part in 100
solution to a third small plastic cup. Add 9 drops of water to
this solution. Stir well. The concentration has again been changed
by a factor of ten. The new concentration is one-tenth of 1 part
per 100 or 1 part in 1,000 (10 x 100) parts of solution.
- Transfer one drop of the 1 part in 1,000 parts of solution
into the next small plastic cup. Add 9 drops of water. Stir well.
The new concentration is one part in 10,000 parts of solution.
- Continue to dilute 1 drop of each solution by adding water
as before to obtain 1 part in 100,000 and then 1 part in 1,000,000.
Your final solution is one part per million.
Interpretive Questions
- In which cup do you first observe no visual evidence that food
coloring is present?
Possible Answer: This generally occurs in the final container,
or one part per million.
- Since you cannot see any evidence of color present in the cup,
how do you know there is food coloring in the cup?
Possible Answer: The students placed it there.
- Can you think of an experiment
that you could do to prove there
is food coloring present in each cup?
Possible Answers:
Compare each cup with a cup containing only water.
Place several drops of the water from each of the cups on a piece of filter paper
and let it dry.
This activity was adapted from "Science Demonstration Projects
in Drinking Water (Grades K-12)," published by the U.S. Environmental
Protection Agency.
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