One in a Million
Objective: Students will perform a dilution experiment demonstrating
how a substance in solution can have differing concentrations.
Grade Level: 5-6
Groupings: Pairs
Materials: (per pair) Seven small cups; tablespoon measure; a cup of
mouthwash (choose one that is strongly colored and flavored); quart of
tap water; cup for rinse water.
Time Allotment: 20 minutes
Directions: This activity is best done after Pollution Solutions to help
clarify the concept of a pollutant's having a certain concentration in parts
per million of treated water.
1. Explain to the class that tap water can contain various substances besides
the water itself. For example, certain minerals, including iron, chloride and
sodium, may be in tap water in different concentrations. Water is often
tested to see what minerals it contains. In these tests, scientists look closely
at the concentrations of substances in the water. They determine whether the
concentrations of certain substances are at safe levels or are too high. The
amount of any mineral or other substance in water is often expressed in parts
per million or ppm. Tell the class that they will be doing a series of dilutions
to see just how small a concentration of a few parts per million is.
2. Divide the class into pairs and give each pair a set of materials. Have
each pair label their cups (1 through 7) and line them up in a row. Have each
pair place ten tablespoons of mouthwash in cup 1. They then remove one
tablespoon of the mouthwash from cup 1 and add it to cup 2. Have them
rinse the measuring spoon with tap water, then add nine tablespoons of water
to cup 2. Mix thoroughly.
3. Have the students use their measuring spoons to remove one tablespoon
of dilute liquid from cup 2 and add it to cup 3. Rinse the measuring spoon
thoroughly with tap water and add nine tablespoons of clean water to cup 3.
Mix thoroughly. Have the students observe what is happening to the color of
the solution as the mouthwash is diluted. Ask them when they think the
solution will appear clear. Have them record their predictions.
4. Repeat the dilution procedure for cups 4-7. Each cup receives one
tablespoon of liquid from the preceding cup and nine tablespoons of clean
water.
5. Have the students look carefully at the seven cups. What do they notice
about the colors of the solutions? Record observations on the Dilution Chart.
How did the
observations compare
with the predictions?
6. Ask the students if
the undiluted
mouthwash has a
strong odor and / or
flavor. Can they detect
any scent in cup 2? In
cup 3? In which cups
are they unable to
detect any odor? What
about the taste?
7. Have the class work together to determine the concentrations of the
various cups in parts per million. Explain that cup 1 is 100% mouthwash (a
million parts per million). In cup 2, one of the ten tablespoons are
mouthwash; it is a 1/10 concentration (100,000 parts per million). Cup 3 is
one tenth the concentration of the preceding cup. Can they figure out the
concentration of cup 3? (1/100 or 10,000 parts per million.) Have the
students continue until they have figured the concentrations of all seven
cups.
8. Relate this dilution experiment to the actual concentration of substances
in tap water. Find out some acceptable concentration levels for common
minerals and other substances (see Maximum Allowable Levels chart).
DILUTION CHART
Cup Number
Color & Odor
Concentration (ppm)
Maximum Allowable Levels of Some Common Minerals in Drinking Water in parts per million
(ppm)
Chloride
Iron
250.00
.30
Flouride
4.00
Sodium
Manganese
250.00
.05
Source: Environmental Protection Rules, Chapter 21: Vermont Water Supply Rule, September 1992.