Date: May 1994

Grade Level(s): 9, 10, 11, 12

Subject(s):

• Science/Process Skills

With a medicine dropper, let one drop of water fall in front of class. Pose the question, "If this water molecule evaporates at the rate of one million (10^6) molecules per second, how long will it take for the molecule to be gone?"

PURPOSE:

This may be used either as a synthesis project where students are expected to bring together possessed knowledge to solve a problem, or as a demonstration either for the concept of large numbers or as a fairly complex example of what can be done with dimensional analysis (factor/label or whatever you may call it).

OBJECTIVES:

By participating in this activity the students will:

1. Be required to make "provable estimates".
2. Review important basic facts of chemistry.
3. Use concepts of metric system.
4. Consider large numbers with "real world" application.
5. Be exposed to the power of dimensional analysis in solving a complex problem.
6. Say "Wow!"

RESOURCES/MATERIALS:

None other than the eyedropper are required, however you may wish to have a metric ruler, a chemistry book and a balance close by in case.

ACTIVITIES AND PROCEDURES:

This activity may be used either as a "class or small group synthesis project" or as a "class participation demonstration". I usually work it in toward the end of the dimensional analysis unit. It can also be used as a test question for a fairly quick group.

Use a medicine dropper to drop one drop of water on a surface where all students may see it. Then pose the problem, "If one million molecules evaporate each second, how long will it take the drop to completely evaporate?"

As a demonstration, the steps for solution would be:

1. Estimate the volume of a drop of water. It is quite easy to drop a drop of water on a balance but the discussion and the logic of estimating is interesting. (A common estimate is that it would take four drops side by side to measure one centimeter, thus 64 drops to the cubic centimeter. Not very accurate, but good enough. You may be lucky enough to have those who insist on weighing the water)
2. Layout the general format for D.A., start with drop of water. Call for estimates of proper ending unit, seconds??, minutes??, hours??
3. Fill in canceling units: Drops of water to ccm of water, ccm of water to grams of water, grams of water to mole, mole to Avogadro's number. of molecules, one million molecules to one second (the evaporation rate), sixty seconds to one minute, etc.
4. In a demonstration, this will naturally lead to solving for the number of molecules which evaporate from a drop of water at room temperature.

Ask each student to design a problem of somewhat this type.

May 1994

These lesson plans are the result of the work of the teachers who have attended the Columbia Education Center's Summer Workshop. CEC is a consortium of teacher from 14 western states dedicated to improving the quality of education in the rural, western, United States, and particularly the quality of math and science Education. CEC uses Big Sky Telegraph as the hub of their telecommunications network that allows the participating teachers to stay in contact with their trainers and peers that they have met at the Workshops.