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Lesson Plan
Molecular Polarity
This is an example of using an interactive student directed demonstration
supported by questions to introduce a topic on polar molecules in chemistry.
The objective of this lesson is to understand the role of molecular structure
in molecular polarity.
The class begins with the formation of a Rayleigh Fountain (J. of Chem.
Ed., Volume 65 #1, 1988, page 69), where a charged plastic comb is placed
near a thin upward stream of water.
- The comb causes the formation of an umbrella type pattern of extra
large water droplets. First, I do the demonstration and then allow
a student to repeat the demonstration with several variations until
it is completely understood.
- This is followed by questions used to develop an understanding of
polarity.
- The questioning starts by having students explain what they
have just seen in detail.
- Answers come from several students describing in detail not
only the demonstration, but also clueing in on the location of
the comb and drop size.
- The "why" type of questions are avoided because the students
are not ready at this point to be able to answer them.
- The students are then asked, "If the comb is moved, what happens
to the spray?"
- The students respond, after a little more group experimenting, that
the charge on the comb controls the type of spray and that since like
charges repel, the water droplets does the same.
- A student drawn water molecule, on the board, showing the sharing
of the bonds and the bond angle, information that was studied earlier,
follows this answer.
- Using the drawing the students quickly put together the concept
of a dipole, a positive and a negative end, which is followed by a
definition of molecular polarity.
- The question now becomes can they apply the information, so they
are asked, "Where can we find an example of this in the real world?" One
student responds that on a trip to Disney World they had seen a fountain
that looked like the demonstration, but larger. It doesn't take students
long to figure out how the fountain works.
- The next question is looking for speculation when the students are
asked, "How does water's polarity affect the chemistry of rain?"
The students' feel that the early fall electrical storm causes the raindrops
to increase in size allowing more water to enter the ground. They recall
an article read earlier stating most light rain is used by tree or evaporates.
The class decides that the larger raindrops allow more water to interact
with the soil dissolving more carbon dioxide for the solution chemistry.
As the end of class approaches the main principles of polarity are reviewed
and one final rubric graded quiz question is asked, "How does your explanation
of polarity fit the demonstration?"
Most students are able to put together a good explanation with help from
their cooperative group.
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