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How to Use the Internet in Your Classroom: Lesson Plan: Molecular Polarity

Eric Hendrickson is a teacher at Presque Isle High School, ME,  and contributor to
How to Use the Internet in Your Classroom.  Contact Eric.

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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. 

  1. 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. 
  2. This is followed by questions used to develop an understanding of polarity. 
    1. The questioning starts by having students explain what they have just seen in detail. 
    2. 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. 
    3. The "why" type of questions are avoided because the students are not ready at this point to be able to answer them. 
  3. The students are then asked, "If the comb is moved, what happens to the spray?"
  4. 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. 
  5. 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. 
  6. 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. 
  7. 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. 
  8. 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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