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Grades 9-12


In this lesson the students cause some chemical compounds to form from clear solutions and discover why the components stick together. The students mix pairs of compounds in solution form, note which combinations produce a precipitate, identify the precipitate, and explain the cause of ionic attraction in each. This activity can be used to introduce the concept of double replacement reactions and precipitate identification using solubility charts. This activity would usually be done during a "double lab block" in a chemistry class, but could be simplified and divided into segments to be done in a lower level science class.
ITV Series
Chem Studies Series: Electrical Interactions in Chemistry (Ward's)

Learning Objectives
Students will be able to:

Each student should have an ion/solubility chart, safety goggles and per group of two or three high school students:
per two to five groups a set of the following 0.1M solutions in dropper bottles:

Pre-Viewing Activities
Begin this lesson with a magic trick (I usually don a magician's hat and try to make the demonstration seem "extra mysterious"). Show the class solutions of blue copper II sulfate and yellow potassium chromate (each should be about 100 milliliters and 0.1M so the reaction will be very visible and dramatic), and ask them what happens when blue and yellow are mixed. Pour some of one into the other (a brown solid forms!), then ask the students what makes this new chemical form. Do not respond to answers. Tell them that they will be making some of these magical new substances themselves. Distribute activity reprint Micro Magic. Pair the students randomly, assigning each to a lab bench area. Instruct them that some of the chemicals are toxic and harmful to eyes so goggles are a must. Also remind them that if one dropper touches another chemical, then the dropper and bottle get contaminated.

Note to teacher: These instructions assume that the students already know how to write balanced equations and use a solubility chart. However, I have used this activity to introduce the concepts and to raise the curiosity level so the student develops a "need to know." The use of charts and equation writing could be done after this activity.

Focus Viewing
The focus for viewing is a specific responsibility or task(s) the students are responsible for during or after watching the video to focus and engage students' viewing attention. The students should write answers to questions on the Focus for Viewing reprint after each segment is stopped and discussed. After watching this video, the students should be able to develop a theory explaining why precipitates form and state a mathematical formula describing the relationship between force and distance involving two ions.

Viewing Activities
State to class: "You are about to see four phenomena that have a common property. See if you can identify what it is, but do not make your theory or say anything out loud until you see all four. There will be total silence."

BEGIN the video with the sound off (this will raise the level of curiosity and sharpen the focus) after the title and credits have ended, and a light bulb is viewed on the screen. (If you do not have time to cue this up in advance, simply face the monitor away from students' view to do it, since it is much more dramatic to begin in silence with the light bulb.) PAUSE after about 50 sec. where the screwdriver is shown and ask the class if anyone can identify the common property. After a few comments, ask if anyone can identify some subatomic particle involved in all four. After a few responses, rewind to the same starting point. RESUME the video with sound for about 2 min. 40 sec. then PAUSE where the model of the negatively charged sphere is shown and the narrator says, "If we add more electrons, the sphere becomes negatively charged...it holds more electrons than protons."

Have two students pass out the Focus for Viewing reprint and ask all students to answer question #1. (It is best to wait until this point to pass these out so it does not stifle the initial curiosity.) After you see that they have the answer and you are confident they understand what they wrote, ask them if they know the answer to the second question (some will incorrectly say "add protons," but make no comment).

RESUME the video for 10 sec. then PAUSE where the positively charged sphere is shown and the narrator says, "There are more protons than electrons." Ask one student to answer question #2 orally. Have all the students write the answer (assuming it is "By removing electrons"). Then ask them if anyone knows the answer to question #3. After a few responses, RESUME the video for 10 sec., then PAUSE when the sphere is shown with equal positives and negatives and the narrator says, "it is uncharged." Ask the class if neutral means no charge. Wait for the correct response of "equal numbers of positive and negative charges." Ask the class if anyone knows the answer to question #4, then ask if anyone knows the answer to question #5. (You need not discuss nor wait for any particular response at this point.)

RESUME the video for 25 seconds then pause where positive and negative spheres are shown and the narrator says, "...the attractive force between the unlike charges causes the spheres to move together." Ask the students to write responses for questions #4 and #5. If there are no questions on #4 and #5, ask if anyone has an idea how this force of attraction could be measured. Whether you get responses or not, tell them that the video will now show a very creative experiment designed to measure this force. RESUME the video for 20 sec. until the sensitive balance is shown and the narrator says, "... connected to a sensitive balance." Point out that holes were drilled into the bottom of the balance and a Christmas ornament was hung by fish line to the balance pan. RESUME the video for 2 min. then PAUSE when you see the spheres at a 10 cm distance on the ruler and the narrator says, "The spheres are initially placed 10 cm apart."

Tell the students that the experiment will now be done in a hurry, so watch carefully. FAST FORWARD with the picture viewable for about 3 min. and pause when you see the scientist write the number "51" in the Fr2 column of the data table. Ask the class to describe what they have just seen. After a few responses, ask them to describe the graph (if no one has mentioned this). REWIND until you see the scientist write the word "none" in the force column again, then RESUME the video until the scientist again writes the number "51" in the Fr2 column of the data table and pause. Ask the class to describe this "constant" rounded to one significant figure. After they have decided it is 50, suggest that they write this on their paper.

Raise the question: "What does all this have to do with chemical behavior?" Whether you get responses or not, after a few seconds RESUME the video for about 3 min., then PAUSE when you see the yellow solution being poured into the center of the horizontal tube and the narrator says, "We will pour the chromate solution into the center section of another tube." Ask the students what they think is going to happen when the wires are reconnected. After a response or two, RESUME the video for about one min. until you see the scientist pour the blue solution into the left compartment and the yellow into the right. PAUSE precisely after the narrator says, "...as you would expect..." and ask the students, "What do you expect?" Wait for 5 sec. then RESUME the video for about 40 sec. and PAUSE when you see the blue and yellow solutions in two beakers and the narrator says, "What is this new substance?" You now ask the class, "What is this new substance?" (This may seem silly, because it is. The students will chuckle at the "echo" if you time it right.) After a few responses ask them, "If we simply mix the two solutions, will this same brown substance form?" After a response or two, resume the video then pause when you see the symbol CuCrO4 printed on a brown background while the narrator says, "Electric forces hold the solid together."

Ask the students to think about the last question on the focus sheet "What makes a precipitate form?" and be sure that they now understand that an appropriate response includes the theory developed in the video about the attraction of oppositely charged ions. Ask them if they know what charge a precipitate has. Be sure it becomes clear that a precipitate is neutral with no net charge, yet there are charges present, simply equal numbers of each.

Note to the teacher: If you have used this video before, it may seem like making a mountain out of a molehill (a 20 min. program) to do all this, but I have found it worth the extra time. The students show increased motivation and learning by becoming active participants. You can use some of these techniques with old technology of 16mm film. Just have clear tape and scissors handy for numerous film breaks!

Post-Viewing Activities
Ask the students to refer to their Micro Magic data sheet and count the number of precipitates that they have observed. Ask a student to identify one of them by name and to explain what makes that compound form and what holds it together. Be sure it is clear that the electrical force of attraction of oppositely charged ions is the essential part of the explanation.

Action Plan
Suggest that the students might contact a large hospital or local chemical company to find out how precipitate formation or solubility is used in blood testing, urinalysis, synthesis of new compounds, etc.

Suggest that the students contact a hospital or medical books to find out how electrolytes (ionic solutions) play a part in our bodily functions and health.

Have the students write to commercial battery companies or shampoo companies to see how electrolytes are used in their products.

Have the students do library research to investigate how precipitates are used to analyze unknown compounds. Information could be found under "Qualitative Analysis."

Have the students investigate why precipitates do not form in all combinations of positive and negative ions. What makes something soluble or insoluble? What makes a good soap?

Social Studies: Discuss the difference between micro-scale and macro-scale lab experiments. Have the students write to industries operating on a macro-scale and inquire about recycling waste products.

Health and Biology: View the CHEM STUDY video Molecular Structure and Health to see how the concepts in this lesson are put to use in cancer research.

Art: How are ionic compounds used in ceramic glazes? Which ions cause color? (Art books often have some of this chemistry which art teachers avoid like the plague.)

Social Studies and Science: Have the students research electrochemistry to find out what is actually happening, chemically, at the negative wire in the video experiment and apply this to metallurgy. How are precious metals made?

Math: Have the students plot data from the video experiment on a graphing calculator to analyze the 2nd degree inverse relationship.

Click here to view the worksheet associated with this lesson.

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