MICRO-MAGIC WITH STICKY IONS
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.
Chem Studies Series: Electrical Interactions in Chemistry (Ward's)
Students will be able to:
- construct a data table suitable for observations on 10 combinations
- label combinations of compounds which produce a precipitate;
- identify the new compound (precipitate) by name;
- demonstrate the ability to write a balanced chemical equation,
- including a precipitate label, for a reaction produced in the lab;
- describe a theory that explains how both positive and negative ions
- describe a theory that accounts for the bonding between the particles
in a precipitate;
- state symbolically a quantitative relationship between force and distance.
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
- silver nitrate
- barium chloride
- potassium sulfate
- iron III chloride
- sodium carbonate
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.
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
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
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!
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
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
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
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
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
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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