CYCLE 5
Developing Ideas
ACTIVITY 6 HW: Dissolving and Polarity--KEY
Name: ________________________________ Date: _______________ Group: ______
How can solubility be explained in terms of
polar and nonpolar materials?
Simulator Exploration #1 What does it mean to be polar or nonpolar?
What does “like dissolves like” mean?
Scientists generally classify solvents and solutes in two categories—polar and
nonpolar. The table below summarizes some of the basic macroscopic
properties of polar and nonpolar solvents.
Table 2: Properties of Polar and Nonpolar Solvents
Polar Solvents
Have a higher surface tension—that is,
more tendency for a droplet to “bead”—
compared to nonpolar materials with
particles of a similar mass
Have a lower vapor pressure—that is,
evaporate less— compared to nonpolar
materials with particles of a similar mass
Have higher melting and boiling points
compared to nonpolar materials with
particles of a similar mass
Dissolve other polar materials and salts;
do not dissolve nonpolar materials
Nonpolar Solvents
Have a lower surface tension—that is,
more tendency for a droplet to spread
out—compared to polar materials with
particles of a similar mass
Have a higher vapor pressure—that is,
evaporate more—compared to polar
materials with particles of a similar mass
Have lower melting and boiling points
compared to polar materials with
particles of a similar mass
Dissolve other nonpolar materials; do
not dissolve polar materials and salts
Classify water as either polar or nonpolar. List any evidence that you
may have gathered so far throughout this cycle.
Water is polar. We observed water bead up on wax paper, we suggested that it has a lower
vapor pressure than ethanol and hexane; we observed that it has a higher boiling point than
methanol, and observed that is dissolves salts.
© 2007 PSET
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Cycle 5
Classify hexane as either polar or nonpolar. List any evidence that you
have gathered so far throughout this cycle.
Hexane is nonpolar. We observed hexane spread out and evaporate immediately when placed
on wax paper, we suggested that it has a higher vapor pressure than water; we suggested that
it has a lower boiling point than water, and observed that it does not dissolve salts but
dissolves oily, waxy materials.
In Cycle 3 Activity 3 HW you investigated polarization of objects and electriccharge interactions. Scientists think about the polarization and electric-charge
interactions of particles in similar ways.
STEP 1: Open Cycle 5 Activity 6 HW
Setup 1. In this setup, you will see an
uncharged hanging sphere and two
uncharged objects that are close together,
but not touching. These objects represent
non-polar particles like hexane.
(Note: the two objects and sphere should be
repositioned further to the right.)
STEP 2: Run the simulator.
What happens? Does this make sense?
Nothing seems to happen, except that the two uncharged objects seem to have taken on a
magenta due.
STEP 3: Activate the Select tool and double-click on
the sphere to open its properties box. Change the
charge on the sphere to ‘100.0’. Click on ‘OK’ and the
sphere should turn red.
As you may recall from Cycle 3 Activity 3 HW, in our model of electric
charges we In the simulator we use red to represent positive charge and blue
to represent negative charge.
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Activity 6 Homework
In terms of electric charge, what happened to the two sides of the closer
object when it interacted with the positively charged sphere?
The side closest to the red sphere turned blue and the side furthest turned red.
Use our model of electric charges to explain how the side of the object
closest to the red-charged sphere becomes blue-charged, while the other
side becomes red-charged. (Remember, our model assumes the negative
charges in the object are free to move, but the positive charges are not.)
The negative charges of the object are attracted to the positive charges of the sphere and
move to the side nearest the sphere, giving that side a net negative charge (which appears as
a blue band). Since the positive charges are unable to move, the negative charges moving
away from the side furthest from the sphere leaves that side with a net positive charge (which
appears as a red band).
When the negative charges in an object move closer to one side of the object,
so that this side becomes negatively charged, while the other side is left
positively charged, the object is said to be electrically polarized. (You saw
an example of that when a charged balloon was brought near a wall in Cycle
3 Activity 3 HW.)
What happens to the other uncharged object? Is the effect as large as on
the first object?
The other object becomes electrically polarized also (the side nearest the red side of the first
object is blue and the side furthest is red). However, the magnitude of the net charges on both
sides of the object must not be as great as the first object because the red and blue lines are
not as thick.
What is the overall charge on each of the objects—positive, negative or
neutral (neither positive nor negative)? How do you know?
The overall charge is neutral. We have not rubbed anything together, so no negative charges
have not actually left one object and gone to another, they have just redistributed themselves
within each object.
While they are polarized, do you think these objects are attracted to each
other? Why or why not?
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Cycle 5
Yes, we saw in Cycle 3 Activity 3 that positive charges are attracted to negative charges
(opposite charges attract).
Imagine that you were to move the charged sphere to the left, far away
from the two objects. Do you think the objects would remain polarized
or not? Explain your reasoning.
Probably not. The further away the charged sphere, the less influence it has on the objects. If
the sphere is moved far enough away, it would have almost no influence on the objects and
they would be not be polarized.
STEP 4: To check your thinking, select the sphere and move it away from the
objects.
When the sphere is on the opposite side of the simulator, do the objects
remain polarized or not? How can you tell?
No, the red and blue disappear and they return to a grey/pink color. [Note: The sphere needs
to be moved far enough away for this to happen. If the sphere is only moved a short distance,
the amount of polarization decreases, but does not disappear all together.]
Suppose the sphere in the simulator were blue-charged. If a bluecharged sphere were brought close to the objects, would they be
polarized in the same way? Would they still attract if they were free to
move? Draw what you think will happen.
The side closest to the blue sphere would turn red and the side furthest would turn blue. The
other object will become electrically polarized also (the side nearest the blue side of the first
object will be red and the side furthest will be blue). However, the magnitude of the net charges
on both sides of the object will not be as great as the first object so the red and blue lines are
not as thick.
STEP 5: Activate the Select tool and double-click on
the sphere to open its properties box. Change the
charge on the sphere to ‘-100.0’. Click on ‘OK’ and
the sphere should turn blue.
STEP 6: To check your thinking, select the sphere and move it toward the
objects. Be careful not to touch them.
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Activity 6 Homework
Do the two objects become charged in the way you predicted? If not,
sketch the simulator arrangement below.
Yes.
Use our model of electric charges to explain how the objects became
polarized in this way.
The negative charges of the first object are repelled by the negative charges of the sphere and
move to the side furthest from the sphere, giving that side a net negative charge. Since the
positive charges are unable to move, the negative charges moving away from the side closest
to the sphere leaves that side with a net positive charge.
The negative charges of the second object are repelled by the negative charges of the first
object and move to the side furthest from the blue side of the first object, giving that side a net
negative charge. Since the positive charges are unable to move, the negative charges moving
away from the side closest to the blue side of the first object leaves the side closest to the first
object with a net positive charge.
Like the uncharged objects in this simulator exploration, hexane particles are
neutral and unpolarized. However, like the object closest to the sphere, when
a hexane particle interacts with a polarized or charged object, it becomes
polarized. The polarized hexane particle subsequently affects those hexane
particles closest to it, causing them to become polarized as well, just like the
second object in the simulator. When the polarized or charged object moves
away, the hexane particles return to their neutral unpolarized state. Many
times, random fluctuations in the arrangements of the charges of a particle
cause it to become temporarily polarized. It then polarizes its neighbors
before its charges return to an unpolarized arrangement. For nonpolar
materials like hexane, this “induced” or temporary polarization of particles is
responsible for the electrostatic attractive forces between particles.
STEP 7: Open Cycle 5 Activity 6 HW Setup 2. In this setup,
you will see two uncharged, but polarized objects that are
close together, but not touching. These objects represent
polar particles like water.
STEP 8: Run the simulator.
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Cycle 5
How are the charges arranged in the two objects? How is this similar to
the objects from the previous simulator setup? How is this different?
Alternating red-blue-red-blue or positive-negative-positive-negative. This is how the objects
looked in the previous simulator when they were polarized. In the previous simulator, they were
only polarized when a charged sphere was near, but in this simulator there is no charged
sphere is necessary to make them polarized.
What is the overall charge of the objects? How do you know?
Zero. The width of red and blue lines are equal, signifying equal magnitudes of positive and
negative charges.
How strongly are do you think these objects attracted to each other
compared to the objects from the previous simulator setup? Explain
your reasoning.
Stronger. The electrostatic attraction always exists because they are permanently polarized, as
opposed to just when a charged object is nearby and polarizing them.
Particles of polar materials, like water, are permanently polarized due to the
arrangement of charges of in their particles. Because water particles are
permanently polarized, water particles have longer and more frequent
interactions that are responsible for the electrostatic attractive forces between
particles.
The macroscopic observations and simulator explorations you have made in
this cycle can help you make inferences about the particles of polar and
nonpolar materials. You have already investigated some of these inferences
using the Small Particle Model simulators.
Does a polar material have weaker or stronger electrostatic attractive
force between its particles as compared to a nonpolar material with
particles of similar mass? Explain your reasoning.
A polar material would have stronger electrostatic attractive forces between particles because
they have longer and more frequent interactions (they are always polarized and are equally
polarized). The particles of nonpolar materials are not always polarized. The polarization of
one particle depends on the polarization of another particle, etc. and the magnitude of
polarization appears to not be the same for all particles.
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Activity 6 Homework
Are the electrostatic attractive forces between the particles of a polar
material and a charged object weaker or stronger than those of a
nonpolar material with particles of similar size?
A polar material would have stronger electrostatic attractive forces with a charged object
because they have longer and more frequent interactions (they are always polarized and are
equally polarized). We saw this in the experiment with the liquid streams and the charged
wand (water is polar but hexane is non-polar).
Classifying materials as polar or nonpolar allows scientists to predict which
materials will dissolve readily in water and which will dissolve readily in
other, nonpolar solvents. A general rule for solubility is: like dissolves like.
That is, materials with permanently polarized or electrically charged particles
dissolve readily in water and other polar solvents; and materials with
temporarily polarized particles dissolve readily in hexane and other nonpolar
solvents.
Given that salts (e.g. table salt, Epsom salt, etc.) dissolve in polar
solvents but salts are not polarized, what could you infer about the
particles of salt when they are dissolved in water? (hint: read the
preceding paragraph carefully!)
They must have electrically charged particles.
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