Download 8. Attraction - Calvin College

2 - 8 ATOMIC AND MOLECULAR ATTRACTION
ATOMIC AND MOLECULAR ATTRACTION
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PART 1
DROPS ON A PENNY
Assemble the following materials: three pennies, two eye droppers, several paper towels, a small cup
of water, a small cup of alcohol, and a small cup of liquid soap. Rub the pennies “clean” with a
paper towel to remove any oil that might be on their surfaces. After this, try not to handle the
pennies with your bare hands as this will deposit additional oil on their surfaces.
1. Place one of the pennies on a paper towel. Predict how many drops of water you will be able
to place on the penny without getting the paper towel wet, and write your prediction here:
2. Now place drops of water on top of the penny, one drop at a time, and note the shape of the
water on the penny as the water increases in volume. Continue adding(and counting) water
drops until the water spills onto the paper towel. Record the number of drops here:
3. Draw the shape that the water took on top of the penny as the drops began to build up.
4. You can explain the shape of the water on top of the penny based on a model which says that
liquid water molecules attract each other. Make such an explanation below:
5. Repeat the procedure given above, only this time place alcohol drops on a different penny.
Record your results below. Make sure the drops of alcohol are about the same size as
the drops of water you used before. (Note: the alcohol drops tend to be smaller than the
water drops. You might have to compensate for this by occasionally adding an extra drop of
alcohol to make the comparison fair.)
6. Based on your observations, which has a greater attractive force: one water molecule on
another, or one alcohol molecule on another?
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7. Now mix several drops of liquid soap into the cup of water. Using the third penny, determine
how many drops of the soap/water mixture can be piled on top of the penny before it spills on
to the paper towel. Record your results below, and comment on what the soap seems to have
done to the attractive forces between the water molecules.
PART 2
SURFACE TENSION
Using a set of tweezers, obtain two small metal pins from the supply area. Be very careful NOT to
touch these pins with your hands, as you will get oil on the pins and ruin the next activity. Place
these pins on a paper towel until you are ready to use them. Rinse out your cup of water
thoroughly so as to remove all soap traces, and then refill the cup with water.
1. Using the tweezers, carefully place one of the pins on top of the water in your cup. If you are
very careful you can get the pin to stay on top of the water and not sink to the bottom of the
cup. (Please note: your pin is not floating on the water. Solid metal is too dense to float.
Something else is occurring. Notice how the surface of the water around the pin is being
distorted. In order to prove that the pin is not floating, you may push the pin under the surface
of the water and see that it does not come back up.)
Explain this surface tension effect based on your results in Part 1.
2. Based on your results in Part 1, do you think it will be just as easy or perhaps harder to get a
pin to stay on top of the alcohol in your other cup? Make a prediction below, and explain your
reasoning.
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3. Now use your tweezers and see if a pin will remain on top of alcohol. Discuss your results
below.
4. Which do you think might evaporate quicker at room temperature, water or alcohol? Use your
model of molecular attraction to explain your prediction. (Note: There are other factors which
affect the rate of evaporation of a liquid besides the magnitude of the molecular attraction.
Included in these factors would be the mass of the molecules involved.)
5. Place one drop of water and one drop of alcohol on the table and see which evaporates first.
You may wish to lightly blow over the tops of the drops to speed the process. Write down
which one evaporated first, and compare this result with your prediction.
PART 3
DISRUPTING SURFACE TENSION
You have already seen in Part 1 that you can sometimes change the magnitude of the molecular
attraction existing between two molecules by introducing a third, different molecule. You can
demonstrate this dramatically with soap and water.
1. Again using the tweezers, carefully place a pin on top of the water in your cup.
2. Dip one of your fingers into a small bit of liquid soap, and then gently touch the surface of
the water with your soapy finger. Describe what you observe.
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3. Did the soap increase or decrease the surface tension of the water?
4. Did the soap increase or decrease the molecular attractions of the water molecules?
5. Give an explanation of how adding soap to water helps you to clean things better than when
you use water alone.
6. Obtain a clean cup containing no soap residue. Fill it half full with water, and sprinkle pepper
on the top of the water. Next, dip a finger into some liquid soap, and then gently touch the
surface of the water near one edge of the cup with your soapy finger. Watch the pepper move
around for a minute or two.
7. Prepare a Share Sheet with others at your table which explains what you observed with the
pepper, soap, and water. (Do not simply describe what you saw, but give a reason for what
you saw based on molecular attraction.)
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Optional
Tie a piece of thread into a loop, and float the loop in a cup of water containing no soap
residue. Add a drop of liquid soap inside the loop and describe what happens. What is the
explanation?
Rinse out the container, and float another loop of thread on top of the water. This time add a
drop of liquid soap outside the loop. Again describe what happens and offer an explanation.
PART 4 LIQUID TO SOLID ATTRACTION
So far you have seen that molecules in the liquid state attract each other. However, attraction
between molecules tends to occur whether the molecules are in the solid, liquid, or gas state.
The next activity demonstrates the attraction between molecules in the liquid state and those in
the solid state.
Activity #1
Spill a drop of each of the following
liquids down a waxed paper incline:
a. Vegetable oil
b. Water
c. Soapy water
d. Alcohol
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Draw a diagram to show the kind of trail left by each drop. Describe which of the liquids
seems most attracted to the waxed paper, and which of the liquids seems least attracted to the
paper.
Activity #2
Dip one end of a long strip of paper towel into a cup of water, and watch the water “climb” up
the towel. Explain what you see in terms of the solid/liquid attraction between water and paper,
and the liquid/liquid attraction between water molecules.
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Particle Attraction Content Overview
The tendency of water molecules to form droplets shows that the individual molecules of water tend
to attract each other. Another example of the attraction molecules have for each other can be
demonstrated by trying to stick your hand through your tabletop. See how well those molecules in
the tabletop stick together?
Many of the features of solids, liquids, and gases can be explained by combining our knowledge of
this attraction between molecules with what we know about temperature and motion. To illustrate,
let’s consider the three phases of water. At low temperatures, water forms ice. Water--and many
other solids--form crystals in which the individual molecules are arranged in very specific patterns.
In a solid the attraction between the molecules holds each individual molecule in its “assigned”
place. The particles can vibrate some, but they can not move around throughout the solid. In other
solids (not crystals) the molecules are arranged somewhat randomly, but they are still held in place.
If a solid is heated, the particles increase their random motions, and in particular they vibrate more
vigorously. If the temperature is increased enough, the molecules can escape their “assigned”
locations and begin to move about more freely. However, because of their continual attraction for
each other, they may not yet be moving fast enough to escape completely from other molecules.
Instead, in a liquid, the individual molecules can move anywhere within the fluid, but they may not
leave the region of the fluid. (One model of a liquid is bowling balls rolling around on a
trampoline. Each ball is free to move around on the trampoline, but because of the collective weight
of the bowling balls, they will tend to stay near the center of the trampoline. Of course, for
molecules, it isn’t gravity that causes them to stay together. Molecular attraction is an electrical
effect.)
If the temperature is increased still more, the individual molecules can get moving fast enough to
escape the attraction to other molecules, and they can begin to move completely independently.
(Think of the bowling balls on the trampoline now moving fast enough to roll off the outer edge of
the trampoline.) In this case we have a gas, and if the particles are in a container they will fill the
entire container.
It is reasonable at this point to ask what causes the attraction between atoms and molecules. The
answer to this question is electric charge. In order to understand electric charge, you must first
modify the particle model you have been working with and note that atoms themselves are made up
of even smaller particles called neutrons, protons, and electrons.
Protons are said to have a positive charge, and electrons are said to have a negative charge. Our
extended model for atoms then says that two different kinds of charges (a positive and a negative)
will attract each other, but, two similar charges (two negatives or two positives) will repel each other.
Neutrons (which is a term derived from the word neutral) do not have any charge. Therefore, they
do not cause any attraction (or repulsion) between atoms. It is the protons and the electrons which
cause the attraction (and sometimes repulsion) between two atoms or two molecules.
An atom’s protons and neutrons are held tightly together near the center of the atom (called the
nucleus). The electrons form some sort of cloud whirling around the nucleus. The electrons
usually can not stray very far away from the nucleus because the positive charges of the protons
attract them towards the nucleus.
Most atoms are electrically neutral - having equal numbers of electrons and protons. However,
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sometimes an electron or two can be pulled away from the atom, and then the entire atom is left with
an excess positive charge. On the other hand, sometimes an extra electron or two can be made to
attach to an atom. In this case the atom is left with an excess of negative charge.
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