WC-13 Water's Skin
MATERIALS:
Water molecule model
Container of water
Plastic plates
Water tray
Paper towels
Sponge
Cheesecloth-covered jar
Plastic basket
Pennies/eye droppers
Capillary tubes
Light wood piece to represent a boat or toothpicks
Liquid soap
Photo of water bug
WATER SKIN
WC – 13
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WATER’S SKIN
WC – 13
SUMMER 1998
Jane Hall, Ralph Johnson
OBJECTIVES:
To introduce the idea of cohesion—the property of water that causes molecules to
"stick" together and adhesion—water's property of wetting or adhering to other
materials.
GRADE LEVELS:
4–6
FOCUS WORDS:
Cohesion, adhesion, surface tension, water skin, molecule, atom, oxygen,
hydrogen
ACTIVITIES:
TIME NEEDED: 15–20 minutes
EXPLAIN:
DO:
If you take a drop of water and divide it up until you have the smallest
possible water particle, you would have one molecule of water.
Show them a water molecule model.
SAY: This is a model of a water molecule.
EXPLAIN:
This model gives you an idea of what a water molecule might look like.
Of course it is made this large so we can see it. The actual size of a
molecule of water is so small that it would take more than you could
ever count to form a tiny drop of fog, and even that is so small that it is
hard to see.
EXPLAIN:
This molecule has three parts. This larger black ball is the oxygen
atom, and these two smaller white balls are hydrogen atoms. I can
break this molecule down further by removing the hydrogen atoms.
DO:
Remove the two hydrogen atoms.
ASK: When I remove the hydrogen atoms do I still have water?
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RESPONSE:
No.
EXPLAIN:
To make a molecule of water I must have two atoms of hydrogen and
one atom of oxygen. The scientists write this as H2O and it is
pronounced H-two-O. Any other combination would not be water.
EXPLAIN:
If you take several water molecules and put them close together they
tend to attract each other. They want to stick together. This is the
property of water called cohesion.
DO:
Place all six molecule models on the table. Bring them closer together
until the magnetic black (or blue) ball (oxygen) attracts another one,
then continue until they are all in a tight bundle.
SAY: This is similar to how water molecules cohere (stick to each other).
SAY: When enough water molecules join together, they begin to become
visible. You can see them as in mist or a droplet of water.
DO:
Have pairs of students add a couple of drops of water on to a Styrofoam
or plastic plate.
SAY: Notice that the droplets are round.
SAY: Raindrops are round, like a ball or a marble. So are droplets of mist,
and any other drop of water in the air.
SAY: If a few drops of water form a small puddle on a flat, level surface, the
puddle will tend to take a circular, (round) shape.
SAY: If you look at that small puddle or your drop of water from the side you
will see that the edges are round and higher in the middle.
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Drop of water seen from top
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Side view of a puddle or water
drop. Notice how the
water bulges at the center.
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SAY: If I pour a little more water into the puddle it will spread out, but it will
stay together.
DO:
Drop a few more drops into the puddle.
SAY: As you add drops of water to this puddle watch how the shape of the
puddle changes.
ASK: Can you describe what changes are taking place as more drops are
added to the puddle?
RESPONSE:
DO:
Have the children express what they see.
Point out that even as the circle becomes uneven the edges remain
smooth.
EXPLAIN:
Each molecule of water in this puddle attracts all of the other molecules
in the puddle, almost as though they were all magnets. They attract
those molecules alongside of them, above them, below them, and
everywhere around them. This attraction is called cohesion. The
molecules on the surface of the puddle have no molecules above them,
so they use all of their strength pulling on the molecules below them.
This gives additional cohesive strength on the surface of the puddle.
That extra cohesion behaves like a skin or film holding the puddle
together, We call this skin surface tension.
EXPLAIN:
Even though the cohesion is strong enough to hold the puddle together,
it only has a certain amount of strength. If it was stronger the puddle
would be held higher. If it was less strong the puddle would be less
high.
ASK: Have you ever seen an insect walking on water?
SAY: Next time you are near a slow moving creek, or a sheltered part of a
pond, look for these insects. They are called water striders.
EXPLAIN:
WATER SKIN
These insects actually do walk on water. They can do this because they
are so light that their feet do not break through the "skin," or surface
tension. They stay high and dry as they skate on the water.
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DO:
Put the plate aside. Take the Pyrex or plastic dish and fill it with an
inch of water.
SAY: Let's take another look at the water skin.
DO:
Place one of the dry plastic baskets on the water in the dish.
SAY: As you can see, even with the holes in the bottom of the basket, it does
not sink.
ASK: Why does it seem to float?
RESPONSE:
Like a water strider, it is not heavy enough to break the water's skin.
SAY: It is not actually floating. If you look closely at the water's surface near
each plastic strip, you will see that the weight of the basked bends the
surface skin down a little bit.
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EXPLAIN:
The basket is resting on the skin much like you could rest on a water
bed. The fabric of the water bed works the same as the surface tension
of the water, except it is very much stronger. The basket is using the
same principle as those insects we call skaters.
Wa ter
su rface
Wa ter
su rface
Surface Ten sion dep resse d
by wei ght of b asket
ASK: How can we make the basket sink without touching it?
RESPONSE:
DO:
Break the skin.
Ask a student to tear off a small piece of paper towel or facial tissue.
Gently place the tissue in the bottom of the basket so that at least a
small amount touches the water surface. Allow some time for the paper
to wick up water. Then watch the water start to "leak" into the basket.
The water will wet the inside of the basket bottom, and then the basket
will sink. This process could take some seconds.
SAY: Now you can see that the basket was not actually floating. When the
skin was broken the basket sank.
EXPLAIN:
Another of the properties of water is its ability to "wet." We call this
adhesion. Water wants to wet or adhere to some things more than it
wants to hold them up with surface tension. When the paper was placed
in the plastic basket it also touched the water. The water starts to wet
the paper. In wetting, the water wicks up the paper like the wick of a
lamp brings fuel to the flame. When enough water has been wicked up,
it wets the floor of the basket. This is like a leak in the skin, and water
keeps flowing in until the basket sinks.
SAY: Now we have a trick that we will try to figure out.
NOTE: This is often done as a magic trick.
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SAY: I have this glass jar which is covered with cheese cloth. I can pour
water through the cloth and into the jar.
DO:
Have one or two students pour some water into the jar. About two
inches deep should be maximum.
DO:
Hold your hands and the jar over a dish. Mumble a couple of
'abra kadabra's' and turn the jar over. The water should not pour out.
ASK: Why can we pour water into the jar, but we can't pour it out?
RESPONSE:
There will be many guesses, such as "No air can get in, so no water can
get out.
EXPLAIN:
The idea of creating a vacuum is a good one; a vacuum is created. But
we are studying surface tension. The smaller the openings, the stronger
the surface tension. The space between the threads of the cheesecloth is
small. The cheesecloth supports the weight of the water, but the water's
skin holds the water in.
DO:
Turn the jar on its side and pour out some of the water.
ASK: Why does the pour water pour out when I turn the jar on its side?
RESPONSE:
Because when the jar is tipped on its side the surface tension is broken;
the water can get through.
ADDITIONAL ACTIVITIES:
If you have time, have students make a hypothesis (guess) of how many
drops of water they can get on the head of a penny. Each student will
write their guess on a scrap of paper. Then, they each get a penny,
eyedropper and some water. They keep count of the drops they can put
on the penny until no more can fit on the penny.
Did everyone get the same number of drops? Why or why not? Did
they get more or less drops than they predicted? Why or why not?
What causes so many drops to “stick” to the penny’s surface? (Surface
tension, water’s cohesion.)
Can they think of any things that might change the number of drops
that will stay on the penny’s surface. (They could change the penny’s
surface.)
Place a plain toothpick or piece of wood in the tray with water in it.
Ask them if they can think of a way to get a toothpick or piece of wood
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to move through the water in the tray without touching it or blowing on
it.
After they give some answers, if no one has thought of it, take the
wood piece or toothpick and, before you place it back in the water, put
a drop of detergent on one end. Gently place it at on the water at one
end of the tray, it will move forward for a short time. (Or, using an
eyedropper, drop a small amount of soap on one end of the wood. ) Ask
them why it moves. (The soap breaks the water’s skin and this helps
push the wood [toothpick] forward. When the soap has been dissipated
too much the skin won’t be broken and the wood stops moving
forward.)
How does water move in plants? (Through small tubes – xylem cells)
How doe water defy gravity and move from roots up to leaves? (Partly
through cohesion, water skin, adhesion, water adheres to the sides of
the tube and from the change in pressure caused by transpiration and
osmosis.)
Use the capillary tubes to demonstrate how water can “climb” the walls
of the capillary tube. It is helpful to use colored water for this exercies.
As the students put a tube in the tray of water they will notice that the
water’s level goes higher than the water in the tray. Ask them to
explain this.
Another activity is to float a clean, rust free needle on top of clean
water. Use tweezers to place it flat and gently on the top of the water.
See extra activities Defying Gravity or Water Polo
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