WC-12: WATER IS WET
MATERIALS LIST
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Plastic plates
Container of water
Paper towels
Sponge
Spray bottle with water
Capillary tubes
Test tubes
Water tray
Rocks, sponges, corks, painted pencil pieces, wood pieces, and styrofoam bits
Eye dropper
Small containers
Plastic bottle
1 jar
Pennies
Red food coloring
WATER IS WET
WC – 12
page 1
WATER IS WET
WC – 12
FALL 1994
Jane Hall/Ralph Johnson
OBJECTIVES:
Introduce the water concept known as adhesion and the concept of capillary action.
GRADE LEVELS: 2 – 6
FOCUS WORDS:
Adhesion, cohesion, capillary action, wetting, evaporation, hypotheses
BACKGROUND INFORMATION:
Water molecules are attracted to each other – cohesion, but water may also combine with
certain other molecules – adhesion.
Capillary action is the name given to raising or depressing the level of liquid in a small bore
tube relative to that of the liquid reservoir it is placed in. It rises because the liquid is more
attracted to the tube surface compared to the liquid itself. The water molecules are also
attracted to the oxygen atoms in the glass tube. Thus, for water or blood in glass capillaries,
the fluid readily wets the glass and is pulled up into the tube. For mercury in glass tubes, the
mercury does not wet the glass tube, and its level in the tube is lower than the reservoir
level. It would not go into the tube at all, but the pressure difference between the reservoir
and tube levels forces some mercury into the tube. Coating a glass capillary with oil or
grease will decrease the water-to-glass attraction, leading to a lower level of liquid in the
coated tube. Thus, strong capillary action depends on strong attraction between the liquid
and the tube surface. Weak interactions give very little capillary action, while repulsive
interactions give capillary depression. Notice that a smaller capillary has a very large ratio
of surface area to contained liquid (surface to volume).
Thus, a small capillary can pull liquid much higher than a large tube, since the attractive
forces are the same but the volume/weight of the lifted liquid is much reduced. This is how
water gets to the top of tall redwoods.
WATER IS WET
WC – 12
page 2
ACTIVITY:
TIME 15 – 20 minutes
SAY:
Water has so many different properties that we take for granted. One of these
properties is water's wetting ability or adhesion.
ASK:
What does it mean to be wet?
RESPONSE: Encourage all types of responses.
EXPLAIN: When we say that something is wet what we are saying is that water is sticking
to it or adhering to it. If I spray your hands with water, your skin gets wet
because water is sticking to them.
DO:
Spray your hands with water (using the spray bottle) and show the children
how the water sticks to your hands. Spray the children's hands with water only
if they want to be sprayed.
DO:
Tell them to study (observe) the water sticking to their hands.
EXPLAIN: Water likes to wet certain things more than others.
ASK:
If you wipe your hands with a paper towel what will happen to the water on
your hands?
RESPONSE: It will wet the paper towel.
DO:
With a paper towel, slowly wipe your hands, showing the children how the
water wants to stick to the paper more than your hands.
DO:
If you have wet any of the children's hands then hand out paper towels.
DO:
Have the children wipe their hands slowly and have them pay close attention to
what is happening to the water.
SAY:
It seems that water likes to wet paper towels more than skin.
NOTE:
This is partially due to capillary action which we will discuss later.
ASK:
What would happen if water did not stick very well to paper or cloth?
EXPLAIN: Water does not wet plastic very well. Think about trying to dry your hands with
a plastic bag. It wouldn't work very well. If water spilled on the floor, and
water would not stick to your mop, we couldn't just mop it up. We would have
to vacuum it up or heat it until it evaporated.
SAY:
Let's try an experiment. let's see what water likes to wet most.
DO:
Have a few plastic plates partially filled with water and a few dry plates ready
for the children.
DO:
Have the children fill out the "Which is Wetter" form. The children should try
to guess which item water will wet or adhere to the most.
DO:
Put a #1 by this item. Then pick out the next item that water will wet the most
and mark this #2. Continue making the list.
DO:
Have the children dip their 'trinkets' in water.
DO:
Conduct the experiment along with the children.
WATER IS WET
WC – 12
page 3
DO:
After wetting the items begin pulling them out of the water. Shake the items off
and place them on a dry plate.
NOTE:
The wettest item will be the sponge, then comes the rock and/or the stick, then
the cork, and finally the pencil and/or the Styrofoam bits.
DO:
Have the children compare the results of the experiment with their hypotheses
(their marked guesses) on the sheet of paper.
DO:
Discuss with the children their responses. Compare observations.
ASK:
Why do you think people paint their houses?
RESPONSE: To keep the rain from wetting the wood and rotting it.
ASK:
Why do people use sponges to clean up water?
RESPONSE: Because water loves to wet sponges more than it loves to wet most other
things.
DO:
Pass out small test tubes with about one inch of water in them.
NOTE:
You put the water in ahead of time and use them for each group.
SAY:
Look at the surface of the water in the test tube.
ASK:
What does it look like?
RESPONSE: It's curved and looks like a contact lens. – It looks like the water is trying to
climb up the glass.
EXPLAIN What we see happening here is due to a combination of properties of water.
The water sticks to its self (cohesion) and forms a skin which is strongest
around the edges of the glass (see Water Skin) and the water coats (adhesion)
the glass. We call the movement of the water capillary action. A capillary is a
small hollow tube that water climbs up.
SAY:
I have here a little thin glass tube called a capillary tube.
DO:
Place one capillary tube in each student's test tube.
SAY:
Look carefully at what is happening to the water in the little tube.
ASK:
What is happening to the water in the tube?
RESPONSE: The water is climbing up the capillary tube. – The water in the capillary tube
is higher than the water in the test tube.
SAY:
Yes, it is creeping up the walls of the capillary, and if the tubes were thinner, it
would climb up even higher.
SAY:
Remember how water wetted our sponge more than it did the other items we
sprayed?
ASK:
Can you think of why the sponge becomes wet and soaks up more water than
the other items we experimented with?
RESPONSE: Because the sponge is filled with millions of capillary tubes.
EXPLAIN: When water touches the sponge, it immediately starts to move through these
capillaries like it did with our capillary tubes here.
WATER IS WET
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page 4
ASK:
How do you empty a sponge?
RESPONSE: Squeeze it.
ASK:
What does squeezing do to the sponge?
RESPONSE: It squeezes the water out of all the little capillaries in the sponge.
ASK:
Can any of you think of how trees get water to their leaves?
RESPONSE: There are capillary tubes in the roots, stems and leaves of a tree.
ASK:
What would happen if you scraped your knee on the ground?
RESPONSE: You would bleed.
EXPLAIN: Even though you didn't cut or scrape through your veins, you still bleed. This
happens because your body is able to get blood around to all the cells in your
body through very tiny capillary tubes. Your heart is not strong enough by
itself to get the blood everywhere it needs to be. Without capillary tubes in our
bodies we could not live.
EXPLAIN: Other uses of capillary action include wicks in candles, water in soils, sponges
soaking up water, etc.
DO:
Collect back the test tubes with the capillary tubes. Between stations, remove
the capillaries, and use a new dry set. After the presentation you can blow out
the water and dry them off.
Have students rate the "wetness" of the different items listed on the table on the next page.
Which do they think will get wettest, absorb or cause water to adhere to the surface?
Students will rate the items according to how wet they guess (hypothesize) the items will
get. Mark the item you think will get most wet a #1. Then mark the item you think will get
almost as wet a #2. Continue to rate all the items. Be prepared to discuss why your have
rated the items the way you have.
WATER IS WET
WC – 12
page 5