Have you ever seen a small insect walking on the surface of water in a pond?  If you looked carefully, you may have noticed that the water seemed to bend downward at the bug’s feet.  The bug wasn’t “floating” on the water.  Instead, it was walking on a thin film of tightly packed water molecules.  This thin film is called “surface tension”.  This next series of experiments explore the property of surface tension of water.

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Observing Surface Tension

Materials Needed: Small container such as bowl,  jar or glass; straight pin; small piece of paper towel, fork.

Procedure:   Rinse the container with clean water to remove any soap film that may have been left from washing.  Fill the container almost full of water.  Make sure the pin is clean and dry.  Place a piece of paper towel just a little larger than the pin on the surface of the water.  Carefully place the pin on the paper towel.  Push down gently on the paper towel edges with the fork to make the towel sink, being careful not to touch the pin.

Remove the pin with the fork. Dry both the pin and fork, but do not touch them with your hands.  Now carefully try to place the pin on the surface of the water without using a paper towel, by gently lowering the pin onto the water’s surface with the fork.  This may be a little more difficult, but it can be done.

What To Look For: Carefully notice the water around the pin in each case.  What do you see?

What Happened: When the paper sank, the pin stayed on the surface of the water. If you looked carefully, you saw that the water appeared to be pushed downward around the edges of the pin.  If you had a steady hand, you were also able to make the pin rest on the water without the paper towel.  In fact, the only thing the paper towel did was to hold the pin level on the surface.

In both cases, the pin rested on the surface of the water because of surface tension.  The specific gravity of iron is far greater than 1, and if it were not for surface tension, it would sink.

Going Further: Can you place other objects on the surface using surface tension?  Try a paper clip or a plastic strawberry basket.

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Observing Surface Tension Another Way

Materials Needed: Two drinking glasses or similar containers; dropper or drinking straw; water.

Procedure: A little water will be spilled in this experiment, so it is a good idea to do this on a counter top near a sink.  You should probably also have a towel handy to wipe up small spills.

Fill one of the containers all the way to the brim with water.  Put some water in the other container and begin adding water from this container to the first one by drops. 

If you are using a straw, stick the straw in the water, place a finger on one end, and remove the straw.  With a little practice, you can drop water one drop at a time by quickly removing your finger and replacing it.  You may want to practice this first until you get the hang of it.

In either case, keep adding water until water begins to run down the side of the container. Pay close attention to the surface as you add water, by looking at the surface at eye level.

What Happened: You were able to add a surprising amount of additional water to the already full glass.  As you added water, the surface began to bulge upward.  This bulge upward was caused by surface tension.  The surface tension acts like a “skin” to hold the water together until the weight of the water becomes stronger than the surface tension, and causes it to spill over the side of the container.

Going Further: Try adding clean paper clips or pennies instead of water drops to a full container of water.  See how many you can add before the container overflows.

Materials Needed: Small bowl or similar container; water; ground pepper; bar of soap.

Procedure: Fill the bowl about 3/4 full of water.  Sprinkle some ground pepper on the surface.  Place a corner of the bar of soap near the edge of the bowl.  What happens to the pepper?

What Happened: The pepper rested on the surface due to surface tension. Soap cleans in part by weakening surface tension. When the soap was placed in the water, it weakened the surface tension immediately surrounding it, and the pepper was drawn away to where the surface tension was still strong.

Going Further: Try this same experiment using baby powder instead of pepper.  Also, what happens if you leave the container undisturbed for a while?

Materials Needed:  Small bowl or similar container; water; ground pepper; cooking oil, tooth pick.

Procedure:  Fill the bowl with water and sprinkle some ground pepper on the surface, as you did in the last experiment.  Dip the toothpick into the cooking oil and place the oily end near the edge of the bowl.  What happens to the pepper this time?

What Happened: Just as soap weakened surface tension, so did the oil.

Going Further: Try this experiment using baby powder instead of pepper.  Does it make any difference?

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Making a Surface Tension Boat

We can use what we have learned in the last two experiments to make a neat little boat powered by weakening surface tension.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Thin cardboard; scissors; soap; tooth pick; cooking oil; bath tub or sink filled with about 3 cm (1 in) or so of water.

Procedure:   Cut out two small boats from the pattern shown.  Use your fingernail to scoop out a pea sized piece of soap from a bar of soap.  Push this soap into the notch in the back of one of the boats.  Place the boat in the water.  What happens?

Remove the first boat and place the other boat in the water.  Dip the toothpick in cooking oil and place a drop of oil in the back notch.  What happens?

What Happened: Both boats moved forward through the water because they rested on the water due to surface tension.  Both the soap and the oil weakened the surface tension behind the boats, and the stronger surface tension pulled the boats forward.

Going Further: How long will these boats keep moving in a bathtub or sink?  What will cause them to stop moving?

Materials Needed: Small bowl or similar container; new rubber band; bar of soap.

Procedure:   Most new rubber bands are curved at either end and are shaped a little like a race track or a flattened oval.  This is the type you want to use.  Fill the bowl about 3/4 full of water and place the rubber band on the surface.  Touch the bar of soap in the center of the rubber band.  What happens?

What Happened: The rubber band spread out into a circle or nearly so.  The soap weakened the surface tension inside the rubber band, but could not get to the water outside.  The stronger tension on the outside pulled the rubber band outward in all directions, giving the rubber band the circular shape.

Going Further: You probably can predict the result without doing the experiment, but just for fun, try this using cooking oil instead of soap.

Materials Needed: Small container such as a bowl,  jar or glass; straight pin; small piece of paper towel, fork; liquid soap such as dish detergent; spoon.

Procedure:   Fill the container almost full with water.  Using the paper towel and fork, place the pin on the surface of the water as you did earlier.   Add a drop of soap to the surface.  Keep adding soap until the pin sinks.

Remove the pin with the fork. Rinse both the pin and fork to remove the soap.  Stir the water gently with a spoon.  Try to rest the pin on the surface.  Can you do it now?

What Happened: When you added the first drop of soap, the pin may have moved away from the soap, and you should have expected that to happen.  If the soap dissolved quickly enough, the pin may have even sunk without any more soap being added.  In any case, two or three drops was probably enough. 

Once the soap was mixed with the water, the surface tension was weakened to the point that you could not rest the pin on the surface.  Soap cleans in part by acting on water to reduce surface tension which tends to hold dirt. 

Going Further: Do this experiment with cooking oil instead of soap.  What is the difference?

These next experiments will show an interesting property of water known as “capillary” action.

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Where Does It Float?

Materials Needed: Small container with straight sides such as a plastic film container or test tube; paper; tweezers; water.

Procedure:   Fill the container almost full with water.  Notice how the water curves up around the edges.  Tear off a small piece of paper and use the tweezers to float it on the water.  Where does the paper move?  Remove the paper.

Carefully fill the container so that the surface of the water is exactly level.  (A dropper may help you here.)  Again, float a piece of paper and observe where it goes.  Remove the paper.

Finally, fill the container so that the water’s surface is bulging slightly over the top of the container.  Carefully float another piece of paper and observe where it goes.
 
What Happened: When the water level was below the top of the container, the edge of the water curved upward due to a property called “capillary action”, and we will learn more about it in the next two experiments.  When the paper was floated the first time, the paper was drawn to the highest water level, which was along the edge.

When the water level was even, the paper was not drawn in any particular direction.

When the water level was above the top of the container, the paper was again drawn to the highest point, but this time, the highest point on the water’s surface was the center.

A floating object will tend to move to the highest point on the water’s surface.

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Capillary Action

Materials Needed: Several small tubes of different diameters (see procedure); clear container such as a glass or jar; water; food coloring or instant coffee (optional).

Procedure: For tubes you can use soda straws that are clear enough to see liquid through, a clear ball point pen barrel, or clear plastic tubing or glass tubing (borrowed from your school lab).  The important thing is to get at least two different inside diameters.  (The diameter is the width of the tube from one side to the other.  The inside diameter is the width from one side to the other as measured on the inside of the tube.)

Fill the container almost full of water.  If the tubing you are using is difficult to see through, you may want to add a drop of food coloring or a pinch of instant coffee to make the water easier to see.

Place the widest tubing you have down into the water.  Hold it straight up. Observe the water level inside the tube.  Also notice the shape of the water inside.

Do the same thing for the other tubes you have.  If you like, you can place them all in the  container at the same time to make it easier to compare.

What Happened: You should have noticed that the water level in the tubes were slightly higher than the water in the glass.  The smaller the diameter, the higher the water level in the tube.  What you have observed is called capillary action.   Water molecules are attracted to molecules of many different materials such as glass or plastic, and they tend to be drawn up the sides of a container made of such materials.  This process of water being drawn up by other materials is called capillary action. In the case of a tube, the smaller the diameter of the tube, the higher the water will be drawn. 

Capillary action explains in part how a plant is able to get water from it’s roots to through the stem and to the top of the plant.

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Capillary Action Along a Flat Surface

 

Materials Needed: Two glass microscope slides or two small rectangles of flat clear plastic cut to the same size as a microscope slide from a “blister” pack; rubber band;  toothpick;  clear container at least as wide as the pieces of glass; water; food coloring or instant coffee (optional).

Procedure:  Place the two pieces of glass or plastic together, with the toothpick between them along one edge.  Place the rubber band around the glass plates as shown.

Put about an inch of water in the bottom of the container.  You can and add a drop of food coloring or a pinch of instant coffee to make the water easier to see if you want. Place the glass assembly into the water and observe the water level inside the glass pieces.

What Happened: The water level between the glass pieces rose as a result of capillary action.  The water level was higher where the glass pieces were closer together, just as the water in the previous experiment rose higher in the smaller diameter tubes.

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Capillary Action In Plants

   

Materials Needed: Celery stalk with leaves; small jar; water; food coloring; knife.

Procedure:   Fill the container about 3/4 full of water and add a few drops of food coloring.  If you don’t have food coloring try using unsweetened tea or coffee that has been cooled.  Cut about 1 cm (1/4 in) off the bottom of the celery stalk and place it in the colored water. Leave it in a well lighted place for a day.  What do you see?

Remove the celery stalk and rinse with water.  Cut the celery stalk about half way up.  Examine the cut area. What do you see?

What Happened: Water was drawn up into the plant through tiny tubes by capillary action.  The colored water allowed you to see this clearly.  When you cut the celery stalk, you saw colored areas inside the stem.  These colored areas are bundles of even smaller tubes that are called “vascular bundles”.  These vascular bundles move the water up the stem of the celery.

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Using Capillary Action to Clean Water

Materials Needed: Two jars; water; dirt; strip of cloth; support for one of the cans (see procedure).

Procedure:   Fill one of the jars about 3/4 full of water.   Add some fine dirt to make the water muddy.  Place this jar on a support such as a brick, a block of wood or several books.  Place the other jar below the first jar as shown.  Put one end of the cloth in the dirty water and the other end in the empty jar.  Watch what happens as you allow this to sit for a few hours. 

What Happened: Water was drawn up into the cloth by capillary action.  The particles of dirt were not.  Once the water got to the top of the cloth, gravity took over and pulled the water down into the other can.  The water in the bottom can was much clearer.

Do not drink this water!  Capillary action does not purify the water,  so the water is not safe to drink.  It only separates the water from the solid dirt.  If there are any poisonous chemicals dissolved in the water, or if there are harmful germs in the water, they will very likely still be there.  Other processes are needed to purify water so that it would be safe to drink.

These next experiments will explore the property of water that we call evaporation.

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Evaporation

Materials Needed: Dinner plate; water; measuring cup.

Procedure:   Measure 50 ml (1/4 cup) of water.  Pour this water into the plate and place the plate in sunlight.  Observe the plate every 15 minutes or so for the next several hours.

What Happened: You almost certainly were able to predict what would happen.  After a period of time, the water disappeared from the plate or “dried up”.  This process is known as “evaporation”.  The liquid water doesn’t really disappear.  Instead, it becomes a gas called water vapor.  You can’t see the vapor in the air, but it is there, and it can be made to appear as we will see later on.

When water is exposed to air, it will usually evaporate, but it does not always evaporate at the same rate or speed.  How fast water evaporates is affected by four things - temperature, surface area of the water, wind, and humidity.  Because evaporation is so familiar to you already, these next experiments may seem very simple at first, and you may be able to predict what will happen in each without even doing the experiment.  However, don’t let that stop you.  Carefully observing each experiment will help you understand just how each of these four factors affect evaporation.

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Evaporation and Temperature

 

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed. 

Materials Needed: Two food tins; water; measuring cup; alcohol lamp or candle with safety pan.

Procedure:   Measure 50 ml (1/4 cup) water into each of the food tins.  Set one aside.  Heat the other one over a heat source until the water begins to boil.  Observe the water carefully as it is heated.  Allow the water to boil until none is left in the can.  Examine both cans.

What Happened: As one can was heated, you probably saw steam began to form.  As the temperature of the water increased, steam continued to form as the water began to boil.  The water in the can which was heated evaporated very quickly.  In the time it took to boil all the water away, some small amount of water evaporated from the other can, but almost certainly not enough to notice.  Generally, raising the temperature of a liquid will increase the rate at which it evaporates.

Materials Needed: A small food tin such as a soup can; dinner plate; measuring cup or beaker; water.

Procedure: Place 50 ml (1/4 cup) of water into the food tin.  Place another 50 ml (1/4 cup) of water on the dinner plate.  Place both in a warm sunny place and observe each container about every 30 minutes or so, until all of the water has evaporated from one of the containers.

What Happened: The water in the dinner plate was the first to evaporate.  In fact, it probably evaporated much more quickly than the food tin.  Even though the same amount of water was in both containers, the surface area of the water (the part of the water that was exposed to the air) in the plate was much greater.  This greater surface area allowed much more water to be in contact with the air at one time.  Since more water was exposed to the air in the plate at one time than in the tin, the water in the plate was able to evaporate much faster.

Materials Needed: Two food tins; two identical wash cloths; measuring beaker; water; electric fan; clothes pins; string.

Procedure: Fill each food tin with 50 ml (1/4 cup) of water.  Place one wash cloth in each food tin and allow each to soak up all of the water in it’s tin.

Make a clothesline in a bathtub or over a large sink using the string.

Hang the two wet wash cloths on the clothesline.  Place the electric fan so that it will blow directly in front of one of the cloths, but will not blow on the other.  Turn the fan on and observe each cloth about every 10 minutes until at least one is dry.  Which is the first to dry?

What Happened: The cloth in front of the fan dried much more quickly.  A current of moving air increases the rate of evaporation by moving the water vapor away from the cloth and allowing the water left in the cloth to evaporate at a faster rate.

Going Further: Why did you need two identical wash cloths?

Materials Needed: Two food tins; two identical wash cloths; measuring beaker; water; clothes pins; string; damp bathroom; another drier room (see Procedure).

Procedure: Fill each food tin with 50 ml (1/4 cup) of water.  Place a wash cloth in each food tin and allow the cloth to soak up all the water in it’s tin.

Hang one of the cloths in a damp bathroom where someone has just taken a shower.  Leave the door closed.  Hang the other cloth in another room in the house some distance away from the bathroom.  Observe each cloth every 15 minutes or so until one is completely dry.  Which dries first?

What Happened: The cloth that was left in the drier room should have dried more quickly than the one in the damp bathroom.  The humidity, or amount of water vapor already in the air, was much greater in the bathroom than in the other room.   When you take a shower, some of the warm water evaporates into the air and raises the humidity.  The more water vapor there is in the air, the less additional vapor it can hold, so it takes longer for the cloth to dry in the humid air in the bathroom.  This makes sense if you think about it.  Things dry faster in drier air.

Let’s sum it all up.  You can increase the rate (or speed) of evaporation by (1) raising the temperature of the water, (2) increasing the surface area of the water, (3) creating an air current around the water, or (4) evaporating the water in drier air.

We have seen that we can put water into the air by the process of evaporation, but can we get water out of the air?  We sure can, as these next two experiments will show.

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CONDENSATION

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Getting Water Vapor from the Air

Materials Needed: Glass container; ice; water.

Procedure: Fill a glass container with ice and water.  Allow it to set for a few minutes and observe the outside of the glass.  What do you see?

What Happened: Droplets of water formed on the outside of the glass container.  The air surrounding the glass was cooled by the ice and water inside the glass.  As the air temperature surrounding the glass dropped, the air was forced to give up some of it’s water vapor.  The water vapor turned into the liquid water you saw on the outside of the glass.  The process of water changing from a gas (water vapor) to a liquid is called “condensation”.
 
Going Further: You may be tempted to think that this water came from inside the glass.  To prove to yourself that it did not, repeat this experiment, except use you favorite soft drink instead of water.  Taste the water that forms on the outside of the glass.  Do you taste any of the soft drink?

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“Do It Yourself” Rain Storm

CAUTION!  Always be careful to follow all safety precautions when using a stove, and use with adult supervision only!  

Materials Needed: Large pot; frying pan; ice; water; oven mitts; stove.

Procedure: Fill the large pot about 1/3 full of water.  Place it on the stove and allow it to come to a boil.  While the water is heating, place a layer of ice cubes in the frying pan and just cover them with water.  When the water in the pot is boiling, carefully hold the frying pan about a foot over the top of the pot.  Notice what happens on the underside of the frying pan.

What Happened: As the water boiled, it produced water vapor.  When that water vapor came in contact with the cool underside of the frying pan, the vapor condensed and formed drops of water which fell back into the pot.

When the temperature of the air drops, the water vapor in the air will condense into tiny droplets.  If this happens high in the air, you see these droplets as clouds.  These droplets are suspended in the air by air currents inside the cloud. When these droplets bump into each other, they join together and become heavier.  Eventually, they become so heavy that the force of gravity is greater than the air currents, and they will fall as rain or snow.

If the water vapor condenses near the ground, you see it as fog.  You have probably noticed that fog usually forms in the early morning when the air has cooled rapidly overnight.

And one other thing.  The the visible wisps you see coming off the boiling water are not water vapor.  Water vapor is invisible.  What you see are tiny droplets of water thrown off by the boiling water that are carried upward with the water vapor by convection currents.  These droplets will quickly disappear as they also evaporate and become water vapor.

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Distilling

In an earlier experiment, you may recall that you were able to clean dirty water by using capillary action.  However, capillary action may only be used to separate water from solid particles like dirt.  If you want to separate water from another substance which has been dissolved in it, such as salt, capillary action won’t work.  However, it is possible to separate the two by combining evaporation and condensation in a process called “distillation”.  In this experiment, you will construct a simple still to separate water from food coloring which has been dissolved in it.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.