• Water needs energy to evaporate.
• Objects cool when water takes energy from them and evaporates.
• The larger the surface area, the more evaporation takes place.
• Evaporation leaves dissolved substances behind.

Condensation
Condensation occurs if the air is saturated and if dust or salt particles are present. During condensation, water vapor collects around the dust or salt particles called condensation nuclei, and forms tiny droplets of water. Condensation is a loss of energy.

• Energy must be removed from water vapor for it to condense.
• Objects on which water condenses gains energy.
• Condensed water is nearly pure water.

Advanced Preparation:

Some of the experiments take more than one day to see the results. Read through all of the experiments first. You will find that you can have a whole class doing a couple of the experiments while waiting for the results of other experiments.

1. As with any demonstration or lesson, read the entire lesson first.
2. Some of the materials may take a few days to get ready.
3. Prepare all of the materials ahead of time.
4. Try out all of the experiments before you do them with your students. Humidity will play a significant role in the results of these experiments so you should test them out first.

Science Vocabulary:

1. Evaporation: is the change of state from a liquid to a gas.
2. Condensation: is the change of state from a gas to a liquid.

Procedure:
Evaporation

Teacher Demonstration: Experiment #1

1. Put a wet object (towel, glove, etc.) in a warm place. (A sunny window will do.)
2. Put an identical object where it is cooler.
3. Compare the two objects in an hour.

Class Experiment #2

1. Take a shoelace and cut it into several short lengths. (About 6 cm.)
2. Slip equal lengths of cut shoelaces around the bulbs of two similar thermometers. (Most shoelaces are hollow. Prewash the shoelaces because they sometimes have a stiffening agent (sizing) present.)
3. Tie the shoelaces with a string, or just make sure they do not slip off of the bulbs. (Tying the shoelaces is hard for small fingers. If the students are careful, the shoelaces will not slip off.)
4. Wet one shoelace with water at room temperature. Keep the other one dry.
5. Hold both thermometers while a friend vigorously fans the bulbs.
6. Stop the fanning and read the temperature every two minutes. (I sometimes have the students count to a given number such as 50 as they fan.)
7. Now have the students write or answer these questions:
   • Does fanning a dry bulb change its temperature?
   • How about fanning the wet bulb?

Results
Fanning a dry bulb does not change its temperature. (The students may see a small temperature change, but it is due to vigorous fanning and is not a true change. It goes back when the student stops fanning.) Fanning a wet bulb causes its temperature to decrease. Vigorous fanning should be kept up for about two minutes.

When water evaporates from objects, it cools them; it takes heat energy away. (A good example would be putting on a wet bathing suit; it feels cold to the skin if it is still wet and you have had it off.) Each escaping water molecule carries some heat away with it. The experiment may not work well on a humid day. On such days, when you feel sticky and uncomfortable, the difference in wet and dry bulb readings may be small, only a few degrees or so. Repeat the exercise on the first pleasant day.

Class Experiment #3

1. You will need a simple elementary balance scale.
2. Wet two paper towels with the same amount of water.
3. Crumple one into a ball.
4. Place the crumpled one on one of the pans on the balance scale.
5. Leave the other paper towel flat.
6. Place it on the other pan of the balance.
7. Use a medicine dropper to drop water on each paper towel until the two paper towels are completely balanced.
8. At the end of a half hour, which towel has lost more water?

Results
At the end of half an hour, the flat towel will weigh less than the crumpled one. Since both towels weighed the same to begin with, the flat towel has lost more water.

This is because the flat towel has a larger surface exposed directly to the air. Escaping water vapor can be carried away more easily. Students should try different types of cloths. They can do this at home with their families.

Class Experiment #4

1. Put some drops of salt water on a clean glass slide.
2. Leave the slide in a warm place.
3. The next morning, examine the slide.
4. What do you see? What does it taste like? Do you think the water took any salt with it when it evaporated?

Results
The next morning, the slide will have a glazing of salt crystals where the drops were. They will taste like salt. The water took no salt when it evaporated.

Condensation

Class Experiment #1

1. Fill a shiny metal can with ice cubes. Then watch what happens on the outside of the can.
2. Carefully out a thermometer inside the can, but do not use it to stir the ice. The thermometer may break.
3. Take a temperature reading from the thermometer every five minutes.

1. How long does it take for water droplets to appear?
2. What happened to water vapor in the air that caused condensation?
3. Does the ice inside the can begin to melt?
4. What causes this to happen?

Results
Almost any clean metal can will do. Shininess just makes the fine condensation droplets more visible. If the day is a very dry one and droplets do not appear on the outside of the can within a reasonable time, let students gently pant their breath onto the can.

Water vapor in the air cools, loses energy on contact with the can, and so condenses into liquid water. The ice inside the can begins to melt. The ice is gaining energy, partly from the air, partly from radiation (sunlight) within the room, and partly from the molecules of water vapor condensing into liquid outside the can.

Insert a thermometer into the crushed ice. Let the students monitor it from time to time. Does the temperature of the ice-water mixture rise while the ice is melting? (No!) Have them note when it finally begins to rise. (It will rise when all of the ice is melted.) If the ice is gaining energy, shouldn't its temperature rise? (Students may or may not recall from prior learning that it takes a certain amount of energy—80 calories per gram—just to melt the ice with no temperature rise. Early elementary students may not know this but older students may have learned this.)

Class Experiment #2

1. Wet a paper towel and crumple it.
2. Put it and a thermometer inside a jar. (A large mayonnaise jar with a lid.)
3. Put the lid on the jar.
4. Then lay it on its side in the sunlight. Fix it so that it won't roll. Placea thermometer outside, but next to the jar.
5. After a half hour, examine the jar.

Questions

1. Has any condensation occurred?
2. How much cooler is it outside the jar than inside?
3. Must a container be ice cold for water to condense on it?

Results
A wet sponge may be substituted for the paper towel. Place the jar so that the thermometer inside is shaded from direct sun. Shade the outside thermometer with a piece of cardboard so that it does not absorb radiant energy from the sun. If left in the sun, it will read too high. It will show its own sun-heated temperature, not air temperature.)

After half an hour, condensation will be visible on the inside of the jar. Yet it will be slightly warmer inside the jar than outside. (Heat energy has been trapped in the jar, held both by air and water vapor.) Condensation occurs because the glass of the jar, being in contact with the cooler air outside the jar, is cool enough to cause the water vapor inside to condense on it.

The container doesn't have to be ice cold for water on condense on it as we see with this jar. Ask the students if they suppose it can get foggy inside a car even if it is not winter. (Yes.) Why do they suppose it happens?

In a closed car, passengers breathe water vapor into the air of the car. Some water also evaporates from their skin. The windshield and the windows are usually cooler than the inside the car, so water will condense onto the glass as droplets of "fog."

Class Experiment #3

1. Collect a drop of condensed water on a microscope slide. (You can use a medicine dropper to collect the condensation by squeezing the bulb and placing the tip of the dropper next to a drop of condensed water. It will suck into the medicine dropper tube. Then drop the drop onto the slide. Repeat several times to get a large enough sample.)
2. Leave the slide in a warm place overnight. Then examine the slide with a magnifier.

Questions

1. Is anything left where the drop evaporated?
2. From your observation, does condensed water have anything dissolved in it?
3. Would you call it pure water?

Results
On a really clean slide, condensed water should leave no visible trace. Condensed water seems to have nothing dissolved in it.