Transcona Community Learning Centre
CHEM 30S
LESSON 4: EVAPORATION
If you leave a glass of water on the counter, eventually the water will disappear: either
someone drinks the water, or it will evaporate. Drying wet clothes is also and example of
evaporation. This lesson will describe evaporation from the particle level and explain
why evaporation occurs.
When you have completed this lesson, you should be able to:
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Use the Kinetic Molecular Theory to explain the process of evaporation.
Describe the process of evaporative cooling.
Describe practical applications of evaporative cooling.
Evaporation is the conversion from liquid to gas from the surface of the liquid (at a
temperature below the substance’s boiling point). Why does evaporation occur?
Not all particles have the same amount of kinetic energy. The distribution of kinetic
energies for any sample of liquid at a constant temperature would look like the diagram
below:
Evaporation occurs when some particles on the surface of the liquid have
enough energy to overcome the forces of attraction between the liquid particles.
Particles on the surface are subject to fewer forces of
attraction than other particles, as shown in the figure.
Some particles will have enough energy to overcome these
forces and be freed into the air to become gas molecules.
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Evaporative Cooling
In any given sample, the particles with the most energy are the particles evaporating,
leaving behind particles with lower energy. If the particles with higher energy leave, the
average kinetic energy of the particles left behind is lowered. Since temperature and
kinetic energy are directly related, lowering the average kinetic energy lowers the
temperature of the substance. The lowering of a substance's temperature caused by
evaporation is called evaporative cooling.
You can try this demonstration at home. Take two thermometers and wrap the
bulb of one loosely in some gauze or cloth. Dampen the cloth with water. Then
blow air over the two thermometers with an electric fan or a hand held fan (a
blow-dryer on "cool" would work too) as in the diagram below. Watch the
temperature on both thermometers. What do you notice?
The dry bulb thermometer's temperature does not change, while the wet bulb
thermometer's temperature decreases! That's evaporative cooling!
Try this with other liquids like alcohol or nail polish remover. Compare these with
water. What do you expect will happen? Why? We'll talk about the differences
more later.
Applications of Evaporative Cooling
Ever wonder why sweating cools your body or you feel cold climbing out of the
shower? In both cases, water covers your skin. This water absorbs heat from
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your skin and evaporates. Only the higher energy molecules evaporate, leaving
the lower energy, or "cooler" particles behind. Since energy is removed from your
skin and particles with lower energy are left behind, you feel cooler. Standing in a
breeze is cooler on a hot day because the water molecules on the surface of
your skin absorb heat from the air and evaporate. The higher energy particles
("warmer" particles) evaporate leaving behind the particles with lower energy
("cooler" particles). The moving air also prevents the air surrounding your body
from becoming saturated with water vapour, increasing the rate of evaporation. It
always feels warmer on a humid day than a dry day because the air does not
hold much more water vapour, so there is little evaporation of sweat.
Early natives in the southern U. S. and Mexico took advantage of evaporative
cooling to store perishable food (this was all prior to refrigerators and freezers of
course). The natives would place perishable food into clay pots soaked in water.
The water in the walls of the clay pot would absorb any heat from the outside or
inside of the pot. As the water absorbed the heat, it would evaporate and the
food inside the pot would remain cool. It was said that vegetables like lettuce
could last for several days in these pots without wilting.
Before the advent of Styrofoam coolers and the more advance coolers,
picnickers would pack lunches in boxes covered with wet towels. They called
these "swamp coolers". The water in the wet towels would absorb any heat and
evaporate, keeping the food fresh for hours. Ask your parents or grandparents
about "swamp coolers".
Refrigerators, Freezers and Air Conditioners
Refrigerators, freezers and air conditioners perform the same functions: cool things down
and keep them cool. They also work in much similar ways. We will discuss how a
refrigerator works.
There are five basic parts to a refrigerator: the compressor, the refrigerant or
coolant, the expansion valve, heat exchanging tubes inside the unit and
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CHEM 30S
exchanging tubes outside the unit. The refrigerant is usually a substance that
evaporates easily. Freon was common before it was discovered that Freon
damaged the ozone layer. It has since been substituted with less harmful
substances. In fact, ammonia is used in most industrial applications, even
making ice at your local indoor arena.
The gaseous refrigerant enters the compressor where its particles are forced
closer together causing it to form a liquid. The heat produced by the
condensation of the refrigerant is released as it moves through exchanging tubes
on the outside, usually at the back, of the refrigerator. The refrigerant then moves
through an expansion valve (a narrowing in the tubing, then a sudden widening)
and immediately begins to vaporize, absorbing heat from the inside of the
refrigerator, keeping the inside cool. The refrigerant is then cycled back to the
compressor. This continuing condensation-evaporation cycle cools the materials
inside the refrigerator.
Assignment
Answer the following questions.
1. Do all particles in a container of a liquid have the same amount of energy?
Explain.
2. In your own words, explain the process of evaporative cooling.
3. How does sweat cool your body?
4. Using sources in addition to the lesson, explain how a refrigerator or freezer keeps
food cool.
5. Do an Internet search to find information on commercial evaporative coolers.
Briefly describe how they work.
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Answer Key
1. All particles do not have the same amount of energy. Some particles will have a
high amount of kinetic energy and others will have a low amount of kinetic
energy. Most will have a moderate amount of energy as shown by the Kinetic
Energy Distribution Diagram below:
2. In a liquid, some particles have more energy than others. The higher energy
particles on the surface evaporate, leaving behind lower energy particles. The
average energy of the liquid decreases, results in a lower temperature.
3. Sweat cools your body through evaporative cooling. The body releases water onto
the surface of the skin. The water absorbs energy from the skin and some particles
use that energy to evaporate. The particles left behind have lower energy,
therefore a lower temperature.
4. Refrigerant is moved through a long winding tube by a compressor. The
refrigerant is compressed into a liquid by the compressor and moved through a
narrowing in the tube called an expansion valve, or a needle valve. The narrowing
suddenly widens, and the refrigerant begins to vaporize. As the refrigerant
vaporizes, it absorbs heat energy from inside the refrigerator or freezer, keeping
the temperature low.
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