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Ask the students why they think that their wet hand felt cooler when
the wind (air) blew. Their explanation will probably simply restate the
fact that the wind was blowing. You will need to ask: "What happened
to the water that was on your hand?" ("It disappeared"
is the most likely response.) Help students to refine this response to
one that explains that liquid water can become part of the air by the
process called evaporation. In this investigation, we found that as liquid
evaporates, it removes heat from the surface, lowering the temperature.
(A sophisticated explanation involving the energy required for the breaking
of hydrogen bonds between water molecules is well beyond the scope of
elementary learning goals.) With the evaporation model in mind, students
can think about sweating on a hot day, dogs panting to cool their wet
tongues, and even the fact that it is not possible to make a fire with
wood or paper that is soaking wet.
Next, encourage your students to explain their observations of the two
thermometers in the second part of the investigation. Why did the thermometer
with the wet cotton on its base show a cooler temperature when the fan
blew on it? They should be able to connect what they have learned about
evaporation to these observations.
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