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To assess your students' understanding of the concepts in this investigation,
ask them to explain why the cooled balloon becomes smaller. They may try
to explain the result as leaking air. In that case, you might need to
suggest ways you could test a balloon for leaks (such as immersing
it in water and looking for bubbles). You can also let the balloon
warm up, in which case it will return to its original circumference. If
they think that the balloon got smaller because it was wet, you can point
out that the other balloon was also wet, and it did not get noticeably
smaller. Once it is clear that the chilled balloon didn't lose any air,
they can agree that the air itself must be taking up less space. Extend
this to a discussion of air pressure. As air spreads out or contracts
due to temperature changes, the pressure it exerts changes.
When the concept that warm air takes up more space than cooler air is
clear, challenge the students to invent a way to test the concept. They
will probably suggest putting the chilled balloon back into warm water
(not hot!) to see if it expands.
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