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Procedure
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Opener (5 min.)
Set up the experiment before class, having the materials out and
ready for the students to see as they walk in. As class starts,
ask for some volunteers to help you. Have one student be in charge
of lighting the candle, and let another inflate a balloon to about 6
inches in diameter. Most will have guessed what's about to
happen! Ask them to guess for how long the balloon can survive if
you put it in the flame.
Development (5 min.)
Ask for someone to be the timer, and have him or her measure the
time the balloon can survive in the flame. Then give the go ahead
and let your volunteer place the balloon in the flame. As most of
your students probably guessed, it pops right away! Thank your
volunteers and have someone else come up to try again, only this time,
pour a few milliliters of water into the bottom of the balloon before
inflating it. (Try a few times before class to figure out exactly
the right amount for your setup, so that the balloons don't pop too
quickly, or not at all.) As before, have the students guess for
how long the balloon will survive, and time it in the flame. This
time, it should survive about a minute. (Make sure all the water
stays at the bottom of the balloon as it is placed on the fire, as it
otherwise will pop pretty quickly.)
Closure (10 min.)
Now, open up a discussion to the class. Start the discussion
slides and go through the slides, stopping at slide 6. At this
point, ask for an explanation as to why the second balloon survived for
so much longer. Most of them will suspect the water, and some
might even guess the specific heat capacity has to do with it. (If
not, nudge them along by asking what was different between the two
balloons, and by asking what happens to the water as it sits in the
flame.) Some often think the water evaporates, but you will notice
the water splashing onto the table as the balloon pops, disproving this.
Move on to slide 7-9, and ask why, when the specific heat capacity of
water is only 4 times that of air, did the balloon not just live 4 times
longer. (The mass must be taken into account.) This is done
in slides 10-12. (To as large extent as possible, have the
students deduce the answer before revealing the slides.) The mass
of air in the balloon is about 1 g, if you inflated it to 6"
diameter. At this point, however, it still looks like 2 mL of
water would only absorb 8 times the heat of the balloon as it contained
just air. Yet, the balloon lived more than 8 times longer.
The remaining slides explain this. In fact, only the bottom
portion of the air gets heated, after which it has to move out of the
way for new, colder air. This convection is slow compared to the
conduction of heat in the water. (This conclusion is much more
difficult and advanced, but you can lead the students along by asking
them if they think all the air in the original balloon was heated up,
and if they know what hot air does - most will know it rises.)
As an alternative closer, leave out the slides until after the class has
had time in groups to discuss what happened, and to try and explain it
on their own. (Adds more time to the lesson.)
Variation
Set up several little rigs and have students volunteer for a
competition. Have each of them compete against the "cool
teacher" to see who can best inflate their balloon to withstand the
fire. Inflate yours with a few milliliters of water in the bottom
without letting them know ahead of time. Then, present it as a
puzzle to them to figure out what about your balloon made you win.
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