|I will be doing an experiment that will show a
balloon filled with air and a balloon filled with
water over a flame. Witch one will pop first?
My first question is: Can you think of the
importance of this in the wider world? Who else
will care about the results, other than people
who try to pop balloons? I was told that I also
have to consider and think about natural water
(lakes, oceans) and how their heat capacity might
affect their surroundings (would those
surroundings be warmer/colder than areas without
large water bodies?).
My next question is:
Will it matter how big the balloon gets when you
blow it up or fill it with water?
|Question Date: 2007-09-30|
The balloon filled with air should pop first. The
reason for this is heat capacity: it takes a great
deal of energy to heat up water, and a lot less to
heat up air, so the rubber will get to the burning
point and pop the balloon.
Here's why this
matters: because water has such a high heat
capacity, it can act as a colossal heat reservoir-
or for that matter a colossal heat sink, to any
system you may be considering. In nature, it goes
beyond even that, however: the amount of energy
required to vaporize water is almost six times
again what is needed to heat it from freezing all
of the way to boiling. This is why the Earth is a
habitable planet: water evaporates from the
oceans, carries it up into the upper atmosphere
where it condenses out and falls back to the
surface, but the water vapor condenses to form
clouds and rain, it leaves the heat from the
oceans up in the upper atmosphere where it
radiates away into space. This effect controls the
temperature of the Earth so that it is within the
range in which life is possible: add more energy
in,the water cycle just dumps it right back out.
Also,water is a greenhouse gas (much more powerful
than carbon dioxide), and this keeps the Earth
from getting too cold.
In addition to its
natural importance, there are a lot of engineering
uses for water's high heat capacity as well. This
is how an automobile radiator keeps the engine
from melting itself: the water takes heat out of
the engine, and then dumps it outside of the
vehicle. The same principle is also used to keep
nuclear meltdowns from occurring in a nuclear
Whoever told you to "consider and think" was
right. Simply put, it takes a lot more energy to
heat water than air. This means that the oceans
are effectively soaking up a lot of the energy
associated with global warming and slowing the
process considerably. This is debatable, but
generally true. However, because water does not
change temperature quickly or drastically, a few
degrees of warming in oceans or lakes may have
dire consequences to organisms who are adapted to
a narrow range of temperatures that they
experience throughout the year. One example of
this is in the polar oceans, where the water temp
doesn't change much (generally -2 to +2 degrees
C). If the global ocean warms 1-2 degrees this is
like it getting 20% hotter (or like Santa Barbara
suddenly turning into Arizona and you not having
an air conditioner). Water also cools much slower
than air, so that in fall and winter lakes
actually give off more heat than they take in.
This also makes our climate "temperate"....look up
the word as well as it's root: temper. Remember
that the air is actually full of water (humidity)
and so changes in air temperature can be less
drastic in places where humidity is high. Ever
been to the high mountains? It gets really cold at
night and really hot during the day in part
because of the low humidity of the air. Same is
true in the desert. If you go camping, camp under
a tree or next to a lake: trees breathe water in
and out of the air so they have the same tempering
effect as the lake on local temperatures. The
balloon trick will be cool and hopefully will
illustrate the point well
Thinking about natural water sources is a good way
to think of the greater importance of this lab. If
the water balloon pops last, why does that happen?
There's a lot of heat coming from the flame, so
where does it go? More importantly for the natural
water sources, what happens to the heat once it
gets into the water? Because of the fact that
water can store a lot of heat energy, natural
water sources are what we might call "heat sinks".
If we have temperate weather, and then all of a
sudden it gets really hot out, it might feel
colder next to a lake because that lake can "suck
up" a lot of the heat, leaving the air around it
cooler. If we all of a sudden got a cold air front
coming in winter, a lake that's been heating up
for the whole summer could have enough stored
energy to warm up the surrounding area by a few
degrees (depending on the size of the lake, of
course). This is also important in thinking about
oceans - the reason why Britain is rather warm is
because of ocean currents that bring warm
Caribbean water up to Britain and warm it up. It's
also the reason why coastal areas in southern
California are a lot cooler than areas that are
more inland - the water in the Pacific ends up
coming from the arctic.
As far as the size
of the balloon - more stuff in the balloon means
there's more stuff to "suck up" the heat from the
flame. Since water can hold a lot of heat energy,
adding more water will make it take longer to heat
the whole thing up. What about air? It can hold
much less heat, so it probably won't make much of
a difference how much air is in the balloon.
There are a couple of useful and interesting
questions to consider here. One is the effect on
the balloon material itself: which balloon will
pop first, and why? A related experiment is to
hold a plastic bag full of water over a flame.
Most people are surprised to find that you can
boil water in a plastic bag over an open flame, at
least for a few minutes, as long as the flame or
heat doesn't reach above the level of the water.
That's a basic first experiment, which should lead
you to ask: why does this work? How does water
keep the bag or balloon from melting or popping?
Does the material matter? Does the thickness
matter? Does time matter? What is the
temperature of the water, and does it matter?
These are the kinds of questions that would allow
you to create a hypothesis ("I think the thickness
of the plastic matters because...") and test it
with an experiment.
This question is
important for a variety of reasons. For example,
when your car engine burns gasoline, the
temperature is about 2500 degrees C. But iron
melts at 1370 C, yet your engine doesn't melt.
Why? Also, nonstick cooking pans emit toxic gases
when they're heated above 150-200C (roughly
300-400 F), but they're safe to cook most foods,
especially foods with lots of water (eggs,
vegetables, pancakes), even on a hot burner. Why
would that be?
This leads to a kind of "big
picture" question: what effect does the water
play? This is where it becomes related to other
things you mentioned, like water having a large
heat capacity and affecting the temperature of
things around it. If water soaks up a lot of
heat, it's hard to heat or cool something else
that's near the water. So objects in the water,
or places near the water, tend to have the same
temperature as the water, whether that's warmer or
colder than normal.
Will it matter how big the
balloon gets when you blow it up or fill it with
I wouldn't want to give away the answer.
:-) The best thing to do is make a guess, write
down what you think will happen and why, and try a
few balloons: air only, water only but half
filled, water only and extra filled. Always try
to prove your idea wrong--that's the scientific
For safety's sake, try to do these
experiments someplace where you won't spread a
fire or get scalded if the bag breaks suddenly or
catches fire. I find that a campfire or small
fire in a BBQ works well, with a garden hose or
big water bucket nearby in case anything goes
wrong. Have fun!
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