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Heat Expansion
Name: Unknown
Status: student
Age: 13
Location: N/A
Country: N/A
Date: 2000-2001
Question:
You will probably be receiving many similar questions
because this relates to a school project and this site was given to us
by our teacher. We did in in-class experiment as follows:
We took a small beaker and put a piece of paper inside. The paper was lit
on fire with a match. Quickly and firmly, the palm of the hand was placed
against the opening of the beaker. The flame eventually went out and the
beaker suctioned to the palm of the hand.
Why did this happen? If you will be adding this to your website, like you
did a previous question, please tell me where to look in your response.
Thank you!
Replies:
The gas inside the beaker was warm due to the combustion. When oxygen was
no longer available, the combustion ceased. The surroundings cooled the
gas, thus decreasing its pressure in the fixed volume. Since air pressure
outside of the beaker was higher than inside, the atmosphere pushed the
beaker against your hand, keeping it there tightly.
---Nathan A. Unterman
This question is a bit more subtle [at least to me] than it appeared at
first glance. My first thought was you used up the oxygen so there was less
gas in the beaker after the paper burned, and so the pressure was less. But
that explanation is not correct!
If we assume the paper is just carbon, the reaction that occurs is:
C(solid) + O2(gas) -----------> CO2(gas)
There is no net consumption of gas. One mol of CO2(gas) replaces one mol of
O2(gas). No change in pressure.
Even if we assume that the paper (cellulose) has the general formula for a
carbohydrate: C(H2O) (solid) the same argument applies:
C(H2O)(solid) + O2(gas) --------------> CO2(gas) + H2O(liquid)
The water readily condenses on the side of the beaker, so once again there
is no net change in the number of mols of gas resulting from the combustion.
What happens is that the heat evolved from the combustion reaction heats the
gases in the beaker. The increase in temperature causes the gases to expand
according to the ideal gas law: PV =nRT. Your hand seals the warmer gas in
the fixed volume of beaker, so when the flame goes out and the gases,
whatever mixture is present, cools, the pressure inside the beaker becomes
less that the pressure of the atmosphere on the other side of your hand.
Hence you feel the suction.
Unknown
Hello,
The flame in the jar heats and evaporates some water. The water vapor
replaces the air in the jar. When the flame is extinguished by placing
your hand tight on top of the jar, the water vapor condenses (on the
cold walls and water surface). The pressure inside the jar drops, i.e.,
a relative vacuum is created. The air pressure outside pushes the hand
against the jar because of the imbalance in air pressure across your
your palm.
AK
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Update: June 2012
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