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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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