Help on gas pressure?

Question

A common demonstration of heat uses a large can. The large can with a screw on top has a small amount of water in it. The can is heated with the top off, and the water begins to boil. As soon as the can is removed from the heat, the cap is put on, sealing off the contents of the can. Choose which scenario you think will occur from the list below.

a) Water vapor gas will stay as vapor because sealing the container keeps the temperature constant

b) As the can cools down the water vapor turns to liquid, lowering the pressure which crushes the can in on itself.

c) As the can cools down the water vapor turns to liquid, raising the pressure which makes the can explode

d) As the can cools down the water vapor turns to liquid, and ice forms on the outside of the can to compensate for this effect.

Answer 1

b) The water vapor has displaced air. When its removed by turning liquid, the pressure inside the can is less than outside, thus the crushing.

Answer 2

The answer is b), but the water vapor does not necessarily have to become liquid to crush the can. If the vapor is very hot when the can is sealed, then merely lowering the temperature alone will crush the can. Normally, you would expect condensation of the vapor once the temp drops below 100ºC. However, as the can cools, the pressure in the can will drop, which lowers the boiling point of the water to below 100ºC, so it is quite possible for the can to collapse before liquid water appears.

Answer 3

when the water was boiling the pressuree int he can was greater than atmospheric pressur, the gas escaped from the can. When the can is removed from the heat and sealed, thepressure in the can goes down because the temperature goes down . Eventually the pressure inside the can is low enough that the pressure of the atmosphere is able to crush the can