Pressure in gases

[ZiHao]

A teacher heats some water in a drinks can until it is boiling and steam comes out of the can. She quickly turns the can over and puts it into some water. Explain in detail why the can collapses.

My solution will be: By boiling the water, the water changes state from liquid to gas, which is water vapor. The water vapor pushes the air that was originally inside the can out into the atmosphere. So there is less air and water in the can, but the water vapor in the can can still balance the atmospheric pressure, as it is hot means more particle collisions and higher pressure. When the can is turned upside down and placed in the water, all the water vapor quickly condense into drops of liquid in the can, this decreases the pressure. And so the much higher atmospheric pressure crush the can.

My questions is: Why can't we just put the can into the cold water? Why do we need to heat the water in it before? Can someone explain the whole process in detail, I don't completely understand the experiment. Thanks!

 

[kuruman]

My questions is: Why can't we just put the can into the cold water? Why do we need to heat the water in it before? Can someone explain the whole process in detail, I don't completely understand the experiment.

Your second paragraph explains the process very well. As you say, you need to fill the can with steam and displace the air. When you cool the can the steam condenses and creates a partial vacuum. If you just cooled the air without displacing it, the partial vacuum would not be sufficient to crush the can. The change in volume from steam at 100 °C to water at 10 °C is much much greater than the change in volume of air from 100 °C to air at 10 °C.

 

[ZiHao]

The purpose of heating the water in the can is we need to fill the can with steam and displace the air, because the steam has higher temperature than air therefore more particle collision and higher pressure in the can. So when we cool the can , steam condenses to water, it takes up much less volume in the can than just cooling the air down if we have not heat the water in the can. And comparing heated water and place into the cold water, to just straight away put the can into the cold water, creating a low pressure environment ( as the steam condenses to water, there is less particles to collide with the wall of the can ), therefore a higher quality vacuum. Right?

 

[SirCurmudgeon]

The crushing of the can is like the "power stroke" of "Newcomen's Atmospheric Engine."
( http://www.thermospokenhere.com/wp/04_tsh/D5600___atmospheric_engine/atmospheric_engine.html )
It was a stroke of genius to learn to inject a "touch" of water to cause rapid condensation of the cylinder
steam.

JP