By removing the hottest molecules, we're able to freeze liquid nitrogen!
Announcer: Frostbite Theater presents... Cold Cuts! No baloney!
Joanna and Steve: Just science!
Joanna: Hi! I'm Joanna!
Steve: And I'm Steve!
Joanna: Today, we're going to freeze liquid nitrogen!
Joanna and Steve: Yeah!
Joanna: The obvious way to do this is to put the liquid nitrogen into something colder. Something that we have lots of around here! Something like... liquid helium!
Joanna: Yeah, but we're not going to do that. Instead, we're going to freeze the nitrogen by removing the hottest molecules!
Steve: So, how do we encourage the hottest molecules to leave? With a vacuum chamber and a big, honkin' vacuum pump that our Machine Shop was kind enough to lend to us.
Once we begin to remove air from around the liquid nitrogen, it becomes easier for the fastest moving particles to escape. The fastest moving molecules are the ones with the most kinetic energy so, when they leave, the average kinetic energy of the remaining molecules drops. That means the temperature drops. Eventually, it gets cold enough that the nitrogen freezes!
It just takes a minute.
And there it goes...
Joanna: That was close.
Steve: It was!
Joanna: Oh, now it's working.
Steve: Now we've got it!
Joanna: Wow, look at that!
Steve: It's like Jiffy Pop!
Joanna: Uh, huh!
While we've made a nice pile of frozen nitrogen, sadly, there's not much we can do with it. It's not like we can reach in there and make nitrogen snowballs or something. As soon as we let warm air back into the vacuum chamber... the nitrogen melts away.
Steve: There's one other thing we can show you.
Take a look at the liquid nitrogen that was in the vacuum chamber. You'll notice that it isn't boiling. It's actually too cold to boil! This liquid nitrogen is colder than this liquid nitrogen.
Joanna: It won't stay that way, though. As it absorbs heat from the room, its temperature rises until it reaches the boiling point. Now, both cups of nitrogen are at the same temperature again.
Thanks for watching! I hope you'll join us again soon for another experiment!
Steve: So, no nitrogen snowball fights?
Joanna: No, no nitrogen snowball fights.
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