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Tom Booth wrote: IMO gas molecules absolutely must have attractive forces that cause the group of molecules within a container or wherever, to come closer and closer together due to mutual attraction before finally coming close enough to condens into a liquid.
Molecules have a mutual attractive force. True. Gas molecules have relative velocities. If that velocity is above the 'escape velocity' they are gaseous. Below the escape velocity they tend to clump/liquefy. As two molecules get closer their mutual attraction makes their velocities increase. They accelerate towards each other. They get faster. Velocity is related to temperature. The funny thing is that escape velocity is higher when closer. So if below escape velocity, they tend to clump no matter how close they get or speed up from mutual attraction. If below the escape velocity, and they are clumping, they get close enough to have their repulsion kick in and effectively orbit each other, become liquid. Colder, they become solids, orbits become vibrations locked into positions.
There are several types of molecular bonding: ionic, covalent, Vanderwal, metallic, and hydrogen are some. They all have different strengths. Combined with size and pressure they dictate melting and boiling points.
If they are above the escape velocity, even if they are headed straight at each other, they will not clump. They will bounce off from each other's repulsive force, and keep going. This is true of identical molecules or different substances. They tend to trade energy's. Like a Newton's cradle. Trading kinetic energy is the transfer of heat. Heat is not the movement of molecules as often stated. Movement of molecules is internal energy, it is related to temperature. The attractive force, and repulsive force become transparent, and both are replaced by the bouncing force associated with energy and momentum.
The bouncing force added up is called pressure. There is talk of zero pressure in a vacuum, but it never quite gets there.
The velocity added up is called temperature. There is talk of zero temperature, but it never quite gets there
The range of travel, distance, is called volume. There is talk of zero volume in a vacuum, or at zero Kelvin, for an ideal gas, but never for a real gas. Real gasses have finite liquid and solid volumes.
Gasses don't liquefy because they get closer together, they liquefy because they get colder, slower. Pushing them together does increase that liquefaction temperature, but it increases temperature too. So pushing them together isn't likely to liquify them, unless cooled too.
So gases always push, are always present even when the liquid is smaller than the size of the container and also true of solids. That push just gets smaller, but never zero, even at very low temperatures. There is a sudden lowering change in pressure when a gas liquefies, (Can crush). Not so much when going from liquid to solid, but some can produce dramatic events, (Ice bomb).
Tom Booth wrote: As water vapor in a 55 gallon drum will condense (contract, draw together) into a liquid leaving a vacuum, so all gas molecules have mutual attraction and "contract", long before condensing into a liquid.
Yes. It is just overridden by the bounce force caused by being over the escape velocity, being hot and gasified.
Tom Booth wrote: Or do you suppose that a large volume of gas just suddenly ALL turns into a liquid without a gradual attraction drawing them closer and closer before finally condensing?
Yes. Except that gas molecules are always flying around each other, getting closer, bouncing off walls, bouncing off each other, getting further away, until they get cold enough, slow enough, to effectively spiral in, to an orbit. Example, cloud in a bottle.
https://m.youtube.com/watch?v=WeXuKd0vMRk
It may be the wrong video. I just copied it. Didn't watch it. Cloud forms suddenly by all the alcohol vapor turning suddenly into liquid droplets, no longer pushing, or pulling.
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