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So I’m learning gas discharge tube making (neon) and there’s a step where you cork one end of the tube and pull a vacuum on the other end to suck out all the impurities. The same thing was done when I was a mechanic working on an AC system that was opened up. Before you recharge it, you pull a vacuum for x minutes to remove the atmosphere in the system.

I’m having trouble comprehending that this is really possible. When you apply vacuum to a closed system, nothing seems to actually be moving. For instance, if you fill a drinking straw with coke, put one finger at the other end and suck, the coke goes nowhere. Remove your finger from the end and suck, and now you have movement; the coke goes right into your mouth. The negative pressure at the vacuum source (my mouth) works with the positive pressure at the other end of the straw, the air movement created pulls the air and the coke towards me. If vacuum on a closed system can’t pull liquid, how can it pull gas?

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    $\begingroup$ If you could suck hard enough with your mouth, then it would pull that liquid out too. $\endgroup$
    – orthocresol
    Jan 16 at 0:53
  • $\begingroup$ Only if the straw were to collapse…? $\endgroup$ Jan 16 at 0:58
  • $\begingroup$ Well, I guess that's another instance where your examples aren't really comparable. The first point was that the pressure difference is far greater when using a vacuum pump or similar. The second point is that a glass vessel (hopefully) won't collapse under the pressure difference. $\endgroup$
    – orthocresol
    Jan 16 at 1:00
  • $\begingroup$ But my earlier comment stands, really. If you used a very robust glass straw, and you sucked hard enough, you would get the drink out. Fortunately (or unfortunately?), our mouths aren't designed to create strong vacuums. $\endgroup$
    – orthocresol
    Jan 16 at 1:02
  • $\begingroup$ Okay, I hear you. It’s just hard to comprehend or picture that because I’ve never been able to see that principle anywhere in life. Any time I see something sucking something to move it someplace else, it’s utilizing an open system. I can’t think of one example of being able to see the type of action you’re (we’re) talking about. Thanks. If you can think of something practical that does this that I can see a video of, that would probably help me wrap my mind around it. $\endgroup$ Jan 16 at 1:09

2 Answers 2

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Gasses don't get "pulled", they get "pushed".

When you empty a neon tube, you attach a device with almost no air in it to mouth of the tube. The air that is in the tube then pushes itself into the device until the pressure is the same in both. Meanwhile, the device expels its air to produce more low pressure, and more of the relatively higher pressure in the tube pushes more of its air into the device. This continues, with the contained air pressure becoming less and less until you decide "good enough" or the limit of the device is reached. It won't be a perfect vacuum, but it might have say a 99% vacuum, containing 1% of the original air content in the same volume.

For the drinking straw, again it isn't your mouth (the vacuum) that pulls, the liquid actually gets pushed up the straw. What happens is that the weight of the atmosphere pushes down on the surface of the drink in the glass, and since that pressure is greater than the pressure of the partial vacuum you have created at the other end of the straw, the liquid is pushed up through the straw. This is much like the way toothpaste is pushed out of its tube.

Similarly, a hand pump on a well works by producing a partial vacuum at the top of the pipe and allowing atmospheric pressure to push the water up the pipe. There is a limit though. The weight of the atmosphere won't push the water any higher than about 10 metres (11 yards).

In the case of a straw sealed at one end, suppose that it is made of steel or very strong glass so that it won't collapse, and that it is long and thin enough that bubbles of air won't flow from one end to the other. The molecules in the liquid are attracted to each other, so the liquid will tend to act like a solid plug. If a vacuum is applied to the other end, because there isn't any air pressure at the sealed end, no matter how good the vacuum you create at the mouth end, there won't be any pressure to push the liquid plug out. Nothing happens.

However, if there is a small pocket of air at the sealed end, then the partial vacuum at the mouth end will cause some of the liquid to be pushed out. The small pocket will become a larger pocket, but with lower pressure. This will continue until the pocket's air pressure is equal to the pressure of the partial vacuum you are applying to the mouth, which could happen either before or after all the liquid has been removed.

Remember, gasses don't get "pulled", they get "pushed".

Even when you use a vacuum cleaner to clean a floor, the vacuum doesn't actually suck up the dust. Instead, the machine creates a volume of low pressure in its canister, and the air in the room gets pushed into the pipe by atmospheric pressure in order to fill that void, dragging the dust along with it.

The best way to visualize the behaviour of gasses is to imagine a room containing thousands of ping pong balls that continue bouncing forever.

  • Temperature: the hotter the gas is, the faster the balls will move around.
  • Pressure: each ball bounces off a wall by pushing against it, faster moving balls pushing harder than slower moving balls.
  • Volume: the larger the room, the longer it will take for a ball to make the next bounce off a wall, and so fewer bounces will happen in a given time interval.

If a wall is moved outward to make a larger room, there will be a brief period when there are no balls near it (a vacuum), but the balls will quickly fill that space and the larger room will soon reach equilibrium again.

The wall didn't "pull" on the ping pong balls, it simply created a void where the balls were "pushed" into. Meanwhile the larger room means less frequent bounces, which means less pressure.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – andselisk
    Jan 17 at 7:43
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"If vacuum on a closed system can’t pull liquid, how can it pull gas?"

It's back to that old thing: Gasses expand to fill the container, liquids don't. So the gasses will fill the entire container - reaching the vacuum, which will expel them, and then the remaining gas will do the same.

But the liquid (the coke) will stay put, unless the pressure is so low it boils.

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