funny balloon thought experiment

Doc Al

Again, I have no idea where you are getting this.

The bit of water vapor that would "fill" the space is irrelevant to the main point here. (If you fear the "vacuum", what do you think lies between those water vapor molecules?)

A diagram is not needed. The situation is clear; it's your reasoning that I'm trying understand.

How about we go back to your balloon experiment, only this time the water doesn't fill the entire container. Let's say there's an inch of "space" between the water level and the top of the container. Would that make a difference to you? When the balloon--which starts out just below the water surface--is pulled down to the bottom and compresses, the water level will go down. Just like with the diver and the pool. If you think there's a physically relevant difference, please point it out.

physical1

You did not define what this inch of space was composed of.

If this inch of space was air, it would be physically different than liquid water being in that space. Why is it physically different? Because water is not compressible and maybe not decompressible either. In order for a vacuum to originate in that space, something has to "give way". Water does not give way. Air does.

If this inch of space was pure vacuum, I am afraid you have done something no man has done before, and I would like to ask you how you created that pure vacuum since they do not exist.

If you argue, that, well, no it is not a pure vacuum.. it is... composed of some water vapor.. then.. how did you first break down that water into vapor I will ask. If you argue that well, the space got full of a tiny bit of air, then I will just argue back that I said it was WATER not air.

Air can be changed easily (compressed and decompressed) in size. Water cannot. Air allows practical partial imperfect vacuums to exist. Water since it is not decompressible/compressible in liquid form, does not AFAIK, allow a vacuum to spontaneously exist out of no where. It seems that practical imperfect vacuums (ones we can actually create) need some other gas molecules to exist. Water is not a gas molecule, it is liquid.

If one has ever held a science syringe with thumb on end and pulled the syringe handle, one would know that it is very easy to change air volume in the syringe. However if one fills the plastic syringe with pure water and no air, the syringe gets locked when you try to pull it with thumb in place. Then, as you pull, the syringe busts and cracks or the handle rips into two pieces if you are strong enough. Often the syringe plastic will deform and collapse, imploding.

So, what component is the most likely to "give way" in the system I described? Probably the air inside the balloon. Instead of the air inside the balloon being compressed when under hydrostatic pressure, it will just.. I don't know, stretch out and have lower pressure. Still mind boggling me at this point though.

Cyrus

If the pressure at the top of the water is zero when normally full to the top... and then there is an empty space at the top with vacuum (after you reel the balloon down), why would the water 'go up'?

What has changed? - Nothing. Nothing has changed. The pressure at the top is zero. Has been, and always will be.

physical1

What has changed is that the balloon, a soft substance, has been placed under hydrostatic pressure. The air molecules in the balloon get affected by hydrostatic pressure, and there must be an equal and opposite reaction. Since the balloon rubber is sucked in and held in place by a water vacuum lock (forgive my pseudoscientific terms), I suspect the air will stretch out in the balloon and the water level will not go up (which it cannot since it is sealed shut). SOMETHING ELSE must give way since water cannot.

If the air in the balloon gives way, then the balloon size stays the same. But that is why I came here - to find answers and confirmations (and back patting). In my original post, I hypothesize that the balloon will stay the same size.

Cyrus

Look, stop talking about the balloon. I don't care what's going on with the balloon. It's irrelevant. You are confusing too many things in your head. Think about ONE THING at a time. Are you listening to what I'm saying, or are you telling me?

You are wrong.

Dadface

How does boiling point of water vary with pressure?As the pressure decreases so does the boiling point so water will boil most readily into vacuum which will then cease to be a vacuum because it contains water vapour.Even if the water didn't boil it would evaporate and in the closed system being described here it will tend to saturation there being equilibrium being rate of evaporation and rate of condensation.The saturated vapour pressure is not necessarily negligible...as the critical point is approached it becomes more difficult to distinguish between the liquid and vapour phases their densities getting closer and closer.

Cyrus

The boiling point will lower as the pressure decreases. Not relevant here, there is no water/air interface. The water can't boil.

diazona

That is false. A pure vacuum certainly can exist. (well... in the real world, nobody has ever constructed a device that produces a perfect vacuum, but this is a thought experiment)

Whenever you have water in a container with some free space above it, a small amount of the water will spontaneously vaporize. The "vapor pressure" of water is the equilibrium pressure of water vapor produced by this process. Under normal conditions, this pressure is pretty small so we usually pretend it doesn't exist.

physical1

No, you are close minded and have no idea what the experiment is about, apparently.

physical1

A pure vacuum can certainly exist, yet you have no evidence it exists. This sounds like God believer to me.

Cyrus

Don't ask any more questions if you are not willing to listen to the answer. I believe you will be banned from here shortly. Ciao.

This is why I hate "thought experiments" - typically very little thought goes into them, much like the airplane on a treadmill "thought experiment".

m.e.t.a.

The answer to the first half of your question, "Can water be compressed?", is yes. All solids, liquids, gases and so on can be compressed. The reason you will see many PF users refer to water as "non-compressible" though is because it takes a very large increase in pressure (in the order of thousands of atmospheres) to compress a volume of water by just a few percent.

The graph on the following web page shows how the density of water varies with temperature at a variety of different pressures:

http://www.engineeringtoolbox.com/fl...ure-d_309.html

You can see that at, for example, room temperature, an increase in water pressure from 1 bar to 200 bar only results in about a -0.9 % change in volume. This is equivalent to the pressure a submarine feels in diving from sea level to a depth of 2 km. A reasonable depth for the water tank in this question might be 1 m. The density of the water at the bottom of the tank would only be ~0.0005 % greater than that of the water at the top. So you can see why people tend to disregard compression when dealing with water.

The answer to the second half of your question, "Can water be compressed into a solid?", is no* for water but yes for virtually all other substances. At standard atmospheric pressure, water reaches its highest density at about 4 ºC, as can be seen from the graph.

- m.e.t.a.


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* From Wikipedia: "Not all forms of ice are less dense than liquid water however, high density amorphous ice (HDA) and very high density amorphous ice (VHDA), for example, are both denser than pure liquid water."

(It seems that both HDA and VHDA can only be formed from ultra-pure water, which is then supercooled. As for whether compressing, instead of cooling, the ultra-purified water would result in some form of higher-than-water-density ice, I couldn't possibly imagine; the physics of water is extremely complex.)

Dadface

I googled and watched a short film of water boiling at room temperature as the pressure was reduced.

But it does exist and under some conditions it is not negligible

Cyrus

Did it boil in a sealed off container with no air in it?

Dadface

Well the air was pumped out.I googled "how does the boiling point of water vary with pressure?"I remember trying it years ago with a syringe partly filled with water.I simply blocked the open end and pulled up the plunger but I wasn't sure whether any air bubbles were being sucked in through the seal.

Cyrus

That means it had air in it. The OP here is talking about a tank filled to the brim with water. No air is inside the tank anywhere. Not even a little.

Dadface

Water will boil more readily if there is no air.Try googling"boiling water under a vacuum.There's some nice films there