# Simple liquid/gas law question and liquid transfer query

• B
• DonJuane
In summary, the liquid propane in the two half-full bottles will flow down from the top to the bottom tank regardless of the position of the valve on the upside down bottle.

#### DonJuane

I am so embarrassed that I have forgotten so much of my physics and I am at the point of being away from school too long to remember even the most basics (Mom said no one can ever rob your education but I'm living proof otherwise). So please forgive my inability to discuss this at an academic level.

Question 1:
I am having a debate with a friend. We take the example of two propane bottles and each of them is half full. Hang one upside down from a tree with the valve pointing down. Set another one on the ground with the valve pointing straight up.and connect the two with a perfectly straight hose. I told my friend that the only way the liquid will run down from the top to the bottom tank while assuming the tanks are the exact same temperature, is if there is an unrestricted path through the hose for bubbles to travel up to the top with that path constantly rising from the bottom to the top tank and where there is no place in the path where the bubbles will have to travel downhill. He says that this cannot be calculated as to if bubbles travel like this and that I in my own reference as to how this is true with transferring water from one sealed jug to another sealed jug is not accurate when it comes to propane. I say that the liquid propane matches the characteristic of water in two enclosed bottles and that the bubbles serve the same purpose with propane as they do in the water transfer example in that bubble must travel uphill for a transfer. Which of us is right?

Question 2:
Will liquid transfer from one bottle to the other over time? If we set the two half-full propane bottles on a level suface with a hose connecting both valves of both bottles, with one tank inverted and one tank straight up. Assuming the temperature of the two is approximately the same with no energy source to make one hotter than the other over time and assuming the temperature of both at exactly the same two during a cycle of daylight making them both warm and cool at the same rate over time. Will the fact that one bottle is inverted cause the liquid to move from one bottle to the other over time with uniform temperature swings on both bottles?

What has this question got to do with thermodynamics?

Chestermiller said:
What has this question got to do with thermodynamics?
If we posit the propane condensing on the bottom surface of the suspended liquid propane and evaporating from its upper surface and a resulting heat flow through the bulk of the propane, then we have a thermodynamics problem.

How about some assistance with directing me to the right area for an answer to my question rather than further explaining my lack of knowledge and inability to categorize it properly? I think many people get so buried in their niche that they begin to dwell on skills with the mechanics of procedures rather than the grand overall spirit of simply helping a fellow out.

vanhees71 and russ_watters
DonJuane said:
Question 1:
I am having a debate with a friend. We take the example of two propane bottles and each of them is half full. Hang one upside down from a tree with the valve pointing down. Set another one on the ground with the valve pointing straight up.and connect the two with a perfectly straight hose. I told my friend that the only way the liquid will run down from the top to the bottom tank while assuming the tanks are the exact same temperature, is if there is an unrestricted path through the hose for bubbles to travel up to the top with that path constantly rising from the bottom to the top tank and where there is no place in the path where the bubbles will have to travel downhill. He says that this cannot be calculated as to if bubbles travel like this and that I in my own reference as to how this is true with transferring water from one sealed jug to another sealed jug is not accurate when it comes to propane. I say that the liquid propane matches the characteristic of water in two enclosed bottles and that the bubbles serve the same purpose with propane as they do in the water transfer example in that bubble must travel uphill for a transfer. Which of us is right?
My community has a propane system with a cluster of underground tanks. I assume this is pretty much exactly how it's filled...and for that matter, how the little propane tanks for grills are filled. One issue: I don't see why the bubbles would need a constantly rising path. I don't think it would trap the bubbles if they are being forced to flow toward the upper tank by the higher pressure in the lower tank.
Question 2:
Will liquid transfer from one bottle to the other over time?
This looks like the same question and I would still say yes.

Chestermiller said:
What has this question got to do with thermodynamics?
jbriggs444 said:
If we posit the propane condensing on the bottom surface of the suspended liquid propane and evaporating from its upper surface and a resulting heat flow through the bulk of the propane, then we have a thermodynamics problem.
I'm not certain I'm following. Are you saying the pressure increase in the lower tank caused by the flow from the upper tank would cause the gaseous propane in the lower tank to condense? Maybe... I would think that during the transfer there would be come condensation and evaporation but the system would eventually settle out at the same pressure and temperature that it started (assuming the two tanks are at the same temp to start).

I had wondered about this problem myself years ago, and my initial guess would make this a thermodynamics problem: I had guessed that the primary tank would be over-pressurized to keep it all liquid and drive the transfer (with a compressed air bladder?). This would condense the propane in the secondary tank as it filled, making it a (minor) thermodynamics problem.

I will re-phrase my question. I want to forget thermodynamics and think of the subject here only as "my selecting the wrong topic" and let's turn this into an elementary gas/liquid question and answer session that has been miss-assigned. Considering in a perfectly controlled environment, where the temperature on both bottles was exactly the same, without a pump or a source of energy, how might one get the liquid in the bottle above to run into the bottle in the bottom. (I know I can make the tempatures different but I am talking about when that is not possible or practical.) My idea was that the propane would follow the same characteristics as with water in two sealed bottles, which is, that with two water bottles, the only way for the water in the top bottle can run down through a tube to the bottom bottle is if there is a continuously asending path for a bubble to take from the bottom tank, up through the liquid in the tube to the top tank. My assumption that liquid propane would follow the same characteristic of water but only in the case where the temperature on the two bottles of propane were kept exactly equal. True or not true?

DonJuane said:
Net of it is that it doesn't seem logical to debate if something is properly categorized...
Valid criticism, but let's move on. We'll move the thread or change the title if it seems appropriate, but I don't see it as necessary.
I will re-phrase my question. I want to forget thermodynamics and think of the subject here only as "my selecting the wrong topic" and let's turn this into an elementary gas/liquid question and answer session that has been miss-assigned. Considering in a perfectly controlled environment, where the temperature on both bottles was exactly the same, without a pump or a source of energy, how might one get the liquid in the bottle above to run into the bottle in the bottom. (I know I can make the tempatures different but I am talking about when that is not possible or practical.) My idea was that the propane would follow the same characteristics as with water in two sealed bottles, which is, that with two water bottles...
Per my response in post #5, I agree.
...the only way for the water in the top bottle can run down through a tube to the bottom bottle is if there is a continuously asending path for a bubble to take from the bottom tank, up through the liquid in the tube to the top tank. My assumption that liquid propane would follow the same characteristic of water but only in the case where the temperature on the two bottles of propane were kept exactly equal. True or not true?
Per my response in post #5, I don't think it is necessary for the path to be a continuous ascent/descent. Also, this is something that could probably be tested without too much effort with a couple of sealed 5 gallon buckets and a garden hose.

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vanhees71 and jbriggs444
The "hose with no continuous upward path for bubbles" resembles a bit a simple siphon barometre.

Consider the case where the barometre is filled with quicksilver. One leg is open to external air, the other closed with no air in the closed end.
The barometre would show 760 mm Hg.
The Torricelli vacuum is not actually a vacuum. It is full or quicksilver vapour... pressure 0,16 Pa at 20 C. Which is mere 0,0012 mm Hg.
Thermodynamically it would be favourable for air to travel through the quicksilver and fill the Torricelli vacuum, whereupon the barometre would show less than the true external air pressure, and eventually zero. But bubbles cannot do so.
If, however, you had only quicksilver vapour and no air in either leg, then the barometre would also show zero.

Now fill your barometre with water.
Ait supports the weight of 10 m water... but the vapour pressure of water at 20 Celsius is about 2300 Pa. Which means that water vapour supports over 230 mm water, compared to quicksilver vapour supporting just 0,0012 mm.
And now fill the barometre with propane.
At 20 Celsius, the vapour pressure of propane is about 8,5 bar, and density about 0,5. Which means propane vapour supports 170 m of liquid.

If you have a siphon barometre with propane vapour in both legs, position where a quicksilver barometre would necessarily show zero, the propane barometre would be liable to show even tiny temperature differentials between the two surfaces as differences in vapour pressure.

I think jbriggs444 has it right. The head pressure would tend to move propane down until an increase in vapor pressure stopped it. But if the temperature is constant then an increase in vapor pressure would mean it is oversaturated and would condense. Similarly in the top bottle, propane would vaporize to keep the increasing head space at saturation pressure.

In practice it would be very slow because heat has to move out of one bottle into the environment and into the other bottle. I've done LPG fuel transfers, not with propane bottles but with propane/butane mixes (backpacking canisters). The temperature (and fuel proportions in the case of mixes) makes a lot more of a difference than gravity. But I believe gravity will eventually work.

Trying to figure out if gas bubbles in LPG will fit through the valves is something I've thought about and realized I don't know how to calculate.

## 1. What is the difference between a liquid and a gas?

A liquid is a state of matter that has a fixed volume but takes the shape of its container. It has particles that are close together and can move past each other. A gas, on the other hand, has no fixed volume or shape and its particles are spread far apart, moving freely and independently.

## 2. How does temperature affect the behavior of liquids and gases?

Temperature plays a crucial role in the behavior of liquids and gases. As temperature increases, the particles in a liquid or gas gain energy and move faster, resulting in an increase in pressure and volume. In contrast, a decrease in temperature causes the particles to lose energy, resulting in a decrease in pressure and volume.

## 3. Can liquids and gases be compressed?

Yes, both liquids and gases can be compressed, but to different extents. Gases are highly compressible because their particles are far apart, making it easier to decrease the volume. On the other hand, liquids are only slightly compressible because their particles are already close together, making it difficult to decrease their volume.

## 4. What is the ideal gas law?

The ideal gas law is a formula that describes the relationship between pressure, volume, temperature, and the number of moles of a gas. It is represented by the equation PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature.

## 5. How is liquid transferred from one container to another?

Liquid transfer can be achieved through various methods such as pouring, siphoning, or using pumps. The most common method is pouring, where the liquid is transferred from one container to another by tilting the first container and allowing the liquid to flow out into the second container. Siphoning involves using a tube to transfer liquid from a higher container to a lower one. Pumps use mechanical force to transfer liquids from one container to another.