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Say a bottle of natural gas used for heating or cooking. There is a liquid phase, which I believe is the densest part, at the bottom of the bottle. And on top of it, there is presumably gas in a gaseous form (I guess this is not vacuum). Why is it that there is not just one phase, like a super dense gas or a lower dense liquid? In the case of water, I would assume it tends at natural temperature to be liquid because the attraction amongst water molecules is stronger than the random motion would set them apart, although the pressure from air around water might also play a role. But in thr bottle of gas, the temperature would make it more likely for the gas to be not liquid. Is there also attraction between natural gas molecules that would consolidate a liquid phase?

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  • $\begingroup$ actually the gases play a role in preventing the remaining liquid from being a gas .. $\endgroup$ – A Student 4ever Nov 30 '20 at 19:25
  • $\begingroup$ How? I would suppose they exert a lower pressure than the densest liquid on each other. $\endgroup$ – Winston Nov 30 '20 at 19:27
  • $\begingroup$ the compressed gases exert large force on the molecules below them.. Even a little compression of gas can increase the pressure a lot $\endgroup$ – A Student 4ever Nov 30 '20 at 19:28
  • $\begingroup$ Are you saying the liquid phase is less pressurized than the gases above? $\endgroup$ – Winston Nov 30 '20 at 19:32
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    $\begingroup$ It's unlikely that you have a bottle of natural gas that has liquid in it at ambient temperature, unless you live in a VERY cold climate (see critical temperature commentary from the Claudio Saspinski answer below). My guess is that you are using either propane or butane. $\endgroup$ – David White Dec 1 '20 at 2:23
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It depends on the type of gas. In the case of natural gas (basically methane), there is only gaseous phase inside the cylinder.

Bottles of cooking gas for residences use normally a mix of propane and butane. They have a liquid and a gaseous phase.

The difference is a thermodinamic property called critical point or temperature. Above that, it is not possible to get liquid phase, no matter the pressure. For methane, it is 190.6 K; propane: 370 K, butane: 425 K.

As a comparison, the critical point for water is 647 K, what allows the designers of power plants to have inlet liquid water, and outlet steam, after being heated, by adjusting pressure and temperature.

Exactly because cooking gas has a liquid phase at room temperature, its pressure can be lower (about 7 bar), requiring a cheaper container. I have a car with an option for natural gas (methane). The cylinder has the capacity for 14 kg, (against 5 kg for a bottle of cooking gas). But the internal pressure is 200 bar.

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  • $\begingroup$ Thanks. My mistake, this is butane not natural gas. Physically, what prevents butane from being in gaseous form only? I understand that its liquid form is allowed at room temperature, but is it mandatory? $\endgroup$ – Winston Dec 4 '20 at 22:01
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In an undisturbed container of fluid or fluids, fluids in areas of higher pressure will to move to areas of lower pressure unless opposed by gravity, and fluids will move downward unless opposed by pressure gradients. This applies even if a vessel contains multiple fluids that do not mix (such as a bottle that contains water in the bottom and air above it). The pressure of the air at the top of the bottle will be slightly less than the pressure of the air at the surface of the water, but that will be the same as the pressure of the water on the surface. That pressure in turn will be less than the pressure of the water on the bottom.

In a propane tank, the lowest pressure will be at the very top of the tank, and it will be below the pressure needed to liquify the propane. At lower elevations, the pressure of the gas will be greater, until at some elevation the pressure will be sufficient to liquify the propane. Because liquid propane is much denser than gaseous propane, density will increase even more quickly at elevations below that.

If some propane were removed from the tank slowly while enough heat was added that the temperature remained constant, the pressure of gas at the surface of the propane would decrease, causing the pressure of the liquid there to also decrease to the point that it can no longer remain liquid, causing the liquid to evaporate and expand. Although this would slightly increase the volume that could be occupied by propane gas, the mass of gas would increase by an amount more than proportional to the increase in volume, thus increasing the pressure. Likewise if propane gas were pumped into the tank slowly while enough heat was removed that the temperature remained constant, the pressure of the gas at the surface would increase enough for force liquefaction, which would then decrease the volume available to hold gas slightly, but decrease the mass in that volume more, thus reducing the pressure.

The net effect when adding or removing propane slowly, and balancing that with the addition or removal of heat, will be that the pressure at the surface of liquid propane will remain constant, and the pressure at the top of the container will change only as a consequence of the changing difference in elevation between the top of the tank and the surface of the propane.

If gas is added or removed quickly enough to non-trivially disturb the thermal equilibrium (as would be the case in typical applications), the interaction of temperature and pressure gradients will cause things to behave in much more complicated ways. If one quickly removes propane, then as propane starts to evaporate, it will take heat from the remaining liquid directly below it. As the temperature of the liquid below falls, it will become capable of remaining liquid at lower pressures. Below that, however, the liquid will not have cooled as much. While liquid further below the surface will be at a higher pressure than the liquid above, if the pressure of the gas falls low enough, even that higher pressure will be insufficient to prevent the still-warm propane from evaporating and producing bubbles. Such evaporation, however, will cool the liquid at those greater depths. Accurately modeling this is much harder than accurately modeling the scenario where gas is added or removed slowly.

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There is an electrostatic attraction force between molecules that holds them together. When molecules have enough translational(moving from one place to another) energy to overpower this force they become free and reach a gaseous state. As long as the threshold is not crossed, they will remain in a liquid state. This is why there is generally no intermediate between liquid and gaseous states, and the transition is not a smooth continuum but involves discreet steps. When the pressure is high, even if the molecules have a lot of energy, the pressure forces the translational energy to convert into vibrational and rotational energy, and so the molecules cannot escape the liquid and turn into a gas. Furthermore at the top of the bottle, there is an equilibrium. Some liquid molecules are becoming gaseous, but an equal amount of gaseous molecules are also becoming liquid.

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