Why does gas turn into liquid at high pressure?

[DingDongD]

Why does gas turn into liquid at high pressure if temperature is kept constant?

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[NextStepTutor_1]

A liquid is denser than a gas and is thus preferred when an increase in pressure ( and potential subsequent decrease in volume) is exerted on the gas. Think about Le Chatelier's principal when pressure increases - a reaction is going to shift to the side with fewer moles of gas and eventually the gas molecules accumulate upon one another as the pressure continues to increase.

Also think about this, as an example, in the context of melting ice. The densest phase of H2O is a liquid, and when you squeeze ice you'll find that the ice begins to melt into a liquid. This is the same premise. Of course try this using tongs or something other than your fingers since the warmth of your fingers will melt the ice despite any pressure.

 

[sazerac]

PV=nRT. R is always a constant, and you are holding T a constant. So the only way to increase P is to raise "n" (the quantity of stuff in your container) or reduce V (the volume of your container).

Either way, you are making things denser. You have more stuff per unit volume. After a while things get so crowded that the gas decides it is just easier to stick together and become a liquid instead.

Maybe not the most scientific explanation, but I think you get the general idea, which is crowding.

 

[Labrat07]

Another scenario to visualize:

Imagine, gas and liquid in a closed piston container. If you increase pressure, the piston go down. Gas come into liquid and be more soluble = becoming liquid. I have easier time remember this. Hope that help

 

[NextStepTutor_3]

The previous answers have all been awesome, but I just wanted to mention a weird way the MCAT has tested this in the past. Consider the two big assumptions of the ideal gas law - ideal gases have no particle volume and no intermolecular forces between particles. When do gases deviate most from the ideal gas law? At low temperature and high pressure. In other words, if you raise the pressure enough, gas particles start to be forced closer and closer together. At this point, they'll begin to interact with each other via hydrogen bonding, van der Waals forces, etc (depending on the molecules themselves, of course). So, liquids and solids are like the ultimate form of a nonideal gas - external pressure has promoted intermolecular forces to the point where they no longer are gaseous at all.

Can't remember which AAMC it was, but if I remember correctly it mentioned ideal gas deviations at 180 GPa, where the substance in question was decidedly in solid form!