1
$\begingroup$

To my understanding, an increase in pressure (of gases in a container) is proportional to an increase in gases' kinetic energy, and hence temperature. However I read that gases liquify under pressure... However, gases liquify in colder temperatures, so how come gases liquify under pressure, where the temperature increases?

Thanks to all responses.

$\endgroup$
  • 3
    $\begingroup$ Do you know what a phase diagram is? $\endgroup$ – Raditz_35 Feb 20 '18 at 10:32
  • $\begingroup$ Related: chemistry.stackexchange.com/q/80492/31775 $\endgroup$ – Apoorv Potnis Feb 20 '18 at 11:51
  • $\begingroup$ @Raditz_35 A phase diagram describes the phenomenon but does not explain it. $\endgroup$ – owjburnham Feb 20 '18 at 14:06
  • $\begingroup$ @owjburnham I'm aware. The comments are not to give answers but e.g. to ask for information to clarify the question, so asking if the OP knows about a certain thing or is completely new to the field offers valuable information to people trying to answer - imo. You can skip the explanation of phase diagrams for example if the OP knows about them $\endgroup$ – Raditz_35 Feb 20 '18 at 14:10
  • $\begingroup$ chemistry.stackexchange.com/questions/31508/… $\endgroup$ – Mithoron Feb 20 '18 at 23:33
1
$\begingroup$

Think of it simply as spheres in a box. The spheres are your molecules. The higher the temperature, the higher the energy the molecules have and the more they move around. So when they have high enough energy for the molecules to move around a lot and stay away from each other they are a gas. (This is a simplified answer). If you decrease the energy they have (cool the system down), they move about less and there's a certain point the molecules aren't moving far away enough from each other to stay as a gas, so they liquify. As Raditz_35 said, look at a phase diagram. So when you put a gas under pressure, the molecules are getting "squished" together and they don't have a lot of room to move around, so they can't move away from each other enough and thus liquify. This is how they store Hydrogen as fuel. It's too dangerous as a gas and they put the Hydrogen under pressure to store it as a liquid. Hope this helps.

$\endgroup$
1
$\begingroup$

Both statements are correct: Gases tend to liquefy under pressure and gases liquefy in colder temperatures. However, in both cases it is implicitly assumed that the other quantity remains unchanged. This is, if you increase pressure slowly enough to give your gas time to transport the increase in heat to the surroundings, then it will eventually liquefy.

To illustrate, look at the working principle of a chiller. In a first step, a working gas is compressed; it increases in pressure and in temperature. The compressed gas is then lead through a heat exchanger on the outside of the machine, where it cools down to ambient temperature. The heat is transported to the surroundings (this is why your fridge is hot in the back). Then the dense gas is allowed to expand again on the inside of the machine and its temperature decreases below the original temperature. The temperature decrease during expansion is just the inverse the temperature increase during compression. With this mechanism way you can cool down gases and eventually liquefy them. The important thing is to dissipate the heat to another medium.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.