Boyles Law question re-phrased...

It wouldn't.

First of all, atmospheres are a measure of pressure not energy. I think what you mean to say is that increasing the pressure will take work which will raise the temperature of the system (which is true since liquid helium is compressible which means increasing the pressure will involve force acting over a distance). But we really need to keep the concepts clear.

You aren't really explaining the situation very well, so it is hard to say for sure what setup you are envisioning. This makes it very hard to reason through what would happen.

In general, compressing a liquid requires work which will likely create heat (depending on the set up) which you could reasonably expect to raise the temperature of the liquid helium.

You could remove that heat to lower the temperature again. Of course removing heat is something you might reasonably do if you wanted to make a liquid freeze.

I would also, in the interest of pedantry, point out again that since this involves liquid helium, this question has nothing to do with Boyle's Law which involves gasses.

As Max said Boyle's law is for gasses.

But a solid does have less entropy than a liquid and an adiabatic decrease in entropy with a change of state (freezing or condensation) at a constant temperature requires a change in conditions along the pressure temperature phase equilibrium.

I would recommend that you Google phase equilibrium of any pure element or compound--

Rap

Of course it does DrewDad. You are exerting a force over a distance. In other words you are doing work on the gas and you are introducing energy.

Yes I guess I'm confusing things by referring to Boyles Law. Let's forget that law for this question, sorry.

I'll try to rephrase:

If liquid Helium needs a pressure of 25 atmospheres to actually freeze, why would doing work on the liquid - and introducing energy - remove enough heat that the liquid freezes?

I will rephrase the answer.

Doing work on the liquid would most likely increase the temperature, meaning that quite possibly it wouldn't freeze.

You would probably have to do two things to get it to freeze. You would have to do the work to raise the pressure, then you would have to have some way of removing the heat you just caused.

Note I am being vague since we don't have a specific set up in mind here.

Ok thanks. Specifically, I read that liquid Helium does not freeze at temps close to Absolute Zero unless under 25 atmospheres and thought - odd. Why raise the pressure to get it to freeze?

On further reading, it seems as many people as not think that Helium can/cannot be frozen. One explanation given was that the freezing point is below Absolute Zero. Ha ha

Thanks for your reply anyway

Let me clarify this again. Helium will freeze if two things are both true.

1) The pressure is really high (i.e. 25 atm).
2) The temperature is really low (i.e. near absolute zero).

If you do both of these things then you will have frozen helium. This is possible, in fact it has been done. Obviously it takes a lot of know-how and pretty cool laboratory equipment to do so.

As I said increasing pressure will create heat (which is a problem). This means the people creating the frozen helium have to two things at the same time (which is certainly possible). They have to remove heat as they are increasing pressure.

But yes, liquid helium does freeze near absolute zero at high pressure.

As I already said, helium has been frozen by scientists.

W. H. Keesom, a Dutch physicist, is credited with doing it first in 1926.

You need to look a the the phase diagram for Helium and realize that is has a superfluid phase that is beyond the three common phases (solid, liquid, gas).



BTW phase transitions are adiabatic--there is no intrinsic change in the internal energy of the element when it goes from one phase to another.

Rap

Theoretically, increasing the pressure does not create heat; it concentrates heat.

That is completely incorrect DrewDad.

Increasing the pressure requires a force over a distance. That means Work is done which means energy is added to the system. This known as the Work-Energy principle and is basic physics. There is no way around that.

I don't know what you mean by "concentrate" heat. This phrase doesn't make sense. Are you implying that somehow the volume matters for temperature? Temperature is an extrinsic property, and don't forget the 2nd law of thermodynamics. Anyway you will have to explain yourself better on what you mean by "concentrates heat".

But at any rate, compressing a liquid or gas requires force and since the volume decreases that force is exerted over a distance. By definition that means work is done on the system and based on the Work-Energy principle, energy is added.

The reason pressure is used is because the needed temperature cannot be reached (or it is impractical). Increasing the pressure increases the temperature that the phase change takes place (if you want the detail try the Clausius-Clapeyron effect). At 25 atm the increase is such that they can reach the temperature needed for freeing.

This does increase the energy of the system but what choice is there. This added heat that must be removed is just the cost of doing business.