“Freeze” is understood to mean to turn solid due to low temperature, with the implicit assumption of not more than atmospheric pressure. Even chrisk’s site did not say that helium freezes, rather that it turns solid at high pressure. It’s a useful distinction to make.
And part two of my statement:
Water can turn solid at high pressure, even with a temperature of over 100 degrees Celsius. I don’t think anyone would say that it has frozen under those conditions.
I don’t know anyone (other than, evidently, yourself) who wouldn’t say that it has frozen. Freezing is a phase transition from liquid to solid. We say that a sample of water in space “boils away”, or even that a sample of water in Denver “boils”, even though such occurs at less than 100C because of the lowered pressure. Why is a liquid-gaseous transition always “boiling”, but a liquid-solid transition only “freezing” at certain pressure?
This thread seems to be getting a little bogged down in semantics… probably useful to bear in mind that not everybody else is making the same assumptions and/or using the same definitions as you are. It’s always better to ask someone else how he’s defining his terminology than mandating ‘only such-and-such is correct usage,’ in my experience.
As far as mathochists’ question, I am not aware of any substance that would never turn solid under any combinations of temperature and pressure. We’ve run out of elements, and chemical compounds would seem to be LESS likely to exhibit such a behaviour than chemical elements, since they’re made up of multiple connected atoms, which would be more likely to gravitate to a solid state, AFAIK.
Are we allowed to consider ‘substances’ that are not composed of atoms as they are traditionally understood?? That’s probably the only way we might find any such thing… of course, such forms of matter are often difficult to classify in terms of solid, liquid, gas, as I understand it… (and most are theoretical anyway.)
to Gary T on preview: Yes, I would personally consider hot solid water under pressure to be ‘frozen’, frozen-hot, at least in the same general way that I consider iron at room temperature to be frozen. If iron melts at fifteen-hundred odd degrees celsius, then to me it must be considered to ‘freeze’ at any temperatures below that point. (hehe, matho seems to agree with me.)
Yes, we’re into semantics. The dictionaries I’ve seen define freezing as changing from liquid to solid when cold or by the withdrawal of heat. For changing from liquid to solid by the application of pressure, we have the word “soidify.”
http://www.cogsci.princeton.edu/cgi-bin/webwn?stage=1&word=freeze
http://www.hyperdictionary.com/dictionary/freeze
http://www.wordwebonline.com/en/FREEZE
http://dict.die.net/freeze/
hmmm… okay, yes… but the presence of pressure does not necessarily mean that the change from liquid to solid was triggered by an application of pressure. For instance… (and this is just an example, so please excuse any practical difficulties,) what if you pressurized some helium to up about 250 atmospheres, and THEN began to cool it until it was within the pressure-temperature range indicated for solid helium. You are then changing it from a liquid to a solid by withdrawing heat.
Assuming that helium has a liquid phase at that pressure, otherwise I believe the correct term is ‘sublimation’, direct conversion from gas to solid.
Sublimation is direct conversion from solid to gas. Deposition is the conversion from gas to solid.
Maybe you guys are barking up the wrong tree. Are there any substances that are always solid, and therefor never freeze? How about toast?
Okay, so raise the pressure to 40 atmospheres, then drop the temperature until the phase transition. Bingo: phase transition of helium from liquid to solid by the withdrawl of heat.
What is it about the “column 8” / “noble” gases that makes them more inclined to be gases at temps where adjacent elements would be liquid or solid, and liquid at temps where adjacent elements would all be solid?
A simple answer:
In order for something to get colder, we need something to absorb it’s heat, which means that we need something at a lower temperature than what we are aiming for. Because nothing can go below absolute zero, nothing can absorb the heat from what we want to get to absolute zero.
Just got this month’s Scientific American and on p24 we have an article by Graham P. Collins called:
“A Glimpse of Supersolid - Solid Helium Can Behave Like A Superfluid”
I’m lousy at physics once we leave Newton behind but the article discusses some interesting properties of solid helium ("…helium 4 that was compressed into solidity and chilled to near absolute zero"). One nifty observation is that some of the solid helium acted like a superfluid “One solid could somehow move effortlessly through another.”
The author also notes that helium is “ordinarily” only a gas or a liquid. Yes it does become solid (and since we’re down in the teeny tiny kelvin range I’ll call that “freezing”) but it ain’t easy.
Roughly… they have enough electrons to completely fill quantum electron shells around their nucleus, which means that every atom is very stable, in a chemical sense, and there is very little attraction between atoms, which usually depends on either ionization, (noble gases are almost impossible to ionize because of the stability of those electron shells,) the impulse of atoms to stablize their shells through electron sharing, (ditto,) or variations in electric fields caused by forming chemical compounds… (noble gases do not form chemical compounds easily, because they have no impulse to share electrons.)
It is this attraction between atoms that tends to be responsible for the states of matter we know as ‘liquids’ or ‘solids’. Solids are formed when atoms or molecules are so tightly attracted to each other that they form some sort of matrix, with each particle generally remaining in a particular location. Liquids are formed when particles are still free enough in independent motion to move freely throughout the area, but are still tightly enough attracted to each other that each particle will remain close to several other particles, with the result that the matter will occupy a definite volume, and occupy a shape at any given moment, though that shape will change easily.
Did that make any sense??
Helium (He4) becomes 100% superfluid just below T=2.17 K. However, the condensate fraction (i.e. percentage of the atoms which are in the ground state) is only about 8%. If the atoms did not interact, the condensate fraction would be 100% – the iteractions prevent the system from fully occupying the ground state.
There are cases where helium can have a higher condensate fraction – for example, in the liquid-vapor interface where the density is lower than in the liquid phase, the condensate fraction can reach 90%.
The reason everything freezes is that there are attractive interactions between atoms which make a solid configuration energetically favorable over a liquid one once the average kinetic energy (aka temperature) is low enough. Even atoms with filled electronic shells, the noble gases, still have weak attractive interactions (called van der Waals interactions). So when the temperature goes to zero, there is no thermal kinetic energy to prevent the atoms from arranging themselves in a periodic lattice to maximize these attractive interactions (and thus minimize the total energy).
Helium is unique in that it is a noble gas with low atomic mass. Zero point motion can be thought of as quantum kinetic energy, i.e. momentum due to the uncertainty principle which can not be removed from the system. Zero point energy is inversely proportional to mass. Helium’s small mass means that its zero point energy alone is larger than the weak interactions, so the atoms can never sit still enough to freeze without external pressure.
Not quite true – He3 does have a superfluid state, but only below 0.003 K.
The fact that helium has both He3 and He4 isotopes is actually incredibly fascinating. He4 has quantum spin 0 and is a boson. He3 has quantum spin 1/2 and is a fermion. Fermions are subject to Pauli exclusion (where no two particles can be in the same quantum state), whereas bosons are not. Many interesting quantum properties, like superfluidity and Bose condensation, are due to quantum statistics, i.e. the intrinisic properties of fermions or bosons. This makes helium a sort of idealized testbed for isolating the effects of quantum statistics while keeping everything else fixed. It’s oddly fortuitous.
Whoops, never finished this thought. He3, like He4, is liquid down to T=0 K at ambient pressure. Apart from that, it has very different behavior than He4: it boils at a lower temperature than He4, freezes at different pressure, etc. This is partially due to the lower mass and subsequently higher zero-point energy, and partially due to the different spin statistics (He3 is a fermion while He4 is a boson).