Phase of one molecule

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Govalant
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Phase of one molecule

Postby Govalant » Thu Jan 29, 2009 7:22 pm UTC

Can we tell the phase of matter of only one molecule? I was just thinking about this while solving some organic chemistry problems. I guess you can't because the phase is defined is defined by intermolecular forces which are non-existant when there is only one molecule.

Anyway, thoughts?

(EDIT: I changed the title to phase, I think is more appropiate.)
Last edited by Govalant on Thu Jan 29, 2009 7:56 pm UTC, edited 2 times in total.
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Re: State of one molecule

Postby TaintedDeity » Thu Jan 29, 2009 7:27 pm UTC

The temperature of that molecule would give us a hint if we knew what temperatures it changed state at, but if we had no way of knowing the temperature, I guess we'd be stuffed.
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Re: State of one molecule

Postby Govalant » Thu Jan 29, 2009 7:31 pm UTC

TaintedDeity wrote:The temperature of that molecule would give us a hint if we knew what temperatures it changed state at, but if we had no way of knowing the temperature, I guess we'd be stuffed.


Right, but could you actually observe the effects of it being solid, liquid or gaseous?
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Re: State of one molecule

Postby PM 2Ring » Thu Jan 29, 2009 7:34 pm UTC

Temperature is usually defined as proportional to the mean kinetic energy of a whole bunch of molecules, so the temperature of a single molecule isn't well-defined, either. But I agree with the OP that the main issue is with the lack of intermolecular forces.

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Re: State of one molecule

Postby Xanthir » Thu Jan 29, 2009 7:47 pm UTC

Govalant wrote:Can we tell the phase of matter of only one molecule? I was just thinking about this while solving some organic chemistry problems. I guess you can't because the phase is defined is defined by intermolecular forces which are non-existant when there is only one molecule.

Anyway, thoughts?

Well, we can tell whether it's in the plasma state or not fairly easily. Does it have any electrons?

I think you're right about the rest of the phases, though, in that they're defined by intermolecular forces.
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Re: Phase of one molecule

Postby AFedchuck » Thu Jan 29, 2009 8:07 pm UTC

No. a phase is only defined when there are a number of particles. I suppose you'd call it a gas if you had to.
Talk about temperature is neglecting pressure, which is pretty much impossible to define until you have a number of particles.

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Re: Phase of one molecule

Postby skeptical scientist » Thu Jan 29, 2009 8:48 pm UTC

Yeah, in some precise sense, temperature, pressure, entropy, phase, and similar don't actually exist per se, but are useful fictions for describing types of group behavior. Sort of like how a rock isn't actually a single object, but a collection of silicon and oxygen atoms held together by various forces, which are in turn collections of protons, neutrons, and electrons held together by still more forces, and so on. However, thinking of the rock as a single entity is a convenient fiction that lets us better model the world in our minds.
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Re: Phase of one molecule

Postby Yakk » Thu Jan 29, 2009 9:51 pm UTC

Another way to look at it...

Assuming you have a (sufficiently) noble gas that provides a background temperature and more importantly pressure of the system.

You have 1 molecule. How does the behaviour of that molecule change with changes in it's kinetic energy and ambient pressure?

Does the details of the noble gas impact matter that much?

If the answers are "yes, and in ways that sort of line up with standard phase diagrams", and "no, the noble gas details don't matter that much", then you could argue that there is something to the 'phase of one molecule', couldn't you? In particular, if the temperature hit plasma or other extreme states, I'd expect that 'plasma' like effects might happen even in the a 'noble' gas that managed to hold on to it's electrons? (Or would the attraction of + to - be large enough that without the other molecules becoming plasma, that the phase transition would fail?)

Of course, I have no idea what the answer would be...
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Re: Phase of one molecule

Postby BlackSails » Thu Jan 29, 2009 10:58 pm UTC

Thermodynamics only works on collections of particles.

You cant define the temperature, chemical potential, partition function, etc of a single particle.

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Re: Phase of one molecule

Postby Luthen » Thu Jan 29, 2009 11:48 pm UTC

If you wanted to be annoying you could try working with the molecule as a whole being the system. Thus apply the laws of thermodynamics to its constituent atoms, electrons, quarks, etc. Thus the whole thing becomes muddled as the electron clouds are obviously gaseous and the nucleii are more akin to solids. Or if everything has wavelike properties then maybe the molecule is a liquid medium?

I'm sorry that wasn't very scientific (and almost none of those terms actually mean what I implied) but if we've agreed that the question is in a sense nonsensical I think I could say it.
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Re: Phase of one molecule

Postby Yakk » Thu Jan 29, 2009 11:53 pm UTC

Well, you could definitely find it's average kinetic energy. The average kinetic energy of a molecule at a given pressure and temperature and phase should be well known for a given substance, right?

The AVGKE( Tempurature, Pressure, Phase ) function tend to be non-injective?
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Re: Phase of one molecule

Postby BlackSails » Fri Jan 30, 2009 12:35 am UTC

Luthen wrote:If you wanted to be annoying you could try working with the molecule as a whole being the system. Thus apply the laws of thermodynamics to its constituent atoms, electrons, quarks, etc. Thus the whole thing becomes muddled as the electron clouds are obviously gaseous and the nucleii are more akin to solids. Or if everything has wavelike properties then maybe the molecule is a liquid medium?


You cant do that. Electron clouds are wholly unlike gases, and the nucleus is not similar to solids. Yes, the nucleus is very solid, but its behaviour is very different from "solids"

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Re: Phase of one molecule

Postby oxoiron » Fri Jan 30, 2009 6:02 am UTC

Solids and liquids are defined as having the molecules that make up the material in constant contact with each other. If you have a lone molecule floating in space, it is by definition neither solid nor liquid, meaning it must be in the gas phase.
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Re: Phase of one molecule

Postby Rentsy » Fri Jan 30, 2009 7:15 am UTC

So either

1. This is a stupid question, a molecule can't have a phase

2. We can guess based on the molecule's speed and what temperature a similarly kinetic molecule would have, and what phase that temperature has.

3. It's a gas

I think we would have to consider it part of a rarefied gas. What else would you call interstellar hydrogen?

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Re: Phase of one molecule

Postby morvita » Fri Jan 30, 2009 3:20 pm UTC

To follow up for the couple of people who suggested that a single molecule is in the gas phase, that is exactly the terminology used when performing theoretical calculations on molecules. Most of the theoretical calculations that are done are on a single molecule (or complex of interacting molecules) completely isolated from any outside influences. These are called gas phase calculations. I think this is an accurate description of our single molecule because when something is in the gas phase, it spends most of its time not interacting with its surroundings. Yes, it collides with other molecules in the gas or the walls of the container occasionally, but I think it is fair to say that that molecule does not interact with any other molecules. This is exactly the case proposed by the OP.
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