rpenner
18th September 2009 - 10:23 PM
QUOTE (spark+Sep 18 2009, 02:09 PM)
I'm having a lot of problem understanding what exactly happens at the triple point at the molecular level.
That's because nothing happens at the molecular level -- it happens in the statistical equilibrium of a lot of a pure substance. Therefore it is the topic of study in Statistic Mechanics for the molecular picture, and Thermodynamics for the nineteenth century approximation of matter as a continuous medium.
And the reason why a macroscopic equilibrium can exist between two phases is that phase transitions are also changes in energy per atom. So if the situation is close enough to the boundary between two different equilibrium regimes, then of course not all the molecules can be in the same equilibrium state. So, since the main variables in determining equilibrium phase are temperature and pressure, two phases meet in a line, and three phases meet at a triple-point which is where two lines meet in a T shape, which may by distorted like a Y.
Thanks, prometheus.
spark
20th September 2009 - 09:24 AM
Thanks rpenner,though I think I'll take some time to go through and fully understand the links and your replies.
In the mean time,please could you help me out with 'critical temperature' ?
Again,looking at it from the molecular point,can we say that above the critical temperature,the liquid molecules that escape from the body of the liquid on heating gain so much of kinetic energy that when pushed against another such a drop of liquid,it does not form a bigger drop of liquid,as opposed to temperatures below the critical temperature-- where the liquid vapour droplets have excess kinetic energy,but not enough of it --so that when we press two such drops together,we can get a bigger drop.
rpenner
20th September 2009 - 04:14 PM
The heat of vaporization (the amount of energy which is needing to push a small amount of the substance from liquid to gas at the same temperature) trends downward with increasing temperature along the liquid-gas equilibrium curve until it reaches zero, at which point no separate gas and liquid states exists.
http://en.wikipedia.org/wiki/Enthalpy_of_vaporization
spark
20th September 2009 - 05:50 PM
Sorry, I didn't realise that promtheus had sent that second last post.Apologies and thanks!
Well,actually it would be nice if you please explained things as they're actually happening. As rpenner said, the triple point cannot be explained at the molecular level-- we can't exactly describe what's going on with the molecules at that temperature.
But still, you see, I don't have any knowlege about the statistical views on molecules and that kind of thing,since I am only in the first month of engineering college.
I was feeling convinced that just like we explained vapour pressure,boiling point etc in terms of molecules gaining heat energy,flying off,etc at school,we could perhaps explain the triple point and the critical temperature in the same sort of way.
However,it doesn't seem so. Still,if anyone can help me in this regard,it'll be much appreciated.
rpenner,I will read through the wikipedia article on enthalpy of vapourisation and get back to you.
rpenner
20th September 2009 - 08:17 PM
The reason it cannot be understood at the molecular level is because at finite temperature, the molecules all have different energies and what you know as temperature is only a statement about average kinetic energy for equilibrium systems. The triple point is a place where there are three coexisting equilibriums for bulk materials. The molecule doesn't care about the bulk properties. Indeed, in liquids near freezing, intermolecular forces often let molecules clump, which is why oils are more viscous at low temperature. This an be validated by x-ray diffraction studies of liquids.
So you when you have to communicate about large systems, you have to use statistical methods, at which point you are doing statistical mechanics on systems of molecules, not doing detailed physics with individual molecules. Some simulations of a mere thousand or million molecules validate this picture, but from a physics or energy standpoint, if you had to know all the momenta of 10^27 molecules, and details of their interatomic forces in every orientation and distance, then making ice would be a daunting task.