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Le Jeudi 5 Janvier 2017 08:56 CET, Tim Gruene <[log in to unmask]> a écrit: I did not follow all the previous details of this discussion, but I think one may say that the statement "The entropy of molecules arranged in a crystal must be greatly larger than in solution" is incorrect. Of course, there is an increase of entropy (DeltaS = S(solution) - S(crystal) > 0) from the ordered crystalline state to the disordered solution state, hence S(crystal) < S(solution). Te nice thing is that, without knowing anything about the crystallization process, one can also say that DeltaS (per mole) = DeltaH/T, where T is the absolute temperature anf DeltaH > 0 is the heat absorbed by one mole of molecules going from the crystal state to the solution state during the melting of the crystal. Hence, the crystallization process, as with ice formation, produces heat (releases heat in the solution). Would one measure (in a calorimeter) the heat -DeltaH produced during crystallization, and the number of molecules having formed the crystal, then one would obtain immediately the DeltaS of crystallization. To be accurate: all of that is correct as far as T is the equilibrium temperature where the two phases coexist and can exchange reversiby (i.e. DeltaG = DeltaH - T DeltaS = 0). For ice melting at atmospheric pressure this would be of course 273.15 K. Philippe Dumas > Dear Nicolas, > > are you sure this concept is correct? > > The entropy of molecules arranged in a crystal must be greatly larger than in > solution. Hence the driving force for crystallisation is actually a drop in > energy, and I am not sure there is actually a barrier. > > I would take the kids to the play ground and let them run around randomly. > When I blow a wistle they should line up in an orderly manner. > > I am sure it will take some time before some sort of order is achieved, for > the kids could face in various directions, or line up in blocks, or other > shapes. Once a seed is there, i.e. once e.g. 4-5 kids have created a regular > block, it will be much easier for the rest to line up (this is consistent with > Patrick's explanation). > > Cheers, > Tim > > On Wednesday 04 January 2017 05:45:50 PM Nicolas FOOS wrote: > > Dear Evette, > > > > If I was is your situation (explaining nucleation and other concept). I > > will discuss in terms of energy. > > > > I mean obtaining the initial nuclei is the "costly" step in terms of > > energy. To represent that, out the classical curve of energy, I will use > > a metaphoric representation such as jump over a barrier and run after. > > > > With this analogy, it's possible to explain that the first step is > > difficult and the second more accessible. If the barrier is to high, > > it's impossible to continue and run. If you don't have any barrier it's > > easy to run and if you only have a small barrier is not to difficult to > > jump over and run. But It also allow you to explain that if you > > facilitate the apparition of the first "surface" thanks to appropriate > > method (seeding, dust...) you can help the first step (to continue with > > the barrier story, it like you have ladder to help, or the ability to> decrease the size of the barrier. > > > > For why the crystal and how, I will maybe use the example of orange > > pyramid in the food store. Orange are stable together because they have > > enough contact, because they have relatively homogeneous shape. If you > > mixed orange with water melon it's difficult to obtain nice pyramid. > > > > For crystallization experiment which work, I have no Idea out of the one > > you already mentioned. > > > > > > Hope this help. > > > > Nicolas > > > > Nicolas Foos > > PhD > > Structural Biology Group > > European Synchrotron Radiation Facility (E.S.R.F) > > 71, avenue des Martyrs > > CS 40220 > > 38043 GRENOBLE Cedex 9 > > +33 (0)6 76 88 14 87 > > +33 (0)4 76 88 45 19 > > > > On 30/12/2016 11:06, Radisky, Evette S., Ph.D. wrote: > > > Can anyone point to some especially useful resources to help explain > > > to kids (pre-chemistry, ~age 10-12) how and why molecules crystallize? > > > Maybe a good online movie or animation? I am especially needing help > > > with the concept of nucleation, and why nucleation is slower and then > > > crystal growth faster once nuclei have formed. I have been > > > supervising some experiments growing sucrose crystals from > > > supersaturated solutions, which have worked really well, but I am > > > having more difficulty in explaining the underlying fundamental > > > concepts in a way that is understandable to the kids. > > > > > > Thanks! > > > Evette > > > > > > Evette Radisky, PhD > > > > > > Associate Professor of Cancer Biology > > > > > > Mayo Clinic Cancer Center > > > > > > Griffin Cancer Research Building > > > > > > 4500 San Pablo Road <x-apple-data-detectors://3/0> > > > > > > Jacksonville, FL 32224 <x-apple-data-detectors://3/0> > > > > > > tel: 904-953-6372 <tel:904-953-6372> > > > > > > fax: 904-953-0277 <tel:904-953-0277> > -- > -- > Paul Scherrer Institut > Dr. Tim Gruene > - persoenlich - > Principal Investigator > Biology and Chemistry > OFLC/102 > CH-5232 Villigen PSI > > Phone: +41 (0)56 310 5297 > > GPG Key ID = A46BEE1A >