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| From: Min-Zhao Lee |
2/11/99
21:15:44
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| Subject: Crystal Growth |
post id:
4418
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In science discussion today we
observed crystal growth, especially the effect of speed of formation
(rapidly cooled solutions form many small crystals, slowly cooled ones
form few large ones). We noticed that: the centre of the crystals looked
like a mess, there was a water current moving from all directions to the
crystal (there were suspended particles in the water that demonstrated
this quite well), layers appeared in the crystals. It was conjectured that
when cooled quickly, a large number of particles lost enough energy to
become nuclei for crystals, and since there was only a certain amount of
solute, each of the crystals was small. When cooled slowly, only some
particles became nuclei at first, the rest were then formed into these
crystals before they had a chance to form crystals themselves, and so
there were few, but large, crystals. How correct is this
theory?
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| From: Alan |
2/11/99
21:51:40
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| Subject: re: Crystal
Growth |
post id:
4435
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Basically your on the right
track. I know how all this works in metal crystals, I hope I can explain
it correctly for salt crystals.
Rapid cooling force the crystals to
solidify out of the liquid quickly. Basically they will form where ever
they can nucleate.
With slower cooling you have the same number of
nucleation points, but as the crystals the larger one will grow in
prefence to the smaller ones as they attain a lower energy state. In fact
the smaller ones over time will dissolve until a single crystal is present
(this depends on the rate of cooling and how much ionic mobility can occur
at the final temperature).
With the layering effect you observed,
think about it in terms of the concentraction of ions in the solution.
Before any crystals form you have a super saturated solution, as the
temperature decreases the first cystals will form, these will contain the
highest concentraction of the salt. As the temperature decrease the
concentraction of the ions in solution has decreased, subsequent layers on
the salt will contain more will be formed from a solution contain less
ions and hence more water molecoles will be attached to the
crystals.
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| From: kelvin fox |
3/11/99
0:11:55
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| Subject: re: Crystal
Growth |
post id:
4474
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you are on the mark as far as i
acn recall from uni lectures. We grew a number of crystals slowly and also
quickly and looked at them growing under a microscope. The quick growing
crystals apear feathery and form a felted mat. The only crystals that did
not do this were cubic crystals such as salt. Basically if you grew them
fast enough you could not pick the crystal symmetry or which crystal type
it was (crystals form in one of seven groups). When formed slowly
individual crystals of good symmetry were formed. try growing crystals of
lead chromate slowly. They are hollow and form the most wonderful
crystals.
Kelvin fox
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