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I'm going to measure the freezing point of tin by recording the cooling curve. It sounds like a dull experiment because all I have to do is to heat up the tin, wait for it to cool, and the computer does all the job. But in the lab manual it says:"keep a heating voltage of 30V so that the tin won't cool down too fast". What will happen if I just cut off the power source after the tin is melted?


Follow-up: I measured the two cooling curves. The curve without the 30V heating drops faster, which is expected. And the peak due to the latent heat released by supercooled tin is lower. But the melting points of tin calculated from the two curves are almost equal, with a difference of 0.04K. So I guess nothing special happened when I turn off the heater. And the error caused by temperature gradient is not significant in this particular experiment.

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The molten tin won't cool evenly because the cooling is at the surface of the container it's in. Your solidifcation will start at the outside and crystals will grow inwards. You will get a temperature gradient in your tin. This effect is more marked with faster cooling, so keeping the cooling rate as slow as possible will give you a sharper liquid to solid transition. How much difference this makes I don't know - I guess it depends on the experimental design.

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I will ask for two stoves and measure both(+30V, 0V) cooling curve, since I have to wait for the +30V to cool down anyway. The molten tin will be preserved inside an airtight container, which is inside a bigger vacuum container, to prevent oxidation! I think my whole afternoon will be ruined –  Alex Su Nov 18 '13 at 16:40
    
@AlexSu: I have to say I think that's an excellent piece of experimental design :-) Understanding the effects your experimental protocol may have on the results is an essential part of any experiment. –  John Rennie Nov 20 '13 at 8:10

From a measurement point of view, it might happen that by cooling too fast you don't see the transition. Another element could be that you cool down to a quenched state which is not the solidification you are hoping but more like into a glass state.

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The referenced lab manual may have been written by someone (intelligent) who wanted to avoid quenching; or sadly it may have been written by someone (dumb) who just wanted to make sure the students got lots of data points before the tin solidifies :-( . Here's an interesting article: unomaha.edu/tiskochem/Chem3350/Lecture_Notes/PDF_Files/… . –  Carl Witthoft Nov 18 '13 at 15:05
    
I don't think the crystal structure of tin is complex enough to have multiple phases at room temperature, and the formation of glassy metal requires liquid nitrogen as cooling medium. So I guess the temperature gradient is the major concern here. But the answer is inspiring, thanks! –  Alex Su Nov 18 '13 at 16:31

Well I'm not any expert on Tin, but typically, when melted stuff is allowed to cool rapidly (or forced to), the resulting crystal structure, is likely to be affected, and that would also tend to alter transition Temperatures.

Ordinary "metallic" Tin, usually called beta-Tin, is a tetragonal crystal lattice, with just one atom per unit cell; two equal short axes, and one longer one. It follows C, Si, Ge, in the periodic table group 14. The other three, are cubic crystals that form in the diamond lattice, and are semiconductors. Tin also forms a cubic crystal in the diamond lattice, which is a face centered cube, which has four atoms in the unit cell. This form is called Alpha-Tin, and is the stable form at low Temperatures, and also is a semiconductor.

During Napoleon's ill-fated invasion of Russia, the Tin buttons on the French uniforms, morphed into Alpha-Tin, at the sub zero winter Temperatures, and turned into a grey powder. This left the French troops with their coats unable to be buttoned up, and many of them quickly froze to death as a result.

So it appears, that Tin has a mind of its own, and may not act as you expect, if you let it cool too quickly. In a past life, I was involved in the synthesis and crystallization of single crystal Gallium-Arsenide, using the horizontal Bridgeman gradient freeze method. We melted the components in an evacuated ampoule, and then linearly cooled it, with a lengthwise Temperature gradient, so that crystallization started at one end, which was seeded, and slowly progressed to the other end, taking about three days in the process. This formed single crystals in the Zinc-blende lattice (similar to diamond), and in the process, any impurities in the ingredients, were swept along the melt, by the partition coefficient, which preferred to keep the impurities in the liquid phase, yielding a crystal, with high purity except at the butt end, where the impurities were frozen out. This is a case, where the very slow controlled Temperature linear cooling, is actually doing something useful, that simply takes time to happen.

So I wouldn't pooh-pooh the experiment instructions, in the case of your Tin melt, or you might get a surprise result.

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v interesting info about GaAs! In electronics, hand-solder joints are generally allowed to cool directly to ambient, without ill effects. –  Art Brown Nov 19 '13 at 3:05
    
Art, solder is a very special alloy, of Tin and lead; most commonly the pure eutectic alloy, which is 26.1 atomic percent lead (38.1% by weight), and melts at 183 deg. C At that mixture, the material goes directly from liquid to solid, and vice versa. For any other Pb-Sn ratio, the molten material will start to freeze at a higher temperature, and precipitate out, either excess lead or excess tin until the composition has adjusted to the eutectic mixture at which point it will freeze without further change into a robust solid. So the correct mixture can cool at any rate with impunity. –  user26165 Nov 19 '13 at 5:50
    
Well put, thanks. I guess I was offering that data point as evidence that pure tin wouldn't do anything surprising in the question's experiment, even if cooled directly to ambient. –  Art Brown Nov 19 '13 at 6:27
    
So if I let molten carbon, cool directly to ambient (room) Temperature, would I get a diamond, or would I get graphite; or maybe a flock of Buckyballs, or nanotubes ?? –  user26165 Nov 19 '13 at 17:07
    
@Art Brown; Art, an indispensable reference book is Dr Max Hansen's Constitution of Binary alloys. Which shows the phase diagrams of (nearly) every possible binary pair of elements. Lead-Tin is one of the classic ones, that shows a simple eutectic, and the classic maximum solubility "waist" at the eutectic temperature, for lead in tin, and tin in lead. While not exactly applicable to single elements, it does hint at the possible structural variations for elements that have several crystal forms, like Tin. –  user26165 Nov 19 '13 at 20:51

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