Wikipedia:Reference desk/Archives/Science/2015 April 14

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April 14

Eutectic systems

I have a difficulty in understanding two-component solid-liquid phase diagrams. The way it's usually explained, as in here, is that as we cool down the melt, compound A solidifies, then at the eutectic temperature, B starts to solidify at constant temperature, and after that all we have in the system is solids A and B and the systems can only cool down after that. I understand this explanation, but I do not understand that why it has to be this way.

What i don't understand is why one component (B or A) does not start to solidify before the eutectic temperature. Why does compound B "wait" for compound A to solidify completely, and only after that it starts freezing? I mean I have no trouble imagining two compounds solidifying independently and simultaneously at the bottom of a beaker, in separate phases. Why does it not happen that way?--Irrational number (talk) 14:05, 14 April 2015 (UTC)

Read the opening paragraphs of eutectic system to see if that answers your question. shoy (reactions) 14:22, 14 April 2015 (UTC)

It didn't help, really. --Irrational number (talk) 14:39, 14 April 2015 (UTC)

Eutectic alloys are homogeneous mixtures. The atoms are mixed uniformly at the atomic level. When the temperature is sufficient to break the bonds for the lattice, that dis-assembles the entire lattice. There are not two separate lattices: there is no way for one group of inter-atomic bonds to break while "the other" group of interatomic bonds remain intact.

The take-away message is that in complex solid metal lattices, even if there is heterogeneity in the individual interatomic bond energies, the lattice cannot remain in solid form after the weakest of these bonds is broken. The atoms are uniformly distributed; and when the bonds are too weak to hold the atoms together, the atoms can macroscopically move (flow) as we would expect in a liquid. Whether you understand this is moot. We can present you with lots of way to start understanding: here's a bunch of equations vomited ex cathedra from University of Delaware's advanced PHYS624 Solid State Physics course. Here are some videos of soft metal lattices that behave like liquids: Ga-In-Sn; the very noxious Bi-Pb (Rose's metal). This is what happens. You can write voluminous equations for it, and you can play with ball-and-stick models of lattices; and you can watch videos of metals melting; but there's no way to make you understand why it has to be this way. Have you got a better idea of a way it could be? If so, that can be a hypothesis, and you can scientifically test it to verify that it is not that way. Nimur (talk) 16:37, 14 April 2015 (UTC)

You seem to be implying that at the eutectic composition, The two components form a compound. Which isn't true, at least according to my physical chemistry textbook (Levine, 6th edition, page 375). It says "However, a eutectic mixture is not a compound. Microscopic examination will show the eutectic solid to be an intimate mixture of crystals of B and crystals of C". So there is no bonds at the atomic level, it's just two solids.

In fact eutectic systems can be composed of solids that are immiscible in each other, and the formation of compounds in eutectic systems has a completely different treatment, and in fact the compound can have and its constituents can form eutectic mixtures themselves (as explained here. I guess my question boils down to this: why can't there be a stable 3-phase system composed of A,B and their melt? It isn't really trivial.--Irrational number (talk) 17:26, 14 April 2015 (UTC)

They're a "compound" in the same way that bronze is (a crystal lattice with tin atoms replacing copper), not in the way that sodium chloride is. There is some kind of metallic bonding holding them together. (I assume that you're talking about eutectics in metal, if not please disregard) shoy (reactions) 17:45, 14 April 2015 (UTC)

User:Irrational number asks ..."why can't there be a stable 3-phase system"? The answer is, like nearly all questions about stability, because the system prefers to find a lower energy configuration. There are conditions in metallurgy in which a solid metal precipitates out of a molten metal solution: this is called slag or dross or "waste metal byproduct." Those form in conditions where the mixture is not, by definition, at the eutectic point. This is a definition - a material is eutectic if it doesn't do this. Nimur (talk) 02:08, 15 April 2015 (UTC)

Can one use air-conditioning equipment to manipulate different forms of energy (apart from heat)?

Hello, again!

I'd like to apologize in advance if this sounds like a stupid question, but it's one that has been gnawing at me for quite some time.

Thanks to climate-control pioneers such as John Gorrie and Willis Carrier, life in hot climates has become much pleasanter, now, than in ages past. I've read several articles on the expansion valves and compressors of air-conditioning equipment, and how they utilize certain chemicals such as freon and puron to eliminate heat in their immediate vicinity. (Or, more accurately, given the 1st law of thermodynamics, convert said heat into something else).

I wonder, though: Does science know of different substances that—when used in a/c equipment—don't eliminate heat, but another form of energy such as light or sound? Viz., may one (at least in theory) use similar equipment to a compressor/expansion valve (but different, hypothetical, fluids) to generate not cold, but rather darkness or silence?

Thank You. Pine (talk) 16:48, 14 April 2015 (UTC)

Except in unusual conditions, sound-energy and light-energy are not in thermodynamic equilibrium with their surroundings. In those exceptional conditions - for example, when we represent vibrational modes using the mathematics of a phonon, or when we carefully construct laser cooling experiments such that matter- and photons- can exchange energy - you might be able to describe energy-exchanges in the form of thermodynamic processes that are analogous to "refrigeration." But in all normal quotidienne circumstances, this is definitely not applicable. In your most elementary representation of an refrigerator - the carnot heat exchange cycle - there are stages of the cycle that require thermodynamic equilibrium. So, ... no. Light bulbs do not emit photons that are in thermodynamic equilibrium with the room they illuminate. Hi-fi stereos do not emit sound vibrations that are in thermodynamic equilbrium with the noisy room where your jams are playing. If you wanted to try to write out math to make a noise-cancelling headphone set look like a phonon-cooler, ... you'd find that it's a stretch of even the most creative imagination.

The question really is sort of fascinating: but you really need to go back to the fundamentals of thermodynamics: read, re-read, then re-read again, all our articles on the physics of refrigeration, until you really grok it.

Nimur (talk) 16:52, 14 April 2015 (UTC)

And note that A/C doesn't "convert" heat into anything else. All it does is displace it. That is, it moves the heat from inside to outside. And refrigerants don't do anything magical. Many fluids would work, to some extent, but standard refrigerants are just more efficient. StuRat (talk) 17:21, 14 April 2015 (UTC)

As for sound, there are things like noise-cancelling headphones and other applications of active noise control. As for light, the most practical means seems to be a black box ;-). --Stephan Schulz (talk) 17:35, 14 April 2015 (UTC)

• There are no stupid questions here on Ref desk: Wonder if a simpler way to put it, is that we normally think of cooling as by convection. But a A/C is throwing out loads of infrared radiation. That is non-visible light but electromagnetic radiation waves never the less. The international space station's ammonia cooling system can not convect heat away (it is orbiting in a vacuum) so it has to radiate. Point an inferred camera at any heat pump and parts of it will be glowing – that's infrared radiation escaping. So any working gas like freon, ammonia and even butane etc., will radiate infra-red heat.--Aspro (talk) 17:37, 14 April 2015 (UTC)

This is an aside to the main question. I have heard it said that because freon was non inflammable it became the working fluid of choice. However, heat pump technology (ie fridges, A/C etc) has moved on and ones fridge doesn't need to contain any more gas that many homes have in their butane gas lighter refills. Did the math about ten years ago and it seemed almost right expect for -if we don't smoke, then why have this dangerous butane in our homes?. That tickled me, because my cousins lived in an area where that get frequent power outages – so what did the have in their basement – Oh you've guessed it. . A butane camping stove and butane camping lamps. If that lot went up in their garage under their house, they would have been roasted in their beds. “Move it out into the out- house in the yard, I said” – Oh, but when the power goes out that is so inconvenient, to hunt around all those harley-davidson bits – (that one-day he 'might' get around to fixing) when all the butane and gasoline fueled stuff is easily found below where they are sleeping. --Aspro (talk) 18:12, 14 April 2015 (UTC)

• Inferred camera. :-) StuRat (talk) 17:50, 14 April 2015 (UTC)

• If we considering the physics of the subject, it is not useful to say that an AC or a refrigerator eliminates or converts heat (into what?). Rather it transfers heat from one space (home/office) to another (outdoors). And this requires work, even for an ideal machine, as is (hopefully!) explained at Carnot's theorem (thermodynamics), and this work is eventually converted to heat, sound, or, (infrared) light energy. So in effect, an AC converts, at least part, of the electricity it consumes into heat, rather than converting heat energy into another form. Abecedare (talk) 18:34, 14 April 2015 (UTC)

No one mentioned heat pump yet? I think that might be relavant for the OP. Vespine (talk) 22:53, 14 April 2015 (UTC)

The closest thing I can think of is the effect of a car engine in generating pollution. The free energy of nitrogen oxides is higher, but by putting air through a compression cycle, some energy can be transferred from heat/pressure into the rearranged composition of the air; a catalytic converter reverses the process. Wnt (talk) 12:59, 18 April 2015 (UTC)