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Why do things melt?

  1. Jul 19, 2010 #1
    Hey guys,

    I was wondering why I have seen liquid ice, gold, iron, ... but never liquid wood. Seems to me that wood simply does not melt, but turns to ashes. Or maybe the right conditions have to be met?

    Anyway, I was wondering what melting actually does and whether this can explain why some things seem to melt why others don't.

    Thanks!
     
  2. jcsd
  3. Jul 19, 2010 #2
    First note that the first substances you mention consists of a fairly homogeneous mix of molecules. Whereas wood as well your computer are a complex system. Your computer doesn't "melt" either.

    But it's actually a good question what melting means.

    Maybe you can read up some wikipedia articles:
    http://en.wikipedia.org/wiki/Sublimation_(phase_transition)
    http://en.wikipedia.org/wiki/Phase_diagram

    Solid means the molecules attract strong enough to force each other into fixed positions. Liquid means the molecules are disturbed by being hot and hence jiggling around. This way the lose attraction, but the attraction is still strong enough to keep them together. However not strong enough to keep the relative position fixed.
    And in the gas phase the molecules are so fierce due to thermal motion, that they don't clump together but fly apart.

    And surely someone will post some more hints... :)
     
  4. Jul 19, 2010 #3
    When a substance absorbs heat, its temperature (generally) increases--which means that its molecules start moving around faster. They move chaotically, like shaking, and vibrating, and zig-zagging around. When something 'melts' it means that the molecules are moving enough that they wont stay fixed in the crystal-lattice of a solid. Instead, although the molecules are still attached to eachother, they are attached loosely in a 'liquid'. When the molecules absorb even more heat, they're moving too much to even be contained in a liquid--and they evaporate into a gas.

    Wood (and similarly behaving things) don't melt because they burn first. Before the molecules in wood has a chance to 'melt' (separate from eachother), the molecules themselves begin to break apart into smaller molecules (carbon-dioxide, ash, etc). Thats why some things don't melt.
     
  5. Jul 21, 2010 #4
    When we say something 'melts', we mean that it turns into a liquid and also by implication, that the process is reversible.

    Some materials (usually simple ones) are able to maintain their internal integrity when heated. Gold, lead and other metals don't break up into bits until you get them really hot. (Hot like inside a star for example) So they are able to stand being heated past the point where the individual atoms or molecules vibrate rapidly enough to be essentially free of each other. A liquid or even a gas. When they cool, the inter-atomic forces re-establish themselves. The solid re-forms and no harm is done.

    In something like wood and other complex molecules, the molecular bonds are relatively weak and break first. There is no hope of reconstruction when that happens.
     
  6. Jul 21, 2010 #5
    In a vacuum, where there is no oxygen for wood to burn, what will happen to it at very high temperatures?
     
  7. Jul 21, 2010 #6

    russ_watters

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    It will disassociate into some of the more prominent substances in it, such as water, hydrocarbon gases, and solid carbon.
     
  8. Jul 21, 2010 #7
    So eventually you will have a melted solution that you can call melted wood, yet non-reversible?
     
  9. Jul 21, 2010 #8

    Andy Resnick

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    2016 Award

    This is a perfectly reasonable question. "Melting" is an example of a phase transition, and a 'phase' is some region of matter that has (essentially) uniform properties. Homogeneous materials (like ice, gold, etc) have well-defined phases, and so have well-defined phase transitions (melting, boiling, etc). Other materials, like semiconductors, colloids, foams, emulsions, etc, can also have well defined phases and phase transitions (superconduction, glassy states, etc) but since the material is more complex, it's difficult to even make a 'phase'. Monodisperse colloids (synthetic opal), magnetic materials, and polycrystalline materials are some common experimental systems, but there are others:

    http://www.seas.harvard.edu/weitzlab/

    Wood has an even more complicated structure, and the very notion of a 'phase' may not apply. So wood can't 'melt' (or undergo any simple phase transition) due to the fact that the material is not homogeneous on any length scale.
     
  10. Jul 21, 2010 #9

    Born2bwire

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    Maybe, the problem is that the constituent components of wood have different phase behaviors. At one temperature carbon may be liquid but other components, like the esthers and alcohols, would be vapor. It's like asking if I can melt a bronze box that is holding an ice cube. We could apply enough heat so that the water, copper and tin all melt, but to melt the metals we have to apply so much heat that the water will vaporize and thus be lost to the liquid.
     
  11. Jul 22, 2010 #10
    You would have a lump of charcoal, some tar, water and some gas.
     
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