I was reading the Knight textbook, which stated in Chapter 16.1 (A Macroscopic Description of Matter) the following:

"The notion of three distinct phases is less useful for more complex systems. A piece of wood is solid, but liquid wood and gaseous wood don’t exist."

Can someone explain to me what is meant by "more complex" and its relationship to phase transition? That is, why can't one describe a phase transition for a piece of burning wood even though, as it appears to me, that it transitions into a gas (i.e. sublimation)?

  • $\begingroup$ The temperature at which wood melts is much higher than the temperature at which it burns. Therefore, it is not possible to melt wood (at least when it is surrounded by oxygen). $\endgroup$ – Hunter Jan 13 '14 at 22:00
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    $\begingroup$ Another issue is that 'wood' is from a physics perspective not a single material. The resin contained in wood for example can be solid, liquid or in the gas phase. Lignin is a large molecule that will break apart before melting, etc. $\endgroup$ – Alexander Jan 13 '14 at 22:06
  • $\begingroup$ @Hunter: can you explain more why wood melts at a much higher temperature than when it burns. Thanks. $\endgroup$ – Jesus Jan 13 '14 at 22:07
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    $\begingroup$ thenakedscientists.com/forum/index.php?topic=7504.0 $\endgroup$ – Hunter Jan 13 '14 at 22:13
  • $\begingroup$ @Alexander: thanks, this was very useful to my understanding and makes it very clear now! $\endgroup$ – Jesus Jan 13 '14 at 22:24

Aside from water, wood has three main components. Cellulose, a crystalline polymer derived from glucose, constitutes about 41–43%. Next in abundance is hemicellulose, which is around 20% in deciduous trees but near 30% in conifers. It is mainly five-carbon sugars that are linked in an irregular manner, in contrast to the cellulose. Lignin is the third component at around 27% in coniferous wood vs 23% in deciduous trees. Lignin confers the hydrophobic properties reflecting the fact that it is based on aromatic rings

It appears that wood is complex in its chemical state as it contains components of both liquid and solid form. Also when you burn wood many chemical reactions take place, primarily the converion of its carbon content to coke as well as production of $CO_2$, remember that it both goes in smoke and stays behind as ash, this is not a conventional phase transition as there is no clear phase change, it is much more of a chemical reaction with products in different state.


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