Re: why don't certain things liquify?

metaperl@xxxxxxxxx wrote:
> I would like to know why certain things (trees, humans, for example)
> cannot be liquified or turned into gas. It seems that humans only
> reduce to ash while water can be easily switched between water and gas.

Well, I smell homework, but what the heck. Please note that the
following is all very loose and descriptive. If you want any
accurate and quantitative description, you need statistical
mechanics. That is often not taught till university, 2nd year
or 3rd year.

Pure elements and pure compounds often have a melting point
and a boiling point. Though some don't have a melting point
at normal atmospheric pressure, such as CO2. CO2 turns from
solid to gas at normal atmo.

Complicated objects, such as trees and humans, are made of
many compounds. If you heat them, their internal nature
changes in complicated ways. In some cases, some of the
materials will melt, some will turn to vapour. Humans have
some fat in them that will melt. Both trees and humans
have water that will boil.

The different amounts of different compounds means that the
behaviour of different parts of these objects are different
under different temperatures. Bones behave very differently
to soft tissue.

A solid can have very heterogeneous characteristics. The atoms
in it are not moving very far, so complicated structures can
be retained. The bonds that hold structures together are of
energy that is far larger than the typical heat energy of a
molecule.

A liquid is a very different thing. The atoms all have to move
around with much less resistance than in a solid. The interactions
between molecules have to mostly be within some range of energy
for the liquid to be a liquid. If they get much stronger, then the
liquid turns to a solid. If they get much weaker, the liquid
turns to a gas. This range depends in a complicated fashion on
the temperature of the compound, the content of the compound,
and the pressure of the system. As mentioned previously, not all
compounds have a liquid state at any given pressure.

In a liquid, molecules can bump around inside the liquid, and
wander around. But they have a hard time getting out of the
liquid.

A gas is a different thing again. The molecules in a gas are
all interacting mostly by bumping off eachother. There is little
"sticking" of one molecule to another, at least for long times.
If you get a compound hot enough, eventually most compounds will
turn to a gas. They may undergo a variety of chemical reactions
first. But eventually the heat energy of the typical molecule will
be larger than the interaction energy. So the compound will lose
any cohesive interaction. Any tendancy of the molecules to clump
will be overwhelmed by the heat energy of the molecules.
Socks

.

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