Phases and Equilibrium
The States of Matter, Molecular Stickiness, and
Thermodynamics
The phases of matter represent 'classes' of the type of molecular motion found at
different temperatures. When the temperature is low, the motion of molecules is
dominated by the fact that they stick together, and the result is a phase of matter that is
rigid and dense. When the termperature is high, the motion of the molecules is
dominated by their translational energy, so intermolecular forces can almost be
ignored. At intermediate temperatures, molecules translate but still stick together.
Solids (tightly-bound molecules)
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At low temperatures the nuclei of the atoms of a solid vibrate about an
equilibrium position but are trapped in their lattice positions, unable to flow or
diffuse.
The intermolecular forces are stronger than the average thermal energy of the
system.
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Long range radial and angular order (structure) are usually present in single
crystal solids. Even amorphous solids have relatively good spatial ordering,
especially over small distances, (10-100 molecules)
Liquids
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As the binding energy to the lattice site is overcome by thermal energy, the
molecules in the solid may slip past each other but maintain close contact.
The overall substance is fluid, but not very compressible.
Some long range radial ordering persists, but usually only over the size of a few
molecular diameters
Gases (free motion)
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Gases are described by the Kinetic Theory of Gases. In this limit, gas
molecules have negligible size, have no appeciable intermolecular forces, and
are in continous, random motion.
Gases have mean free paths that are larger than molecular diameters, i.e. they
are usually isolated but occasionally have collisions
The state of a gas is universally, if approximately, described by the Ideal Gas
Equation of State.