The underlying concept behind changes of state is kinetic
molecular theory - the idea that all particles in substances are constantly in
motion.
Two factors determine the physical state of any
substance: the relative attraction between particles and the velocity of the
particles.
A change of state where heat is added is called
an endothermic phase change. Endo meaning entering or going in, and therm referring to
heat. When heat is added to a substance the velocity of the particles (their kinetic
energy) also increases. As their velocity increases, the particles move further apart,
and the attraction between particles decreases. This happens during melting, boiling,
and sublimation (the direct change from a solid to a
gas).
A change of state where heat is removed is called an
exothermic phase change. Exo meaning leaving or going out, and therm again referring to
heat.. When heat is removed, the velocity of the particles decreases, they get closer
together, and the attraction between the particles becomes more important and more
likely. Exothermic phase changes are solidification, condensation, and deposition
(direct change from a gas to a solid)
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