Phase Changes

Changing a liquid
from one form
to another
involves a process
called phase change,
phase transition,
or change of state.
An endothermic
phase change
occurs when water
changes to vapor
by adding heat.
An exothermic
phase change
occurs when water
changes to ice
by removing heat.
Thus ice
is an aggregation
of water molecules
with sufficient heat
removed
to enable
that phase change.
Ice changes back
to liquid water
as either heat
or external pressure
is increased.
Phase changes
from liquid to gas,
gas to liquid,
liquid to solid
and solid to liquid
are well-known
chemical processes,
yet the origins
of chemistry itself
remain shrouded
in mystery.

- A.T. Williams

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