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In general any substance is solid at a low
temperature and gaseous at a high temperature.
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Let's take for example
an ice crystal at (-10 ºC) and increase its temperature by slowly
heating it; the water molecules which constitute its crystalline
network increase the amplitude of their vibrations until at 0 ºC,
the vibrations become so intense that the molecules lose their fixed
positions and loosen their mutual attraction, becoming sufficiently
free to be able to move around; we say that the ice has changed its
state.
The water and the ice
in this glass receive heat from the air which surrounds them, this is
why all the ice will end up melting.
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The change of state from
solid to liquid is called fusion.
While there is fusion the heat applied will not raise the
temperature above 0 ºC, but rather will be used to "loosen"
the molecules more and more until all the ice has changed into a liquid
state. The change from liquid to solid is called solidification.
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