When we keep the pressure constant and heat up a piece of solid substance
(e.g. ice) then its temperature rises.
In the figure below we start at the phase indicate by
1.
The process of rising temperature is indicated by the horizontal line.
At a certain point we arrive at the line which separates the solid phase
of the substance from its liquid phase.
At that point, the
melting point
of the substance, the temperature stops rising
although we continue to supply heat to the piece of solid substance.
This heat is used for melting the solid into liquid.
Only after all substance is melted the temperature starts rising again
and we do reach the temperature indicated by 2
in the figure below.
Solid to liquid at constant pressure
The opposite proces, from 2
to 1, happens when we cool a liquid
under constant pressure.
At the melting point, which then is called
the freezing point or crystallization point,
further cooling does not lower the temperature until all liquid
is transformed into the solid state.
Only after all substance is frozen the temperature starts lowering again
and we do reach the temperature indicated by 1
in the figure above.
