Water ice melting
- "Molten" redirects here For the Japanese
company, see Molten Corporation;
or see Molton
or Moulton.
Melting (sometimes called
fusion)
is a physical process that results in the
phase change of a substance from a
solid to a
liquid. The
internal energy of a solid substance is
increased, typically by the application of heat or pressure,
resulting in a rise of its temperature to the
melting point, at which the rigid ordering of
molecular entities in the solid breaks down to a less-ordered state
and the solid liquefies. An object that has melted completely is
molten.
Melting point
Under a standard set of conditions, the melting point of a
substance is a characteristic property. The melting point is often
equal to the
freezing point. However,
under carefully created conditions, supercooling or superheating
past the melting or freezing point can occur.
Water on a very clean glass surface will
often supercool several degrees below the melting point without
freezing. Fine emulsions of pure water have been cooled to -38
degrees Celsius without nucleation to form
ice.
. Nucleation occurs due to fluctuations in the properties of the
material. If the material is kept still there is often nothing
(such a physical vibration) to trigger this change, and
supercooling (or superheating) may occur. Thermodynamically, the
supercooled material is unstable with respect to the frozen phase,
and it is likely to change phase suddenly. This phenomenon is
similar to hysteresis in permanent magnets, as they are heated and
cooled near the
Curie point.
Molecular vibrations
When the internal energy of a solid is increased by the application
of external energy, the molecular vibrations of the substance
increase. As these vibrations increase, the substance becomes less
and less ordered until the stored vibrational energy exceeds the
stabilization energy of the solid arrangement and molecules may
break free of the lattice.This IS Complicated Stuff
From Leah Topp
Constant temperature
Substances melt at a constant temperature, the melting point. As
long as the solid is in equilibrium with its liquid form it does
not increase its temperature. The energy used to transform the
solid into a liquid is a
latent heat.
This characterizes the process of melting as a first-order phase
transition.
Thermodynamics of melting
From a thermodynamics point of view, at the melting point the
change in
Gibbs free energy
(\Delta G) of the material is zero, but the
enthalpy (H) and the
entropy
(S) of the material are increasing (\Delta H, \Delta S > 0).
Melting occurs when the Gibbs free energy of the liquid becomes
lower than the solid for that material. The temperature at which
this occurs is dependent on the ambient pressure. It can also be
shown that:
\Delta S = \frac {\Delta H} {T}
The "T","\Delta S", and "\Delta H" in the above are the temperature
at the melting point, change of entropy of melting, and the change
of enthalpy of melting, respectively.
Other meanings
In
genetics,
melting
DNA means to separate the double-stranded DNA
into two single strands by heating or the use of chemicals.
- See also : Polymerase chain reaction
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