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Water ice melting
"Molten" redirects here For the Japanese company, see Molten Corporation; or see Molton or Moulton.

Melting, or 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 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.[citation needed]. 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.

Lattice vibrations

When the internal energy of a solid is increased by the application of external energy, the molecular and lattice vibrations of the substance are excited. As the lattice vibrations gain amplitude, the kinetic energy of molecules exceeds the lattice stabilization energy of the solid arrangement and molecules escape the lattice and the system becomes less ordered.

Constant temperature

When a substances melts and the solid and liquid phases are in an equilibrium, it maintains a constant temperature, the melting point. The energy used for melting 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 (ΔG) of the material is zero, but the enthalpy (H) and the entropy (S) of the material are increasing (ΔHS > 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","ΔS", and "ΔH" in the above are the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting, respectively.

Driving force

The driving force for the fusion process is directly proportional to the under cooling (δT) if the entropy of fusion (ΔS)and latent heat of fusion (ΔH) don't vary much with temperature.It can be shown as follows:

At fusion temperature (Say T(f)), ΔG=0
=> ΔH(f)=T(f).ΔS(f)

Now ΔG = ΔHTδSΔH(f) − TδS(f)ΔS(fT
Here δT = T(f) - T (Under-cooling)

Related concepts

In genetics, melting DNA means to separate the double-stranded DNA into two single strands by heating or the use of chemical agents, cf. Polymerase chain reaction.

See also


Further reading

From Solid Liquid Gas Plasma
Solid Solid-Solid Transformation Melting/Fusion Sublimation N/A
Liquid Freezing N/A Boiling/Evaporation N/A
Gas Deposition Condensation N/A Ionization
Plasma N/A N/A Recombination/Deionization N/A

Simple English

Melting is the process of changing something from a solid into a liquid (like ice into liquid water). It is the opposite of freezing. The temperature at which something begins to melt is called its melting point. When an object is heated, the object's particles begin to move faster and faster until the structured particles are freed. Now it is a liquid.

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