##  [Melting Point](/melting-point-0) 

 Definition

Temperature at which a pure solid and its liquid are in thermodynamic equilibrium under a specified pressure, so the solid begins to convert to liquid without net temperature change.

 

 

 

 

 

 





## Principle

Principle

At the melting point the Gibbs free energies of the solid and liquid phases are equal for the substance at the given pressure; energy input goes into phase change rather than raising temperature.

 

 

 

 

 





## Demonstration

Demonstration

A sample of pure naphthalene in a sealed capillary at 1 atm remains at 80.2 °C while it partially melts; heating just above this temperature produces a complete liquid phase.

 

 

 

 

## Misapplication

Misapplication

Using a melting point from a mixture or impure sample as if it were the pure-substance value, or reporting the temperature at which melting is first observed during rapid heating rather than the equilibrium temperature.

 

 

 

 

 





## Consequence

Consequence

When correctly measured for a pure substance at the specified pressure, the melting point provides a reproducible thermodynamic property useful for identification, purity assessment, and phase diagrams.

 

 

 

 

## Reversal

Reversal

Freezing point refers to the temperature at which the liquid and solid are in equilibrium on cooling; for pure substances at constant pressure melting point and freezing point coincide but may differ in practice due to hysteresis and kinetics.

 

 

 

 

 





## Boundary

Boundary

Applies to pure substances under defined pressure (commonly 1 atm); excludes glass transitions, polymorphic transitions, metastable supercooled liquids, and cases dominated by kinetic barriers rather than equilibrium.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Tension exists between the equilibrium definition (thermodynamic melting point) and operational definitions used in laboratories (onset of visible melting during heating, which can be shifted by impurities or heating rate).

 

 

 

 

 





## Synthesis

Synthesis

Melting point is the thermodynamic temperature at which a pure solid and its liquid are in equilibrium at a given pressure; it reflects equality of phase free energies and is used to identify substances and assess purity, while experimental values require controlled conditions to avoid kinetic and impurity effects.