Enthalpy of Fusion / Melting
Also: Heat of fusion, Latent heat of melting, Melting enthalpy
The enthalpy of fusion is the amount of energy a substance must absorb to change from the solid to the liquid state at its melting point, without changing its temperature.
The enthalpy of fusion (also called heat of fusion or latent heat of melting) describes the energy that must be supplied to a substance in order to convert it fully from the solid to the liquid state at constant pressure and constant temperature — namely at the melting point. This energy does not change the temperature but breaks the crystal-lattice bonds in the solid. The process is endothermic; on solidification the same energy is released again as the enthalpy of crystallisation.
Physical basis
When a metal is heated, the temperature first rises linearly. At the melting point it remains constant despite further heat input, until the entire substance has melted — only then does the temperature of the melt continue to rise. The energy supplied goes into overcoming the intermolecular bonding forces.
The specific enthalpy of fusion $\Delta H_\text{fus}$ is given by:
Q = m · ΔH_fus
where Q is the required amount of heat (in J), m the mass (in kg) and ΔH_fus the specific enthalpy of fusion (in kJ/kg) of the material in question.
Enthalpies of fusion of important precious metals compared
| Metal | Melting point | Enthalpy of fusion (kJ/mol) | Enthalpy of fusion (kJ/kg) |
|---|---|---|---|
| Gold | 1,064 °C | 12.55 | 63.7 |
| Silver | 961 °C | 11.28 | 104.6 |
| Platinum | 1,768 °C | 22.17 | 113.6 |
| Palladium | 1,555 °C | 16.74 | 157.3 |
| Copper | 1,085 °C | 13.05 | 205.4 |
Both per mole and per kilogram, platinum and palladium have considerably higher enthalpies of fusion than gold, which explains the increased energy input in their processing at the refinery.
Relevance for precious-metal processing and melt value
For the purchase, recycling and refining of scrap gold, broken gold or dental gold, the enthalpy of fusion plays a practical role: the melt value of an object describes the pure material value relative to the current gold price or silver price — independent of form or minting. Refineries and recycling operations must calculate the thermal energy input during melting, which depends directly on the enthalpy of fusion of the alloy mixture.
The fineness of an alloy also affects its effective enthalpy of fusion: alloys such as 585 gold (14 carat) melt over a temperature range (solidus to liquidus), not at a defined point like the pure metal fine gold (999).
In brief
The enthalpy of fusion is a key quantity in precious-metal physics: it determines the energy required for melting, explains the plateau in the heating curve, and underpins the melt-value calculator. A higher enthalpy of fusion means more energy input per kilogram — with a direct impact on processing costs in refining and recycling.