Fusion
Definition and meaning of Fusion in chemistry.
Fusion is the physical phase change where a solid substance turns into a liquid. The solid must absorb heat energy to reach its specific melting point temperature. The overall temperature remains completely constant during this entire physical state transition.
In more detail
During fusion, the absorbed heat energy does not increase the substance's overall temperature. Instead, this energy works to overcome the strong intermolecular attractions holding the solid together. The particles break out of their fixed positions in the rigid solid lattice.
The solid and liquid phases coexist at a single temperature until melting finishes. Only after the solid completely melts will additional heat raise the liquid temperature. The heat required to melt one mole of a substance is a specific value.
Chemists identify this specific energy requirement as the molar enthalpy of fusion. This important value is represented by the symbol ΔHfus in thermodynamic chemical equations. Every pure chemical substance possesses its own unique molar enthalpy of fusion value.
Many beginning chemistry students incorrectly confuse this physical phase change with nuclear fusion. Nuclear fusion involves atomic nuclei combining to form entirely new chemical elements. Chemical fusion simply involves changing the physical state from solid to liquid.
The chemical identity of the substance does not change during the melting process. Chemical fusion is the exact reverse physical process of freezing or liquid solidification. Both fusion and freezing occur at the exact same temperature for a pure substance. The only real difference is the physical direction of the overall heat energy flow.
Key facts
| Common name | Melting |
|---|---|
| Reverse process | Freezing or solidification |
| Key property | Molar enthalpy of fusion (ΔHfus) |
| Temperature behavior | Remains perfectly constant during the transition |
| Energy flow | Endothermic (requires absorption of heat energy) |
| Example value | 6.01 kJ/mol for pure water |
You can easily observe the fusion of water by melting ice cubes in a glass. Melting exactly one mole of solid ice requires absorbing 6.01 kilojoules of heat energy. This specific energy amount represents the exact molar enthalpy of fusion for pure water. The melting solid ice and the newly formed liquid water both remain at 0°C.
Frequently asked questions
Why does the temperature stop rising while the solid is actively melting?
The absorbed heat energy breaks the solid lattice structure instead of increasing the particle speed.
Is the chemical process of fusion related to nuclear fusion in stars?
No, chemical fusion is simply physical melting, while nuclear fusion creates completely new atomic elements.
Does a substance melt and freeze at the exact same temperature?
Yes, for any pure substance, the melting point and the freezing point are perfectly identical.