Heat of Fusion
Definition and meaning of Heat of Fusion in chemistry.
Heat of fusion is the thermal energy needed to melt a solid. This completely turns a solid mass into a liquid at its melting point. The entire melting process happens without any change in the actual temperature.
In more detail
Adding energy during melting only breaks the rigid forces inside the solid. It overcomes the strong intermolecular or ionic forces holding the crystal lattice. The heat does not make the tiny particles move any faster overall.
This means the temperature stays perfectly flat until the melting finishes completely. The temperature only starts rising again after every last solid piece melts. Chemists often call this energy an enthalpy change at a constant pressure.
It is a specific type of latent heat hidden inside the phase change. Freezing the exact same liquid releases an identical amount of heat energy. That reverse process is formally known as the heat of crystallization.
Substances with strong attractive forces usually have unusually high heats of fusion. Water has a huge heat of fusion because of its hydrogen bonds. You must put in enormous energy to break its hexagonal ice structure.
Students often forget that melting requires constant energy input from the room. An ice cube left on a counter actively absorbs heat to melt.
Key facts
| Symbol | ΔHfus |
|---|---|
| Typical units | kJ/mol or J/g |
| Water's heat of fusion | 6.01 kJ/mol (334 J/g) |
| Sign convention | Always positive (endothermic) |
| Effect on temperature | Remains constant during phase change |
| Field | Physical Chemistry |
The heat of fusion of pure water is exactly 6.01 kJ/mol. This equals 334 joules per gram of ice at 0 degrees Celsius. This is why melting a large tray of ice absorbs massive heat. The melting ice water mixture stays at a steady 0 degrees Celsius. It stays cold until the very last piece of ice turns liquid. This is the exact same principle that keeps insulated picnic coolers cold. The melting ice constantly pulls heat away from your packed food items.
Frequently asked questions
Is heat of fusion the exact same thing as latent heat?
Yes, the two terms are used interchangeably in most general chemistry classes. Both phrases describe the hidden energy absorbed during melting at constant pressure. This energy changes the phase without changing the thermometer reading at all.
Why does the temperature stay perfectly flat while a solid melts?
The supplied heat only breaks the strong attractions holding the solid together. The energy does not increase the average kinetic speed of the particles. Since temperature just measures average kinetic speed, the thermometer reading stays constant.
Do all metals have extremely high heats of fusion?
Yes, most solid metals require enormous amounts of energy to melt completely. You have to break very strong metallic bonds holding the atoms together. This is why foundries need incredibly hot industrial furnaces to melt steel.