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Phase changes
When a substance melts or vaporizes, its temperature stays constant while all the heat added goes into breaking bonds between the molecules. The heat needed is called the heat of fusion or vaporization and is often much larger than the heat needed for an ordinary temperature change.
heat for a phase change
Symbols
| heat | J | |
| heat of fusion or vaporization | J/kg | |
| mass | kg |
Example
To melt kg of ice ( kJ/kg) requires kJ – the melting itself, with no temperature change.
Use for a temperature change and for a phase change – never both for the same step.
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