Heat of Fusion

The energy required to change a gram of a substance from the solid to the liquid state without changing its temperature is commonly called it's "heat of fusion". This energy breaks down the solid bonds, but leaves a significant amount of energy associated with the intermolecular forces of the liquid state.

Water phase changesTable of heats of fusionHeat of fusion measurement
Index

Phase change concepts
  HyperPhysics***** Thermodynamics Go Back





Heat of Fusion Measurement

The final expression for the experimental heat of fusion is

The experimental value is seen to be too small since the denominator of the first expression for latent heat above contains the entire mass and is therefore too large. If 100 grams of water is started at 40 C and cooled to 10 C, an ice mass of 33.3 grams of perfectly dry ice would be required for a heat of fusion equal to 80 cal/gm. If one gram of the measured 33.3 gms were already melted, then the water would be cooled to 10.6 C and the experimental value for the heat of fusion would be 77.6 cal/gm.

Water phase changesHeat of fusionSpecific heat
Index

Phase change concepts
  HyperPhysics***** Thermodynamics Go Back









Heat of Vaporization

The energy required to change a gram of a liquid into the gaseous state at the boiling point is called the "heat of vaporization". This energy breaks down the intermolecular attractive forces, and also must provide the energy necessary to expand the gas (the PDV work). For an ideal gas , there is no longer any potential energy associated with intermolecular forces. So the internal energy is entirely in the molecular kinetic energy.

The final energy is depicted here as being in translational kinetic energy, which is not strictly true. There is also some vibrational and rotational energy.

Water phase changesTable of heats of vaporization
Some energy modeling calculationsThe heat of vaporization at body
temperature is 580 cal/gm. Why?
Index

Phase change concepts
  HyperPhysics***** Thermodynamics Go Back