Physics Dept. Demonstration Manual

DHT26 - Latent Heat of Fusion

  • to develop the concept of energy storage (heat of fusion)

  • to demonstrate the physical change of a super-saturated solution.


  • "Zap Pac" heat pack in liquid state
  • ama-digit precision electronic thermometer (optional)
  • thermal coupling grease (optional)

Description of the Apparatus:

The "Kap Pac" is a sealed vinyl pouch containing a super-saturated mixture of water and sodium acetate. The pouch also contains a small, thin, stainless steel disk.

In an open container, the mixture would freeze (crystallize) at 54°C. Sealed in the pouch with the solution free of "sites" to trigger the freezing process allows the solution to be super-cooled to as low as -14°C before freezing. At room temperature the solution in the pouch will remain liquid until activated by the disk, thereby storing heat for an indefinite period.

Flexing the stainless steel disk within the sealed pouch shocks (jars) the solution into forming a single, crystallized molecule on the disk. This causes the entire solution to crystallize (freeze). The bond formation related to crystallization releases heat and the temperature of the solution rapidly rises to the "normal" freezing point of 54°C. The process can be reversed i.e. the crystals thawed, by placing the pouch in boiling water for about 10 minutes or until clear. (Actually any temperature above 54°C will re-liquify
the solid but higher temperatures speed the process of
returning the heat energy to the material.)



  1. Hold the liquid pack in front of the class for all to see and discuss what is expected to happen. Let several students verify that the pack feels cool, i.e., is at room temperature. Optionally the instructor could measure the surface temperature of the solution using the electronic thermometer (thermocouple). To make the temperature measurement, press the brass end plate of the probe on the pouch surface. Use of a small amount of thermal coupling grease aids in this temperature measurement.

  2. Trap the steel disk in the corner of the pack and pinch it between your thumb and forefinger. Bend the disk in the middle. You should feel and hear it click. At this point crystal growth should start expanding rapidly out from the disk and the temperature increase to near 54°C. You may have to flex the disk a couple of times to trigger this freezing process.

  3. You could now remeasure the pack temperature. Pass the "hot" pack around to have the student verify and experience the heat released in the freezing process.