BCIT Physics Department

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.



Description of the Apparatus:

The "Zap 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.)