Latent Heat

Purpose:

To demonstrate the tremendous amount of energy released when a liquid freezes to a solid.

Also to demonstrate the transformation of supercooled liquid to a solid by nucleation on a seed.

Both are shown by analogy to the exothermic crystallization of sodium acetate trihydrate from a supersaturated solution of sodium acetate and water.

Procedure:

If flasks filled with solid sodium acetate trihydrate are left from previous demonstrations (see TA office cabinet), simply heat them to a boil until there is no solid left. It may be necessary to add up to a tablespoon of water. Allow the flasks to cool to room temperature before the demonstration.

If you have to make the solution, fill a flask 1 inch full with sodium acetate trihydrate crystal and add enough water to wet crystals (about a tablespoon). Heat to a boil until there is no solid left. Allow the flasks to cool to room temperature before the demonstration.

Transport the flasks carefully so crystallization does not occur prematurely.

Drop a few crystals of sodium acetate trihydrate into the flask to provide a seed for crystallization. The solution solidifies and the flask becomes toasty warm. Pass the flask around.

Equipment:

Comments:

The heat released is analogous to the latent heat released in a phase transition. The mechanisms for releasing heat is similar - a substantial change in the arrangement of molecules and bonds when the phase transition (crystallization) occurs.

Some commercial hotpacks use this process.

References:

Contributors:

C. M. Bitz,

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