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Supersaturation of Sodium Acetate

Chemicals and Solutions

Sodium acetate trihydrate

Materials

  • Two flasks sodium acetate trihydrate
  • Vial of solid sodium acetate trihydrate
  • Optional: commercial hot packs (instructions on the package)
  • Watch glasses for each flask

Procedure

  1. At least 2 hours before class, remove stoppers & begin heating flasks on a hotplate. Add a little water if they are completely dry. Heat flasks covered with a watch glass until there is no solid remaining.
  2. Set stopper loosely (leave a small gap) upside down on flask. Carefully set aside to cool undisturbed.
  3. Transfer carefully to classroom.
  4. Put out vial of crystals for instructor to "seed" flasks.
  5. Add a seed crystal of sodium acetate to the flask. The supersaturated solution will crystalize and give off heat.

Discussion

When solid sodium acetate trihydrate is heated above 58ºC it loses its water of hydration and begins to dissolve in that water. The salt becomes completely dissolved at 79ºC. When the solution is cooled back down to 20ºC, it is unsaturated with respect to anhydrous sodium acetate, but it is supersaturated with respect to sodium acetate trihydrate. When a seed crystal of sodium acetate triacetate is added, sodium acetate trihydrate crystallizes out. The heat of solution of sodium acetate trihydrate is 19.7 kJ/mole (an endothermic process). The crystallization is exothermic.

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