Crystallization of Supersaturated Sodium Acetate
A supersaturated solution of sodium acetate is poured onto one small sodium acetate crystal. The liquid crystallizes immediately upon contact with the seed crystal. A thermometer shows that the crystallization process is exothermic. A chemical hot pack is shown as an example of a commercially available supersaturated solution.
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Keywords
crystallization, supersaturated solutions, exothermic process, solutions, enthalpy, real life/environment, phase changes, physical properties
Multimedia
Demonstration
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movie (57 seconds, 3.5 MB)
A solution of sodium acetate in water is in a flask. A few small crystals of sodium acetate are placed on the top of the laboratory bench. The solution from the flask is poured over the crystals. As the solution contacts the crystals, its color changes from clear to white very rapidly and it crystallizes into a solid mound of material that builds up into a fairly tall vertical structure. The probe from a digital thermometer touches the solid sodium acetate to test the temperature. It is seen that the material is quite warm. The process of crystallization is an exothermic process in which heat is given off to the surroundings. The crystallization occurred rapidly once a seed crystal was available. The seed crystal provides a structure around which the ions of sodium acetate could form a solid crystal lattice.
Commercial application
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Now a commercial application of this crystallization process is shown. A metal disc is manipulated inside a sealed plastic bag holding the sodium acetate solution. This causes the sodium acetate to crystallize. The digital thermometer shows that the temperature of the heating pack has gone up significantly from room temperature. This heating pack provides an easily portable method of having heat available whenever and wherever it is needed.
Discussion
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Demonstration Notes: Warnings, Safety Information, etc.
Exam and Quiz Questions
1. Based upon the temperature reading observed, is crystallization of sodium acetate an endothermic or an exothermic process?