Crystallization of Supersaturated Sodium Acetate
A supersaturated solution of sodium acetate is poured onto one small sodium acetate crystal. Crystals form from the liquid 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
applications - practical/real-life, crystals/crystallization, enthalpy/heat, exothermic process, solutions/solubility, supersaturated
Multimedia
The Reaction
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(QuickTime 3.0 Sorenson, duration 39 seconds, size 3.1 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.
 Crystals of sodium acetate. |
 Pouring a solution of sodium acetate in water. |
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Measuring the Temperature
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(QuickTime 3.0 Sorenson, duration 19 seconds, size 1.5 MB)
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.
 The material is quite warm. |
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Commercial Application
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(QuickTime 3.0 Sorenson, duration 46 seconds, size 3.5 MB)
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.
 The sodium acetate crystallizes... |
 ... and fills the container. |
 The material is quite warm. |
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Discussion
Without a seed crystal to act as a template for formation of a crystal lattice, crystallization is very slow and a solution can be supersaturated for a long time. Because this crystallization process is exothermic it has practical application as a heating pad.
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?