TOPIC: Solids and Liquids

Demo 009

Supercooled Liquids

This is a remarkably effective illustration of the transformation of a supercooled liquid to a solid brought about by one seed crystal. The liquid is the trihydrate of sodium acetate, which melts around 65 C. If allowed to cool undisturbed, it will remain liquid and one crystal of the trihydrate will cause an immediate, exothermic solidification.

MATERIALS

Two stoppered 500 mL erlenmeyer flasks containing 200 mL of supercooled liquid sodium acetate trihydrate.

A small vial containing crystals of sodium acetate trihydrate.

PRESENTATION

This demonstration should be done after a discussion of the melting of solids and the freezing of liquids. The discussion should include the phenomenon of supercooling liquids below their freezing points. Point out that usually such liquids are very unstable and can often be induced to solidify by a slight mechanical disturbance. Mention that solidification can always be induced by the addition of a seed crystal.

Hold up a sealed flask of supercooled sodium acetate trihydrate before the class and swirl the liquid. Have several students confirm that the flask and liquid are at room temperature. With as much drama and flourish as you wish, open the flask and drop in a crystal or two of solid sodium acetate trihydrate. The liquid will begin to solidify rapidly. Hold up the flask so the class can see the change...students usually like to see the process. It takes about 30 seconds for the liquid to solidify completely, so talk about something while this is happening and then turn the flask upside down to show that there is no more liquid present. The crystallization is also fairly exothermic, so have several students verify that the flask and contents are now hot.

A variation on this procedure is to place a few crystals of sodium acetate trihydrate on a wooden board or in a pyrex pie plate or dish and slowly pour the supercooled liquid onto the crystals. With a little practice, you can make the rate of solidification the same as the rate of pouring so that when finished only a pile of white crystals is on the dish. This is a messier procedure, and it is more work to recycle the materials.

COMMENTS

Whether this is an example of a supersaturated, very concentrated solution or a supercooled liquid is open to debate. The same demo can be used to show supersaturation.

HAZARDS

It is hard to imagine any hazards for this experiment other than burning yourself with the hotplate during the melting of the trihydrate or dropping a flask on your foot. All the solid sodium acetate trihydrate can be recycled for the next time you demonstrate this phenomenon. It is not toxic.

REFERENCES

L.R. Summerlin and J.L. Ealy, Jr., "Chemical Demonstrations, A Sourcebook for Teachers," vol. 1, p 28.

 

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