Supercooled Solution

Location:

Concepts Shown:

heterogeneous nucleation

Equipment:

The only item essential to this demonstration is a little bag of sodium acetate trihydrate (CH3COO Na * 3H2O) with a flat piece of metal inside it. It is simply a reusable hand warmer made by Prism Technologies. They cost $4.95 each or $192.80 for a case of 48. They can be obtained through: Midwest Therapeutic Equipment Co. 419 Leonard, NW Grand Rapids, MI 49504 (616) 530-2120 Attn. Steve Daugherty

Procedure:

When the class is covering reaction rates and nucleation, this would be a great experiment to perform. Prior to each lecture, a bag of this solution should be boiled for a few minutes to transform the solution in its liquid form. This should be done at least an hour before so that the solution has time to cool down to room temperature. the instructor will then show and explain that the liquid sodium acetate trihydrate is well below its melting temperature of 58 degrees C. Then he will bend the piece of metal in the solution and it will quickly (in about 3-4 seconds) transform into its stable solid state. Since the liquid is clear and the solid is not, the crystallization could easily be seen by the entire class if it is shown on the overhead projector.

Science:

This experiment is perfect for demonstrating heterogeneous nucleation. This solution of pure sodium acetate trihydrate is free of any dirt particles that allow the nucleation of its solid phase at its freezing temperature. Because of this, the solution remains in its liquid form far below this temperature. When the metal is bent, it exposes a fresh crevice that has high energy with many dislocations that act as nucleation sites for the solid sodium acetate trihydrate to form. Since the liquid is so far below its freezing temperature, the transformation takes place rapidly. It should be noted that it is the bending, and not the cracking, of the metal that is significant in the nucleation process illustrated in this demonstration. Another principle illustrated here is the heat of transformation. This is why the solution is sold as a hand warmer. When the liquid transforms into a solid, heat is released during the reaction, making the solution warm. The neatest thing about it is that the solid looks and feels like ice even though its temperature after solidification must be near 50 degrees C. Remarks: Thomas Glasgow, Chief of Processing Science and Technology Branch at NASA in Cleveland who sent me the information on this demonstration, had an interesting question that could be asked to the class. What would happen if water solidified at the same high temperature as sodium acetate? Answer: When water solidifies, it expands. If this same demonstration were done with a bag of water instead, expansion of water's solid form might be enough to rip the bag open.

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