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05 December 2003

On Equipment to Study Freezing

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published January, 1968.

Common salt can lower the temperature of water a degree or two, even in the absence of ice. Other salts are more effective. A good freezing solution can be made by dissolving in fresh water sodium thiosulfate (the photographer's "hypo"). The temperature of the solution can drop as much as 25 degrees Fahrenheit. To demonstrate the effect wet the bottom of a thin glass container with fresh water, stand the container on a base of wood or some other material that is a poor conductor of heat and fill the container with equal weights of water and hypo. Stir the mixture with a wooden stick for a minute or two. The wet bottom will freeze and the glass will stick to the base.

A stoppered vial of fresh water placed in the water-hypo solution will also freeze. If the vial is full, the expanding ice may exert enough pressure to break the glass, fresh water having the unusual property of being denser as a liquid than as a solid.