Frigorific mixture

A frigorific mixture is a mixture of two or more chemicals that achieve an equilibrium temperature independent of the temperature that the two chemicals started at. The temperature is also relatively independent of the quantities of mixtures as long as significant amounts of each original chemical are present in its pure form.

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Liquid water and ice, for example form a frigorific mixture at 0 degrees Celsius or 32 degrees Fahrenheit. A mixture of sodium chloride and ice form a frigorific mixture at -17.8 degrees Celsius or 0 degrees Fahrenheit. Other examples of frigorific mixtures include ^[1]:

Uses

The most common use of a frigorific mixture is to melt ice. When salt is placed on ice when the ambient temperature is greater than −17.8 °C, then the salt melts some of the ice and the temperature drops to −17.8. Since the mixture is colder than the ambient, heat is absorbed and the temperature rises. This causes the salt to melt more of the ice to drive the temperature down again. The process continues until all of the salt is dissolved in the melted ice. If there is enough salt present, then all of the ice will be melted.

Frigorific mixtures are commonly used in laboratories as a convenient way to generate reference temperatures for calibrating thermometers.

They are also useful for creating cold temperatures when mechanical refrigeration is not available.

They can be used for shrink fitting two items such as machined pieces of metal. One item is placed in a mixture to get it cold and shrink. The cold item is then placed into the other item. When the cold item warms up, it expands and fits tightly into the other item.

References

1. ^ The Ordnance Manual for the Use of the Officers of the United States Army, Third Edition, 1862, page 462