A frigorific mixture is a mixture of two or more chemicals that reaches an equilibrium temperature that is independent of the temperature of any of its component chemicals before they are mixed. 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.
Liquid water and ice, for example form a frigorific mixture at defining 0 degrees Celsius or 32 degrees Fahrenheit. A mixture of ammonium chloride and ice form a frigorific mixture at about -17.8 degrees Celsius or 0 degrees Fahrenheit.
Other examples of frigorific mixtures include:
|Frigorific mixtures (Temperatures in Fahrenheit)|
|Materials||Parts||The Thermometer falls from|
|Hydrochlorate of Ammonia (NH4Cl)||5||50° to 10°|
|Nitrate of Potassa (KNO3)||5|
|Hydrochlorate of Ammonia (NH4Cl)||5||50° to 4°|
|Nitrate of Ammonia (NH4NO3)||1||50° to 4°|
|Sulphate of soda (Na2SO4)||3||50° to 3°|
|Dilute Nitric acid (HNO3)||2|
|Sulphate of soda (Na2SO4)||8||50° to 0°|
|Hydrochloric acid (HCl)||5|
|Snow||1||32° to 0°|
|Common salt (NaCl)||1|
|Snow||1||30° to -15°|
|Caustic Potash, Crystallized (KOH)||1|
|Snow||1||20° to -60°|
|Sulphuric acid, dilute (H2SO4)||1|
|Snow||2||-4° to -67°|
|Chloride of Calcium (CaCl2)||3|
|Sulphuric acid, dilute (H2SO4)||10||-67° to -90°|
The most common use of a frigorific mixture is to melt ice. When sodium chloride salt is placed on ice when the ambient temperature is greater than −17.8 °C (0°F), the salt melts some of the ice and the temperature drops to −17.8. Since the mixture is colder than the ambient temperature, 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 low temperatures when mechanical refrigeration is not available, for example to tightly fit two parts of machined metal: one part is soaked in a frigorific mixture, causing it to cool and contract, and then placed into the uncooled second part. As the first part warms, it expands to fit tightly within the second part.