Freezing Mixture

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freezing mixture

[′frēz·iŋ ‚miks·chər]
(physical chemistry)
A mixture of substances whose freezing point is lower than that of its constituents.

Freezing Mixture

 

a combination of substances that when mixed lower the temperature of the mixture by absorbing the melting heat or the heat of solution of the system’s components, which can be liquid, solid, or both. To attain the lowest possible temperature, the components of a freezing mixture are used in quantities that will form a cryohydrate. Water, ice, snow, and various salts and acids will form freezing mixtures that attain a temperature of — 50°C. Salts in dilute acid solutions lower temperatures more than salts in a water solution. Substitution of crushed ice for water makes it possible to attain lower temperatures because, in addition to absorbing the heat of solution of the salt, the mixture absorbs the melting heat of the ice (see Table 1).

Table 1. Some freezing mixtures
ComponentsPercent composition by weightTemperature after mixing1 (°C)
1Before mixing, the temperature of combinations of acids or liquid water with salts is 10°–15°C, while the temperature of combinations that use ice is 0°C
Water..............
NH4C1 .............
KNO3..............
Freezing Mixture–12
H2SO4 .............
Na2SO4 ............
Freezing Mixture–20
Ice................
H2SO4 .............
Freezing Mixture–30
Ice................
K2CO3 .............
Freezing Mixture–46

Freezing mixtures of dry ice and alcohols or ethers can reach a temperature of – 80°C.

Freezing mixtures are used mainly in the laboratory to generate and maintain low temperatures. Industrial requirements for low temperatures are fulfilled by a freezing mixture that consists of crushed ice and reagent grade table salt, or NaCl.

V. N. BONDAREV