Freezing-point diagram with compound formation

Sodium chloride Sodium bromide Sodium sulfate Potassium chloride Ammonium chloride 

By permission from A. Findlay, A. N. Campbell, and N. O. Smith, The Phase Rule and Its Applications, 9th ed. New York Dover, 1951, p. 141. 

The invariance of the system at the eutectic point allows eutectic mixtures to be used as constant temperature baths. Suppose solid sodium chloride is mixed with ice at 0 °C in a vacuum flask. The composition point moves from 0% NaCl to some positive value. However, at this composition the freezing point of ice is below 0 °C hence, some ice melts. Since the system is in an insulated flask, the melting of the ice reduces the temperature of the mixture. If sufficient NaCl has been added, the temperature will drop to the eutectic temperature, —21.1 °C. At the eutectic temperature, ice, solid salt, and saturated solution can coexist in equilibrium. The temperature remains at the eutectic temperature until the remainder of the ice is melted by the heat that leaks slowly into the flask. 

The action of rock salt or calcium chloride in melting ice on sidewalks and streets can be interpreted by the phase diagram. Suppose sufficient solid salt is added to ice at — 5 °C to move the state point of the system to c (Fig. 15.10). At c the solution is stable the ice will melt completely if the system is isothermal. If the system were adiabatic, the temperature would fall until the state point reached d. The eutectic temperatures of a few ice-salt systems are given in Table 15.1. 

4 FREEZING-POINT DIAGRAM WITH COMPOUND FORMATION 

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Freezing-Point Diagram with Compound Formation 329... 

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