Aqueous Salt Solutions

 

The sodium chloride - water binary system

Phase diagram for the sodium chloride water system

In the sodium chloride - water system, one eutectic point and one peritectic point is found. These points are shown in the figure to the right.

The eutectic point is the cryohydratic point, where ice and NaCl·2H2O (hydrohalite) both are in equilibrium with the same saturated solution. The eutectic point is at -21°C. The peritectic point is at 0.1°C. The peritectic point marks the transition between hydrohalite and anhydrous sodium chloride as the stable solid phase in equilibrium with a saturated sodium chloride solution. If heated, solid hydrohalite will at this temperature be transformed into anhydrous sodium chloride (NaCl) and a saturated solution of sodium chloride.

The three branches of the phase diagram are:

Hydrohalite is the stable form of sodium chloride at temperatures below 0.1°C.  The use of sodium chloride as road salt to dissolve ice in the winther can be efficient at temperatures down to the eutectic point of the solution. Below -21°C sodium chloride has no effect on ice.