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The Sodium Chloride - Water binary system
In the sodium chloride - water system, one eutectic pointand oneperitectic point is found. These points are shown in the figure below.
The eutectic point is the cryohydratic point, where ice and NaCl·2H2O (hydrohalite) both are in equilibrium with the same liquid. The peritectic point is at 0.1°C and marks the transition between hydrohalite and anhydrous sodium chloride as the 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.
Hydrohalite is the stable form of sodium chloride at temperatures below 0.1°C. It is suspected that hydrohalite forms on heavily salted roads in the winter and provides a slippery, glassy surface that looks like ice, but is "resistant" to further salting.