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.
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.