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Figure 3.09. H2O-NaCl equilibrium phase diagram. Phases at equilibrium for the system H2O-NaCl are shown as a function of chemical composition (weight percent NaCl) and temperature. The blue area indicates temperatures and compositions for which a single phase, brine, is present at equilibrium. Areas that have the horizontal line pattern indicate temperatures and compositions for which two phases are in equilibrium. The brine region is bounded on the right by a halite-saturation curve and on the lower left by an ice-saturation curve. Lowering of the temperature (by cooling) or changes in chemistry (by dissolving halite in the brine or by dissolving ice in the brine) can cause brines to become saturated with halite or ice. Where the two saturation curves meet is a special point, a composition and temperature where brine is saturated both with halite and with ice. Below -21.21°C, ice and halite are in equilibrium and brine is not stable. This special point gives the composition (23.20 weight percent NaCl) of the first liquid that would appear upon heating a cold mixture of ice and halite above -21.21°C.