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 or ice in the brine) or 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. This special point, called a eutectic, 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. Click on the "Show Phases" button and mouse over the diagram to see the equilibrium proportions of phases at each composition and temperature. Hydrohalite and steam been omitted from this diagram to simplify it for students just learning about saturation diagrams. To see diagrams with those phases, select them from the diagram list.