At What Temperature & Pressure Can All Three Phases of Water Exist Simultaneously?

At normal pressures, two physical phases of water -- gas and liquid or liquid and solid -- can coexist at the same time. At low pressure, steam, water and ice can all occupy the same container simultaneously. The combination of temperature and pressure is called the "triple point" of a substance. In this state, all three forms of water are in thermodynamic equilibrium -- the amounts of each phase remain unchanged.

Triple Point of Water

Water has its triple point at a temperature of 0.0 degrees Celsius (32 degrees Fahrenheit) and 610 Pascals (0.089 pounds per square inch). Under these conditions, water molecules freeze, melt and turn to steam all with equal probability. In a controlled chamber at this temperature and pressure, a glass of water containing ice cubes and surrounded by vapor remains essentially the same indefinitely; more water does not form at the expense of ice, or steam at the expense of water.

Phase Diagram

Each substance has a characteristic graph called a "phase diagram" that maps out the temperatures at which the substance melts, freezes and boils. Lines on the diagram separate the three phases. Lines on water's phase diagram meet at the triple point near the center bottom where pressure is low, forming the intersection of water, ice and steam.

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