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 Question 1291

 Why can ice, water and steam co-exist at "triple point"? — CL
Let's start with three simpler problems: the coexistences of ice and water, of water and steam, and of ice and steam. Each pair of phases can coexist whenever the water molecules leaving one phase are replaced at an equal rate by water molecules leaving the second phase. This isn't as hard as it sounds. In ice water, the water molecules leaving the ice cubes for the liquid are replaced at an equal rate by water molecules leaving the liquid for the ice cubes. In a sealed bottle of mineral water, the water molecules leaving the liquid for the water vapor above it are replaced at an equal rate by water molecules leaving the water vapor for the liquid. And in an old-fashioned non-frostfree freezer with a tray of ice cubes, the water molecules leaving the ice cubes for the water vapor around them are replaced at an equal rate by water molecules leaving the water vapor for the ice cubes.

In each case, there is some flexibility in temperature—these coexistence conditions can be reached over at least a small range of temperature by varying the pressure on the system. In fact, at 0.03° C and a pressure of 6.11 torr; pure water, pure ice, and pure steam can coexist as a threesome. At this triple point, water molecules will be moving back and forth between all three phases but without producing any net change in the amount of ice, water, or steam.