How can water co-exist at three phases (solid, liquid and gas)?

Asked by: Kenny Low

Answer

Water exists in three distinct phases at something called the triple point. Zero degrees celsius is defined by the triple point of water which is 273.16K at 611.2 Pa.

At this temperature water is in the process of changing from a solid state into the liquid phase or visa versa. Molecules in the liquid phase can loose a bit of energy and solidify whilst solid water (ice) can gain some energy and melt. This can be seen in melting ice where the solid ice exists for some time while the exposed surface melts.

Molecules in a liquid don't all have the same energy. The energies of the molecules can vary from a finite minimum, which would mark the transition back to a solid phases, up to an infinite energy (although the probability of this occuring is infinitely small). The average energy of the molecules gives us the temperature of the liquid. Statistical thermodynamics can map out the energy distribution of the water molecules. At a certain energy molecules will have enough energy to evaporate, even if the water temperature is 0 degrees C.

Because of these two effects it is possible for the water to exist as solid, liquid and gas at the same time.
Answered by: Edward Rayne, Physics Undergraduate Student, Cambridge UK