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Triple point of water is a single point in P-T phase diagram of water where the three phases of water coexist. For practical applications people use the triple point cells. This video explains how to realize the triple point in this cell.

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My question is:

To get to the triple point, temperature and Pressure should be set to specific values but what he shows in this video is just cooling the cell to make some ice! How is that enough to reach the triple point??

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    $\begingroup$ If the pressure is lower than the vapor pressure at the given temperature then the evaporation of the liquid will establish an equilibrium between the liquid and its vapor at the right pressure. Check out the Clapeyron-Clausius equation: en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation $\endgroup$ – hyportnex Jan 30 '15 at 18:37
  • $\begingroup$ Isn't the triple point only realizable at a very localized region of space? You cannot for example expect it to occur in a solid crystal of ice. It's an interfacial event, and requires existence of all phases at the same time, right? $\endgroup$ – docscience Jan 30 '15 at 21:21
  • $\begingroup$ water has multiple triple points, they are all discrete (points) see upload.wikimedia.org/wikipedia/commons/0/08/… but there is only one with one of the phases being vapor. The reason why the triple points being discrete is that water is a one-component system, see en.wikipedia.org/wiki/Phase_rule . $\endgroup$ – hyportnex Jan 30 '15 at 21:42
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In a sealed constant volume container containing both liquid and gas, the pressure is maintained along the liquid/vapor line. If the pressure were higher than the liquid/vapor line the gas would condense into the liquid until the pressure reached its equilibrium point. Alternatively, if the pressure were lower than the liquid/vapor line, the liquid would boil to increase the pressure until its equilibrium point.

Thus a sealed constant volume container will be constantly be adjusting its pressure as it's heated or cooled. In this case it is cooled all the way to the triple point. At which point if it was further cooled the same effect would keep the gas and solid in equilibrium pressure.

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