engineering question #8649



Justin H, a 29 year old male from Vancouver asks on December 2, 2011,

Q:

If fresh water is pressurized to 1 MPa or greater, frozen solid at this pressure, then completely thawed, will it remain pressurized? [Editors note: a megapascal is 10 times the pressure of air at the Earth's surface.]

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the answer

John Jones answered on December 4, 2011, A:

Yes it will, though the details depend on what mechanism is used to apply the pressure.

If the pressure is created by, for example, putting a stone weighing 1 MN [one meganewton, a measure of force] on top of a one-square-meter piston that fits into a cylinder containing the water, as the water freezes the expanding ice will simply raise the stone. Then when the ice melts, the stone will come down again and the water will still be under pressure.

If on the other hand we compress the water using, for example, a diamond piston fitting into a diamond cylinder, and, having compressed it to 1MPa, lock the piston into place so that the contained volume is forced to remain constant, the pressure inside the cylinder will rise further as the ice attempts to form.  Assuming the diamond is absolutely rigid, the pressure will rise to about 200 MPa, at which point ice-three will form.  Ice-three is a crystalline form of ice that is denser than water.  It forms tetragonal crystals, whereas regular ice forms hexagonal crystals. But when it melts, the melted water will occupy the same volume as it did before, so it will still be pressurised. 

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