|MadSci Network: Chemistry|
Some liquids you can compress into a solid, if they are not too far above their melting point. The reason why you can obtain a solid by compression is that for most substances the solid is more dense than the liquid, and therefore occupies less space. So by forming a solid, some of the excess pressure is accommodated a bit (Le Chatelier's principle). But water is unusual in that ice is considerably less dense than liquid water, so extra space is needed to solidify the water, and that is less likely to happen the higher the pressure! Far from getting a solid when you compress water, you can actually melt ice when you compress it, provided that it is not too far below freezing point. But you are asking a theoretical and hypothetical question -- I imagine your thinking goes something like this: "If you compress any substance, you are forcing its moleculses closer and closer together, and they will have a harder time sliding past one another like they do in a liquid, and will have to form some sort of rigid structure". So you would envisage that any liquid, if sufficiently compressed, would eventually become more viscous, and then set into some sort of solid -- possibly a glassy amorphous one. I do not think there is much experimental evidence to support this reasonable notion. Certainly water is still a liquid at the bottom of the deepest ocean trench. 10 metres depth of water is about 1 atmosphere pressure, and the deepest trench would be about 10 km deep, corresponding to 1000 atmospheres. There are many liquids and gases that do not turn solid at any pressures we know about, and water is one of them. You cannot imagine just increasing pressure indefinitely. Three levels of breakdown will occur. At the first level, the energy produced by the pressure will become roughly equal to the energy of chemical bonds, and at that point the molecules and substances we know will really become something different. That is a level of pressure that is reached in the core of the larger planets. The second level is when atoms themselves get pushed into one another, and form a giant atomic nucleus. That level of pressure is produced when a large star cools down and collapses to produce a neutron star. And the third level is when the forces get so large that a black hole might be produced. As soon as you reach even the first of these stages, you would have to stop talking about "water".
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