MadSci Network: Chemistry

Re: Can You compress water into a solid?

Date: Wed Jun 9 01:45:08 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 927644755.Ch

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 
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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