Temperature and Compressing Water?

  • #1

Main Question or Discussion Point

Does anybody know what happens to the temperature of water when it's subject to a very high pressure? Say you take water at room temperature and apply a pressure of 20MPa, does it get hotter (and thus less dense), colder (more dense), or does it's temperature stay the same?
 

Answers and Replies

  • #2
mgb_phys
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does it get hotter (and thus less dense), colder (more dense), or does it's temperature stay the same?
The density of a liquid doesn't depend on the temperature in the same way as a gas.
Water in particular is complicated by the bonds rearranging themselves - which is why water has a maximum density at a temperature of 4C (at normal pressure)
Like most liquids water isn't very compressible at 20Mpa (say 2000m) the density only increase by 1% or so

If you compress something you are doing work and (ignoring phase changes) the temperature will rise.
 
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  • #3
Yeah, intuitively I feel that compressing something (doing work) will increase the temperature, but with water, won't that will make it less dense?
 
  • #4
russ_watters
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The density of a liquid doesn't depend on the temperature in the same way as a gas.
Clarification: if a fluid (gas or liquid) isn't volume constrained, the density does vary roughly linearly with temperature, along a pretty wide range.

That's not what the OP was asking, though. The OP was aksing about applying pressure, which means you must have a closed container. As a result, the temperature goes up as the pressure goes up - again, just like with a gas.

The only real difference in the behavior between a gas and liquid in these scenarios is the coefficients.
Water in particular is complicated by the bonds rearranging themselves - which is why water has a maximum density at a temperature of 4C (at normal pressure)
Well yes, as it gets close to the point where the phase changes, it starts behaving differently. Outside of that, it can be pretty linear.
 
  • #5
russ_watters
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Yeah, intuitively I feel that compressing something (doing work) will increase the temperature, but with water, won't that will make it less dense?
If you take an unconstrained volume of water and increase the temperature, the volume will increase roughly linearly with the temperature along most of the temperature range available (3-100C, give or take).

But that's not what you asked in your first post. In your first post, you specified (without realizing it) that the volume was decreasing and then asked if the density (and temperature) was decreasing! If you decrease the volume, obviously you increase the density. At the same time, you've added energy to a compressible fluid, therefore you'll increase the temperature.
 
  • #6
Thanks for the suggestions and information. This all makes sense to me for what I'll call 'normal' liquids. I was mainly wondering if the way the molecules fit together to give water it's lowest density around 4C did anything to alter the temperature/pressure/density relationship. I'm still a bit confused, but from what I'm reading I take it that this doesn't really make a difference, and that water is pretty much the same as other liquids when it comes to pressure, temperature, and density.
 

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