CRITICAL POINT OF WATER QUESTION

[dkm0038]

I was looking at a water/vapor table and I noticed something interesting and am curious if anyone can give me a good explanation why.

That is when you increase the pressure of liquid water and you approach the critical point the density of the liquid decreases. It seems weird to me that when you increase the pressure of a volume of liquid the density will decrease.

Why is this?

 

[TheTick]

Pure water in its liquid state (without dissolved gases) is nigh incompressible. This is true for most (all?) liquids. Pressure has almost no measurable effect on density at any temperature.

Meanwhile, liquid water still expands with temperature, regardless of the pressure it is under.

 

[dvd]

Maybe you should look at a p-v or a T-v diagram. There is a line of constant specific volume that lies directly in-line with the critical point. You can travel up and down this line without changing specific volume (thus density). This is a line that falls under the saturation curve, therefore a mixture of saturated vapor and liquid. Points to the left of the saturation curve are liquids. To reach the critical point you need to increase pressure while increasing the specific volume i.e. move up and to the right (thus increasing pressure, decreasing density). So the behavior is not too surprising; I think that is how the salesman who sold water to the Creator intended it to work.

 

[dkm0038]

Isn't the thing about critical point of water that the physical properties of liquid and vapor merge (ie the density) this must happen at high pressure and temperature(22,090 kPa and 374.14 C).

So lets say you have water at 30 C with a density of 996.0 kg/m3 at a pressure of 4.246 kPa and you increase the pressure to the critical point value 22,090 kPa this would mean the liquid will have a density with the critical point value of 317.0 kg/m3.

Thus the density goes down while the pressure and temperature go up????

 

[katmar]

Read TheTick's post again. The increase in pressure has little effect - it is the increase in temperature that is causing the density to decrease.

You cannot investigate the effect of pressure alone because if you set the temperature at 374.14 C and increase the pressure towards P_c you will be in the vapor regime.

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