0
$\begingroup$

I've been thinking about the critical point of water, which has three distinct and specific properties: critical temperature, critical pressure, and specific critical volume. However, when I draw a PV and TV graph I can't seem to think of any way in which to bring the water to its critical point because in order to get one property to its final state you have to change one of the others. For instance, if you already have the water at its critical temperature and pressure, you would have to decrease or increase one of those values to get it to its critical volume.

To this end, I'm wondering whether it's even possible to bring water to its critical point - because once you achieve two of the three intended properties, it seems impossible to me to get the third property to the correct state without changing one of the other two.

$\endgroup$
5
  • $\begingroup$ You’re not using the ideal gas law, are you? $\endgroup$ Commented Jul 11, 2023 at 18:33
  • $\begingroup$ No, simply thinking about the process logistically. $\endgroup$
    – bmjoe
    Commented Jul 11, 2023 at 18:48
  • 1
    $\begingroup$ Related: How to realize the triple point of water? I would think that heating a triple point cell would get you to the critical point in the same way. $\endgroup$ Commented Jul 11, 2023 at 19:00
  • $\begingroup$ renmatix.com/understanding-supercritical-water $\endgroup$
    – mmesser314
    Commented Jul 11, 2023 at 20:06
  • 2
    $\begingroup$ There is no critical volume. It's just critical temperature and pressure. Critical volume may be referring to "specific volume at the critical point" $\endgroup$
    – RC_23
    Commented Jul 11, 2023 at 20:19

3 Answers 3

8
$\begingroup$

The flaw in your reasoning is in assuming that the critical temperature, pressure and (specific) volume are three independent quantities. The temperature, pressure and volume are linked by the equation of state and so, as you reasoned, cannot be varied independently. However this simply means the critical temperature, pressure and specific must also obey this relation, so once two of these properties have been brought to their critical values, the third will be there automatically.

$\endgroup$
2
$\begingroup$

Yes. The use of supercritical water in industrial processes is an active area of research and has uses in waste cleanup through the process known as supercritical water oxidation.

$\endgroup$
1
$\begingroup$

As stated by others, the critical properties of water are all dependent on each other. The procedure to get water to its critical temperature, critical pressure, and critical volume is "easier" than you are assuming. To run such an experiment, acquire a thick walled insulated container, place an electrical heat source inside that container, place a pipe fitting on the container that allows addition of water and a connection to a vacuum pump, and place instrumentation on that container that measures temperature and pressure. Then make sure the container is tightly closed and pull as high of a vacuum on that container as practical. Fill the container half full of water and seal it. Due to the vacuum conditions, the vapor space will be filled with water vapor. Now turn on the heat source and heat the container until it reaches water's critical temperature. Due to the physical property dependencies involved, the pressure instrument will read critical pressure, and if you had a way to measure it, the volume of the vapor inside the container would be at its critical volume. This means that you only have to manipulate one physical property of water (the temperature) and the other two physical properties will "match" that temperature.

Note that this experiment will not work if you attempt to pressurize water up to its critical pressure without heating it. Water can be compressed far beyond its critical pressure, but it will remain a liquid if its temperature is anywhere close to ambient. This fact may be where your confusion arises.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.