Thanks for your question. Answer B is referring to the saturation pressure. As temperature decreases, saturation pressure also decreases. For the example of water, the saturation pressure just below the critical temperature at 373 degrees is 220 bar, but at 100 degrees C, the saturation pressure is only 1 bar. It is easiest to see how saturation pressure changes with temperature by looking at a steam table. Hopefully this helps!
3 years late but here ya go: The question is basically comparing Water in two states: #1 Critical water (above Critical temp) and #2 Non-critical water (Below critical temp). #2 can be thought of as just, regular water, or maybe a pot of boiling water. Critical fluids are characterized by gas and liquid states having no distinguishing characteristics between them, 0 latent heat to phase change, 0 density change, the gas and liquid are indistinguishable when the fluid is critical. Meanwhile, for a pot of boiling water, its very easy to distinguish between the liquid boiling in the pot and the steam vapor rising above it. Final point, "Condensation pressure" is just a fancy way to say that something is at its boiling temperature (for a given pressure). All that said, for a pot of boiling water, we intuitively know that the steam is less dense than the boiling water. Therefore, when a critical fluid (with same density of "liquid/vapor" mix) becomes sub-critical, the liquid becomes more dense, at least relative to its vapor phase. Side note - The question is a little weird, as decreasing temperature CAN increase the density of a gas (PV=nRT), but I can't think of any examples where decreasing temperature decreases the density of the liquid.
Great video! I think you meant to say that the density of the vapor will also decrease as the temperature decreases not that it is increase.
Speaking too fast for me. But i can replay....
i can't understand why answer A is invalid.
good
THANK YOU!!!
I didn't understand why the B is false
Thanks for your question. Answer B is referring to the saturation pressure. As temperature decreases, saturation pressure also decreases. For the example of water, the saturation pressure just below the critical temperature at 373 degrees is 220 bar, but at 100 degrees C, the saturation pressure is only 1 bar. It is easiest to see how saturation pressure changes with temperature by looking at a steam table. Hopefully this helps!
Why option A is false
3 years late but here ya go: The question is basically comparing Water in two states: #1 Critical water (above Critical temp) and #2 Non-critical water (Below critical temp). #2 can be thought of as just, regular water, or maybe a pot of boiling water.
Critical fluids are characterized by gas and liquid states having no distinguishing characteristics between them, 0 latent heat to phase change, 0 density change, the gas and liquid are indistinguishable when the fluid is critical. Meanwhile, for a pot of boiling water, its very easy to distinguish between the liquid boiling in the pot and the steam vapor rising above it.
Final point, "Condensation pressure" is just a fancy way to say that something is at its boiling temperature (for a given pressure).
All that said, for a pot of boiling water, we intuitively know that the steam is less dense than the boiling water. Therefore, when a critical fluid (with same density of "liquid/vapor" mix) becomes sub-critical, the liquid becomes more dense, at least relative to its vapor phase.
Side note - The question is a little weird, as decreasing temperature CAN increase the density of a gas (PV=nRT), but I can't think of any examples where decreasing temperature decreases the density of the liquid.