[danago]

Hey. Ive just started taking my first course in chemical engineering thermodynamics, and come across something that im a little confused about. I am reading about systems that contain two phases of the same pure substance, and how we need only one intensive property in a single phase to determine all other intensive properties of that phase. It then goes on with an example of water in a liquid-vapor state showing that if we know the pressure of the water is 1atm, we can determine the temperature of the water, internal energy etc. What i am a bit lost with is how we can describe the water with a single pressure value. Will the pressure in the water not depend on how deep into the water we are? Am i missing something?

Thanks in advance,
Dan.

[ARGOS++]

Dear danago;

You are right! - that only one parameter is not enough!

Usually pressure and temperature are required, and that’s normal presented in a so called "Phase Diagram", as for example on:

http://en.wikipedia.org/wiki/Phase_diagram

Therein you can see that for one pressure you can have till three phases but all with different temperatures.

I hope to have been of help to you.
Good Luck!
                    ARGOS⁺⁺

[Borek]

What i am a bit lost with is how we can describe the water with a single pressure value.

We are talking about volume of the phase small enough that you can assume pressure to be constant. So not an ocean, more like a glass

[danago]

Thanks for both the replies

What i am a bit lost with is how we can describe the water with a single pressure value.

We are talking about volume of the phase small enough that you can assume pressure to be constant. So not an ocean, more like a glass

Answers my question perfectly  Guess i was imagining the ocean scenario  Thanks.

[typhoon2028]

Dear danago;

You are right! - that only one parameter is not enough!

Usually pressure and temperature are required, and that’s normal presented in a so called "Phase Diagram", as for example on:

http://en.wikipedia.org/wiki/Phase_diagram

Therein you can see that for one pressure you can have till three phases but all with different temperatures.

I hope to have been of help to you.
Good Luck!
                    ARGOS⁺⁺

If I read this problem correctly, he is given two parameters.  The student is told there is a pure substance (water) existing in two phases and the pressure is 1 atm.  If the pure substance is in a sealed container at 1 atm, we can determine the temperature to be 100 C.  This can be determined, because we are told the substance exist in two phases.  Water should exert a vapor pressure of about 1 atm at 100 C.

Do you agree with this line of thought?

[physstudent1]

If the system is at saturation only one variable will be needed, then just go to the steam tables for saturated water look at the pressure and the temperature will correspond to saturated water at that pressure

just read your post typhoon and I agree with what you are saying you should only need one intensive variable if you know the system is saturated shouldn't you?

[Hunt]

Hey. Ive just started taking my first course in chemical engineering thermodynamics, and come across something that im a little confused about. I am reading about systems that contain two phases of the same pure substance, and how we need only one intensive property in a single phase to determine all other intensive properties of that phase. It then goes on with an example of water in a liquid-vapor state showing that if we know the pressure of the water is 1atm, we can determine the temperature of the water, internal energy etc. What i am a bit lost with is how we can describe the water with a single pressure value. Will the pressure in the water not depend on how deep into the water we are? Am i missing something?

Thanks in advance,
Dan.

For any single component system in which two phases coexist at equilibrium, there is only one intensive indepedent variable , could be T or P.  The system can then be described by only one variable. Pressure cannot vary without a change in temperature, or vice versa. You can think logically about this by first assuming that you have two independent variables, say T and P. But two phases , liquid and vapour , are at equilibrium ,so their chemical potentials are equal. The Chemical potential is a function of T and P, so you have an equation that relates T and P for the system. The number of independent variables reduces to one.

This is just a specific case of the more general Gibbs Phase rule that specifies the number of degrees of freedom for a system of C components and P phases.

[physstudent1]

For any single component system in which two phases coexist at equilibrium, there is only one intensive indepedent variable , could be T or P.  The system can then be described by only one variable. Pressure cannot vary without a change in temperature, or vice versa. You can think logically about this by first assuming that you have two independent variables, say T and P. But two phases , liquid and vapour , are at equilibrium ,so their chemical potentials are equal. The Chemical potential is a function of T and P, so you have an equation that relates T and P for the system. The number of independent variables reduces to one.

This is just a specific case of the more general Gibbs Phase rule that specifies the number of degrees of freedom for a system of C components and P phases.

perfect.

[danago]

I have actually decided to take this class next semester instead due to timetable clashes, but it is still good to see people replying

Perhaps i am mis-interpreting some of the replies, but my question was not so much about why we need only 1 property to fully define the system, i get that, but more in the direction of how we can describe the water's pressure by a single number, since pressure is a function of depth. Unless there is more to it, i think Borek sufficiently answered my question.

[Borek]

Note, that pressure/depth dependency means gravitation or noninertial system.