# Thermodynamics, saturated Pressure and temperature

1. Dec 14, 2015

### kidsasd987

I wonder why we can find temperature (or pressure )when the other property value is given.

Lets say we have a rigid tank, which contains 50Kg of water at 90 degree celsius.
If we look up the thermodynamics table, we can find the saturation pressure 70.14 kpa and definite liquid and gas specific volume.

But why should it be? why knowing the state(temp or pressure) of one property determines the state of the other property and specific volumes?

Is there any formula related to this?

2. Dec 14, 2015

### BvU

There is one other thing you need to know: that the stuff is at thermodynamic equilibrium.

Basically it has to do with distribution of energy over the molecules. If we pick temperature as independent variable, the energy is distributed in such a way that the average kinetic energy of the molecules causes a specific pressure. Things are connected in the equation of state of the mateerial (simplest case: ideal gas law pV = nRT).

3. Dec 14, 2015

### kidsasd987

Thanks. it makes sense. so if we change the volume of the rigid tank. it will lower the pressure the tank, and also temperature because energy will be dispersed over extra space. If we heat up the rigid body and let the temperature rise to 90 degree celsius, then pressure will accordingly go up and result in the consistent pressure temperature specific volume relation.

Can I understand this as equal distribution of energy over the space?

4. Dec 14, 2015

### BvU

I'd say yes.

Note that things can become complicated easily: e.g. when there is liquid and vapour in the tank and you increase the volume, then a) the gas does pV work and b) liquid will evaporate.

5. Dec 14, 2015

### kidsasd987

Thanks!

6. Dec 14, 2015

### Staff: Mentor

Have you ever heard of the Phase Rule? If so, please state it.

7. Dec 16, 2015

### kidsasd987

hmm unfortunately no. I will do some reading about phase Rules. I appreciate you for telling me a key word. I actually tried to figure out how we get a phase diagram and whats the relationships between Pressure Volume and temperature.