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?