Because the [[Phase|phase]] of a [[Substance|substance]]is defined by [[Pressure|pressure]], [[Specific Volume and Density|density]], and [[Temperature|temperature]], we can plot these properties against each other.
First we may plot temperature against specific volume, resulting in the T-$\nu$ diagram
![[Tv.png|center|500]]
The two lines represent different constant pressures. At a given pressure $P$, the substance can exist at any combination of temperature $T$ and specific volume $\nu$ along the corresponding line. For each pressure line (up to a critical point), we observe that as the temperature rises, the specific volume initially increases: _the substance warms and expands_.
However, at a specific volume $\nu_f$, the substance reaches a phase where it continues to expand without further temperature increase. The temperature associated with this flat line is called the saturation temperature ($T_{sat}$). This flattening corresponds to the phase change region, where the [[Liquid|liquid]] begins to boil at the pressure-dependent [[Boiling Point|boiling point]]. During this flat portion of the line, the liquid and [[Gas|gas]] phases coexist. Once all the liquid has vaporized into gas at $\nu_g$, the temperature begins to rise again as heating resumes.
When we plot pressure against specific volume we see a similar pattern, however note that the pressure decreases as the substance expands in volume.
![[Pv.png|center|500]]
The value $\nu_{fg}$ is commonly used to represent the change in specific volume between $\nu_f$ and $\nu_g$, defined as $\nu_fg=\nu_g-\nu_f$. By plotting $\nu_f$ and $\nu_g$ for various pressures, we can visualize the behavior of a substance under different conditions more comprehensively.
![[regionsdia.png|center]]
The phase of a substance can be determined by comparing its conditions to the saturation temperature, pressure, or specific volume. Saturation values refer to the constant temperature or pressure within the saturated mixture region and the specific volume limits $\nu_f$ and $\nu_g$.
Criteria to determine substance phase:
| Phase | Pressure | Temperature | Specific Volume |
| ---------------------- | ------------ | ------------ | ----------------- |
| [[Compressed Liquid]] | $P>P_{sat}$ | $T<T_{sat}$ | $\nu<\nu_f$ |
| [[Saturated Mixture]] | $P=P_{sat}$ | $T=T_{sat}$ | $\nu_f<\nu<\nu_g$ |
| [[Superheated Vapour]] | $P<P_{sat}$ | $T>T_{sat}$ | $\nu>\nu_g$ |