The document discusses the phase behavior of pure substances through P-T and P-V diagrams. A P-T diagram shows the relationships between pressure, temperature, and phases (solid, liquid, gas). Key points on the diagram include the sublimation, fusion, and vaporization curves that separate the phases and meet at the triple point. The critical point marks the highest P and T where the substance can exist as a liquid and gas. A P-V diagram adds volume information and shows isotherms representing constant temperature lines that intersect phase boundaries.

1. PVT Behavior
Prepared by:
Awais Sharif
Comsats Lahore

2. P-V-T Behavior of Pure Substances
PT Diagram
• A typical P-T diagram
showing the
relationship between
pressure and
temperature of a pure
substance is shown
below:

3. PT Diagram
• The three lines 1-2, 2-3
and 2-C display
conditions of P and T at
which two phases may
co-exist in equilibrium,
and are boundaries for
the single-phase regions
of solid, liquid and
vapor (gas).

4. Graph Explanation
• Line 1-2 is known as the sublimation curve, and it
separates the solid from the gas regions.
• Line 2-3 is known as the fusion curve, and it
separates the solid and liquid regions.
• Line 2-C is known as the vaporization curve, and
it separates the liquid and the gas regions. All
three lines meet at Point 2, known as the Triple
Point. This is a point where all 3 phases can co-
exist in equilibrium.

5. Critical Pressure and Critical
Temperature
• The pressure and temperature corresponding to
this point(Critical Point) are known as the critical
pressure PC and critical temperature TC
respectively. These are the highest pressure and
temperature at which a pure substance can exist
in vapor-liquid equilibrium.
• The shaded area shows the area existing at
pressure and temperature greater than P and T.
This region is called the fluid region.

6. Explanation
• The gas region is sometimes divided into two
parts, as indicated by the dotted vertical line
through temperature TC.
• A vapor region is the region to the left of this line
and represent a gas that can be condensed either
by compression at constant temperature or by
cooling at constant pressure.
• The region everywhere to the right of this line,
including the fluid region, is termed supercritical.

7. P-V Diagram for Pure Substance
• The P-T Diagram does not provide any
information about volume.
• It merely displays the phase boundaries on as
a function of pressure and temperature.
• On the P-V Diagram, the triple point appears
as a horizontal line, where all 3 phases co-exist
at a single temperature and pressure.

8. P-V Diagram

9. P-V Diagram
• Isotherms are lines of
constant temperature
and these are
superimposed on the P-
V Diagram as shown in
the Figure.

10. Explanation
• Point C is the critical point. VC is the critical
volume at this point.
• The isotherm labeled T > TC does not cross a
phase boundary.
• The lines labeled T1 and T2 are isotherms for
subcritical temperatures, and they consist of 3
segments.

11. Continue..
• The horizontal segment of each isotherm
represents all possible mixtures of liquid and
vapour in equilibrium, ranging from 100%
liquid at the left end (curve B-C) to 100%
vapour at the right end (curve D-C).
• Curve B-C represents saturated liquid at their
boiling points, and curve D-C represent
saturated vapours at their condensation
points.

12. PV Diagram (Continue)
• Subcooled liquid and superheated vapour
regions lie to the left and right, respectively.
• Subcooled liquid exists at temperatures below
the boiling point for the given pressure.
• Superheated vapour exists at temperatures
above the boiling point for the given pressure

13. PV Diagram (Continue)
• Isotherms in the subcooled liquid region are
very steep, because liquid volumes change
little with large changes in pressure.
• The horizontal segments of the isotherms in
the 2-phase region become progressively
shorter at higher temperatures, being
ultimately reduced to a point at C, the critical
point.