P-V Diagram Isotherms in Superheated Region

In thermodynamics, a P-V (Pressure-Volume) diagram illustrates the relationship between pressure and volume for a substance during a process.

For a pure substance in the superheated region (where the substance exists solely as a vapor at a temperature above its saturation temperature for the given pressure):

• A constant temperature line is called an isotherm.
• On a P-V diagram, isotherms in the superheated region exhibit a specific curvature. As the specific volume (V) increases, the pressure (P) required to maintain a constant temperature generally decreases, but the rate of decrease slows down, causing the line to curve outwards or away from the P and V axes.
• This outward curvature means the line expands or moves away from the origin as volume increases.

Therefore, the constant temperature line in the superheated region of a P-V diagram for a pure substance is described as diverging.