pV Diagrams

pV Diagram

A graph of pressure vs volume showing the thermodynamic behavior of gas, including isotherms (constant-temperature curves)

Isotherm

A curve on a pV diagram representing the relationship between pressure and volume at a constant temperature.

pV = nRT = constant

Equation for an ideal gas isotherm

It is a rectangular hyperbola

How does the ideal gas isotherm looks graphically?

Van der Waals Isotherm (Below Tc)

• Shows a “hump” where pressure increases with volume

• An unphysical region corresponding to liquid-gas transition

• Corrected by a horizontal line (constant pressure during boiling/condensing)

Tc

(i)

T < Tc

(ii)

T > Tc

(iii)

Critical Point

(1)

Saturation Curve

(2)

Critical Isotherm

(3)

Tc

(4)

Vapor region

(5)

Liquid-vapor region

(6)

Liquid region

(7)

True but not ideal gas

(8)

Critical Temperature

Maximum temperature at which a liquid phase can exist. Above this, the substance becomes a supercritical fluid with no distant liquid-gas boundary.

Critical Pressure

The pressure required to liquify a gas at the critical temperature.

Critical Point

The specific combination of critical temperature and critical pressure where liquid and gas phases become indistinguishable.

Supercritical Fluid

A phase above the critical pint with liquid-like density but gas-like flow, not condense or by pressure alone.

Niacin (Vitamin B3)

C₆NH₅O₂