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A pV diagram plots pressure versus volume for a gas. For an ideal gas at constant temperature and moles, pressure times volume remains constant, producing a hyperbolic curve called a pV isotherm. This relationship helps visualize how pressure and volume change inversely as described by the ideal gas equation.

At high temperatures and low pressures, pV isotherms remain hyperbolic. As temperature decreases, the curve deviates from this shape. At the critical temperature, the pV curve displays a point with zero slope. Below this temperature, a region of zero slope emerges where volume changes without pressure changes, indicating phase transitions.

The flat region represents the liquid-gas transition where liquid and gas coexist. Volume can increase as liquid vaporizes or decrease as gas condenses without changing pressure or temperature. This region shows combinations of pressure and volume where both phases exist simultaneously during vaporization or liquification.

The critical temperature is the maximum temperature at which a substance can exist as a liquid. Above this temperature, no liquification occurs regardless of pressure. The critical temperature is a property specific to each gas at a given critical pressure, marking the point where the liquid-gas distinction disappears.

Non-ideal gases behave like ideal gases at higher temperatures but deviate at lower temperatures. When cooled to the critical temperature, their pV curves show regions with zero slope, indicating phase transitions. This behavior reflects intermolecular forces absent in ideal gas models, causing real gases to liquefy under appropriate conditions.

The steep left portion represents the liquid phase, which is nearly incompressible. A slight decrease in volume requires a significant increase in pressure. This steep slope reflects the liquid's resistance to compression, contrasting sharply with the flat transition region and the gentler slope of the gas phase on the right.

A supercritical fluid forms when a gas exists above the critical temperature at sufficiently high pressure. At these conditions, the gas has the density of a liquid but will not condense into a distinct liquid phase. Supercritical fluids exhibit unique properties intermediate between gases and liquids, making them valuable in industrial applications.