isothermal

Isothermal describes a process or condition in which the temperature remains constant throughout. The term comes from the Greek iso meaning equal and thermē meaning heat, emphasizing a fixed temperature during the change.

In thermodynamics, an isothermal process is a quasi-static transformation where the system’s temperature does not change. For an ideal gas, this implies PV = nRT with T constant, so pressure and volume are inversely related. Because the internal energy of an ideal gas depends only on temperature, ΔU = 0 during an isothermal ideal-gas process. Consequently, the heat transferred to the system equals the work performed by the system (Q = W), with the exact amounts determined by the path of the process in the pressure–volume plane.

Real systems only approximate isothermal conditions. Perfect isothermal processes require infinite time or perfect heat exchange with the surroundings. In practice, engineers design slow processes or highly conductive surroundings to keep temperature nearly constant, such as in isothermal reactors or certain liquefaction and gas-processing steps.

Isothermal conditions also arise during phase transitions. At a phase-change temperature, a substance can absorb or release energy without changing temperature, as energy goes into latent heat rather than a temperature rise or fall. This is common during melting, freezing, boiling, or condensation at the transition point.

Applications of the concept include modeling ideal gas behavior, designing processes with controlled heat exchange, and understanding natural phenomena where temperature is effectively held constant.