Isothermal Process

P-V graph of isothermal process
By Yuta Aoki (Original) [GFDL (, CC-BY-SA-3.0 ( or CC BY-SA 2.5-2.0-1.0 (], via Wikimedia Commons

Definition: An isothermal process is a thermodynamic process in which the temperature of the system remains constant. The heat transfer into or out of the system typically must happen at such a slow rate that the thermal equilibrium is maintained.

Other Factors in an Isothermal Process

In general, during an isothermal process there is a change in internal energy, heat energy, and work.

The internal energy of an ideal gas, however, depends solely on the temperature, so the change in internal energy during an isothermal process for an ideal gas is also 0.

Isothermal Processes & States of Matter

In a phase diagram, an isothermal process is indicated by following a vertical line (or plane, in a 3D phase diagram) along a constant temperature.

Therefore, if the pressure and volume change, it is possible for a substance to change its state of matter even while its temperature remains constant, if you're careful about how you apply or remove heat from the system.