In an isothermal process, which property remains constant?

In an isothermal process, which property remains constant?

• A. Temperature
• B. Volume
• C. Pressure
• D. Entropy

Answer: (A)

Isothermal means the temperature stays the same throughout the process. That constant temperature is the defining feature. If you’re modeling an ideal gas, the relation PV = nRT ties pressure and volume to temperature; with temperature fixed, the product PV remains constant, so pressure and volume can swap values as the system moves, but temperature does not change. Entropy, on the other hand, typically changes for an isothermal process (for example, a reversible isothermal expansion increases entropy by nR ln(V2/V1)). Volume and pressure are not required to stay constant in an isothermal process, so the only property guaranteed to remain constant is the temperature.

Isothermal means the temperature stays the same throughout the process. That constant temperature is the defining feature. If you’re modeling an ideal gas, the relation PV = nRT ties pressure and volume to temperature; with temperature fixed, the product PV remains constant, so pressure and volume can swap values as the system moves, but temperature does not change. Entropy, on the other hand, typically changes for an isothermal process (for example, a reversible isothermal expansion increases entropy by nR ln(V2/V1)). Volume and pressure are not required to stay constant in an isothermal process, so the only property guaranteed to remain constant is the temperature.