Which thermodynamic process is defined by no change in temperature?

• A. Isothermal - constant volume
• B. Isothermal - constant pressure
• C. Isothermal - no temperature change
• D. Isothermal - no heat exchange

Answer: (C)

An isothermal process is defined by a constant temperature throughout the change. Because the temperature doesn’t change, the internal energy of an ideal gas stays the same, so any heat added goes into doing work (Q = W) rather than changing U. The option that states no temperature change directly expresses this defining feature, making it the best description of an isothermal process. The other qualifiers—constant volume or constant pressure—are additional conditions that can occur during an isothermal process but are not what defines it.