Question

For an ideal gas undergoing an isothermal process, which of the following remains constant?

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

Hint:

{Isothermal = Constant Temperature}. Always associate “iso” with “same” in thermodynamics.

Answer 1

The Correct Option is B

Step 1: What an isothermal process means. 
An isothermal process is a thermodynamic change in which the temperature of the system does not vary at any stage. Note that “iso” means constant and “thermal” refers to temperature.

Step 2: Behavior of an ideal gas.
For an ideal gas, the state equation is:

\[ PV = nRT \]

During an isothermal process, the temperature \(T\) is fixed. Therefore, any change in pressure must be balanced by an opposite change in volume so that the product \(PV\) remains constant.

Step 3: What changes and what does not.
- Pressure changes as volume changes.
- Volume changes as pressure changes.
- Entropy may increase or decrease due to heat transfer.
- Temperature, by definition, stays the same.

Step 4: Checking the options.
(A) Pressure – not constant in an isothermal process.
(B) Temperature – remains unchanged throughout the process.
(C) Volume – varies to keep temperature fixed.
(D) Entropy – may change depending on heat flow.

Step 5: Final conclusion.
The defining feature of an isothermal process is constant temperature.

\[ \boxed{\text{Temperature}} \]