JEE Main 2025 Apr 2 Shift 1, Chemistry Q6: Basic Concepts of Thermodynamic
Two vessels A and B are connected via stopcock. The vessel A is filled with a gas at a certain pressure. The entire assembly is immersed in water and is allowed to come to thermal equilibrium with water. After opening the stopcock the gas from vessel A expands into vessel B and no change in temperature is observed in the thermometer. Which of the following statement is true?
- A
- B
- C
- D
The pressure in the vessel B before opening the stopcock is zero.
The thermometer registers no change in temperature when the stopcock is opened. For an ideal gas, the internal energy depends only on temperature, so:
If gas A had expanded against a finite pressure in B, that would constitute irreversible work against an external pressure, with associated heat exchange to keep constant. The fact that the surrounding water bath transferred no heat (no temperature change) is the signature of free expansion, meaning the external pressure is zero:
A zero external pressure on the gas in A means vessel B was evacuated, i.e. its pressure was zero before opening the stopcock.
Option (4) is correct.
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