Verified Solution • 2025–26 Session moderate

QPage 32: Q2: HC Verma: Concepts of Physics - Volume 2 Kinetic Theory of Gases Page 32: Q2 Solution: While gas from a cooking gas cylinder is used, the pressure…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often apply the ideal gas law to the entire cylinder and expect pressure to drop as gas is used. In reality, the gas is stored as a liquid; as it is drawn, the liquid vaporises to keep the pressure at the saturation vapour pressure, which depends only on temperature. Only when the liquid is almost exhausted does the pressure fall. Remember to consider the two‑phase equilibrium instead of treating the contents as a single ideal gas.

While gas from a cooking gas cylinder is used, the pressure does not fall appreciably till the last few minutes. Why?

Step-by-Step Verified Solution

**Answer:** Because the gas is stored as a liquid (or partly liquid) in the cylinder. The pressure is the saturation vapour pressure, which depends only on temperature, not on the amount of liquid remaining. **Step-by-Step Explanation:** In a cooking gas cylinder, the gas is liquefied under pressure. As gas is used, the liquid vaporises to maintain the pressure. The pressure inside is the saturated vapour pressure at the ambient temperature, which remains constant as long as some liquid remains. Only when the liquid is almost exhausted does the pressure begin to fall appreciably.

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