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GATE 2020 ME (ME2) – Question 26

Thermodynamics · Thermodynamic systems and processes, properties of pure substances · 1 mark · Multiple choice

A closed vessel contains pure water in thermal equilibrium with its vapour at 25°C (Stage 1). It is placed in an isothermal oven with air at 80°C and 1 atm until new equilibrium (Stage 2). What happens immediately when valve A is opened inside the oven?

Diagram for GATE 2020 ME (ME2) question 26
  1. Water vapour inside the vessel comes out through valve A.
  2. Hot air goes inside the vessel through valve A.
  3. Nothing happens; the vessel remains in equilibrium.
  4. All vapour inside the vessel immediately condenses.

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Show answer and explanation

Correct answer: (B) Hot air goes inside the vessel through valve A.

Explanation

Liquid and vapour in equilibrium at80°C have saturation pressure below1 atm (approximately47 kPa). Ambient pressure is higher, so hot external air initially flows into the vessel. **Answer:B**. Heating the water above25°C does not by itself raise saturated pressure above atmospheric pressure.