Engineering
Chemistry
Le Chateliers Principle
Vapour Pressure
Meaning of Equilibrium
Question

A closed jar having waters vapours in equilibrium with liquid suddenly all the vapours of the jar is transferred to another identical jar and is subjected to compression. Assume initial temperature to be the same and negligible volume occupied by the liquid water. Select the observation in the record jar.

temperature of the vapour will increases.

vapour will undergo gradual compression without any condensation.

liquid water will start forming in the jar

final pressure will be the same as initial pressure.

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Solution
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This scenario describes water vapor initially at saturation (100% relative humidity) in equilibrium with liquid. When transferred to an identical jar and compressed isothermally (same temperature), the vapor becomes supersaturated. Since the system is now above its saturation vapor pressure for that temperature, condensation must occur to re-establish equilibrium.

The vapor pressure for a given substance at a constant temperature is fixed. The compression increases the number of moles per volume, but the maximum possible pressure (saturation vapor pressure, Psat) remains unchanged. Therefore, the excess vapor will condense into liquid to maintain the pressure at Psat, which is the same as the initial pressure.

Final answer: liquid water will start forming in the jar AND final pressure will be the same as initial pressure.