Vapour Pressure — Core Principles
Core Principles
Vapour pressure is the pressure exerted by the vapour of a liquid when it is in dynamic equilibrium with its liquid phase in a closed container at a specific temperature. This equilibrium arises when the rate of evaporation (liquid to vapour) equals the rate of condensation (vapour to liquid).
The magnitude of vapour pressure is an intrinsic property of the liquid and is primarily determined by two factors: temperature and the strength of intermolecular forces. As temperature increases, the kinetic energy of molecules rises, leading to more molecules escaping into the vapour phase and thus higher vapour pressure.
Conversely, liquids with stronger intermolecular forces hold their molecules more tightly, making it harder for them to evaporate, resulting in lower vapour pressure. Vapour pressure is independent of the amount of liquid or the surface area.
A liquid boils when its vapour pressure equals the external atmospheric pressure.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Vapour Pressure | Atmospheric Pressure |
|---|---|---|
| Definition | Pressure exerted by the vapour of a specific liquid in dynamic equilibrium with its liquid phase in a closed system. | Pressure exerted by the column of air (mixture of gases) above the Earth's surface at a given location. |
| Origin | Due to the kinetic energy and collisions of molecules that have evaporated from a liquid. | Due to the weight of the air molecules in the atmosphere above a point. |
| Dependence | Depends on the nature of the liquid and temperature. | Depends on altitude, temperature, and weather conditions. |
| Relation to Boiling Point | A liquid boils when its vapour pressure equals the external atmospheric pressure. | Determines the boiling point of a liquid; lower atmospheric pressure means lower boiling point. |
| Measurement | Measured using a manometer in a closed system. | Measured using a barometer. |
Vapour pressure is an intrinsic property of a liquid, reflecting its volatility and the kinetic energy of its molecules, measured in a closed system. It is the pressure exerted by the gaseous phase of a specific substance in equilibrium with its liquid phase.
Atmospheric pressure, on the other hand, is the external pressure exerted by the entire blanket of air surrounding the Earth. While distinct, they are related at the boiling point, where a liquid's vapour pressure overcomes the atmospheric pressure, allowing it to boil.
Why it is tested: For NEET, understanding the distinction is crucial for defining boiling point and solving problems related to phase transitions. Misconflating the two can lead to errors in conceptual questions regarding boiling and evaporation.