Does the amount of liquid affect its vapour pressure?

No, the amount of liquid does not affect its equilibrium vapour pressure. Vapour pressure is an intensive property, meaning it's independent of the quantity of the substance. As long as there is enough liquid present to establish dynamic equilibrium with its vapour in a closed system, the pressure exerted by the vapour will be constant at a given temperature. Adding more liquid will simply mean there's more liquid to evaporate from, but the equilibrium pressure reached will be the same.

Why does vapour pressure increase with temperature?

As temperature increases, the average kinetic energy of the molecules in the liquid phase also increases. This means a larger fraction of molecules will possess sufficient energy to overcome the intermolecular forces holding them in the liquid and escape into the vapour phase. Consequently, the rate of evaporation increases, leading to a higher concentration of vapour molecules at equilibrium, and thus a higher vapour pressure.

How do intermolecular forces relate to vapour pressure?

Intermolecular forces (IMFs) play a crucial role. Liquids with strong IMFs (like hydrogen bonding in water) hold their molecules together more tightly, requiring more energy for them to escape into the vapour phase. This results in a lower rate of evaporation and, consequently, a lower vapour pressure at a given temperature. Conversely, liquids with weak IMFs (like London dispersion forces in diethyl ether) have molecules that can escape more easily, leading to a higher vapour pressure.

Is vapour pressure the same as atmospheric pressure?

No, they are distinct. Vapour pressure is the pressure exerted by the vapour of a specific liquid in a closed system at equilibrium. Atmospheric pressure is the pressure exerted by the entire column of air (a mixture of gases) above the Earth's surface. A liquid boils when its vapour pressure becomes equal to the external atmospheric pressure, allowing vapour bubbles to form throughout the liquid.

What is dynamic equilibrium in the context of vapour pressure?

Dynamic equilibrium refers to a state where two opposing processes occur at equal rates, resulting in no net change in the system's macroscopic properties. In the case of vapour pressure, it means that the rate at which liquid molecules evaporate into the vapour phase is exactly equal to the rate at which vapour molecules condense back into the liquid phase. While individual molecules are constantly transitioning between phases, the total amount of liquid and vapour remains constant.

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Vapour Pressure

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Version 1Updated 22 Mar 2026

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Vapour pressure is a fundamental thermodynamic property of a liquid, representing the pressure exerted by the vapour in thermodynamic equilibrium with its condensed phases (liquid or solid) at a given temperature in a closed system. This equilibrium state is dynamic, meaning that molecules are continuously evaporating from the liquid surface and condensing back into the liquid phase at equal rates…

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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.

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Key Concepts

Dynamic Equilibrium in Vapour Pressure

Dynamic equilibrium is a cornerstone of understanding vapour pressure. It's not a static state where nothing…

Effect of Temperature on Vapour Pressure

Temperature is the most critical factor influencing vapour pressure. Molecules in a liquid possess a range of…

Influence of Intermolecular Forces on Vapour Pressure

The strength of intermolecular forces (IMFs) directly dictates how easily molecules can escape from the…

Definition: — Pressure exerted by vapour in equilibrium with liquid in a closed system.

Dynamic Equilibrium: — Rate of evaporation = Rate of condensation.

Factors Affecting VP:

- Temperature (T): VP $\uparrow$ with T $\uparrow$ (exponentially). - Intermolecular Forces (IMFs): VP $\downarrow$ with stronger IMFs.

Factors NOT Affecting VP: — Surface area, amount of liquid, volume of container.

Boiling Point (BP): — T at which

P_{vapour} = P_{external}

Volatility: — Higher VP

\implies

higher volatility

\implies

weaker IMFs.

Clausius-Clapeyron: —

\ln\left(\frac{P_2}{P_1}\right) = -\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_2} - \frac{1}{T_1}\right)

(qualitative understanding for NEET).

Very Pressured Tigers In Mountains Boil Easily.

Vapour Pressure

Temperature (increases VP)

Intermolecular Molecules (stronger IMFs, lower VP)

Boiling Equals (VP equals external pressure)

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