Vapour Pressure of a Liquid

 What is Vapour Pressure?

Vapour pressure is the pressure exerted by the vapor of a liquid when it is in equilibrium with its liquid phase at a given temperature

👉 In simple terms:

When a liquid evaporates in a closed container, some molecules escape into the air (vapour). These vapour molecules push against the walls of the container — this pressure is called vapour pressure..

How Does Vapour Pressure Develop?

Let’s understand the process step by step:

1. Evaporation begins
   Molecules at the surface of a liquid gain enough energy and escape into the vapour phase.

2. Vapour accumulates
   In a closed container, these vapour molecules cannot escape, so they start building up.

3. Condensation starts
   Some vapour molecules lose energy and return to the liquid phase.

4. Dynamic equilibrium is reached
   A point comes where:

• Rate of evaporation = Rate of condensation
  At this stage, vapour pressure becomes constant.

Factors Affecting Vapour Pressure

(a) Temperature

• As temperature increases, vapour pressure increases.

• More molecules gain energy to escape the liquid.

🔥 Example: Hot water evaporates faster than cold water.

(b) Nature of Liquid

• Liquids with weak intermolecular forces → High vapour pressure

• Liquids with strong intermolecular forces → Low vapour pressure

💡 Example:

• Petrol → High vapour pressure (evaporates quickly)

• Water → Lower vapour pressure

(c) Intermolecular Forces

This is closely related to the nature of liquid:

• Strong forces (like hydrogen bonding) → lower vapour pressure

• Weak forces → higher vapour pressure

4. Vapour Pressure and Boiling Point

• A liquid boils when its vapour pressure becomes equal to external pressure.

• At this point, bubbles form throughout the liquid.

👉 Important relation:

• High vapour pressure → Low boiling point

• Low vapour pressure → High boiling point

💡 Example:

• Alcohol boils faster than water.

5. Real-Life Applications

• 🌧️ Weather & humidity

• 💊 Pharmaceutical formulations

• ⛽ Fuel volatility (petrol evaporation)

• 🧪 Distillation processes

• 🍲 Cooking (pressure cooker works on this principle)

 6.Key Formula (Advanced)

One important equation used is the Clausius–Clapeyron Equation:

$$\ln P = -\frac{\Delta H_{vap}}{RT} + C$$

Where:

• $P$ = vapour pressure

• $\Delta H_{vap}$ = enthalpy of vaporization

• $R$ = gas constant

• $T$ = temperature (in Kelvin)

A vapour pressure curve shows how vapour pressure changes with temperature.

• It increases exponentially with temperature.

• Each liquid has its own unique curve.

Thank you!!!!!

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