Short Answer

Vapor pressure is the pressure created by the vapor of a liquid when the liquid and its vapor are in a closed container. It forms when some liquid particles escape into the air above the liquid and collide with the container walls, creating pressure.

Vapor pressure depends mainly on temperature and the nature of the liquid. Liquids with weak intermolecular forces have higher vapor pressure because their particles escape more easily. As temperature increases, vapor pressure also increases because more molecules have enough energy to evaporate.

Detailed Explanation

Vapor Pressure

Vapor pressure is the pressure exerted by the vapor of a liquid when the liquid and vapor are in equilibrium in a closed container. When a liquid is placed in a sealed container, some of its molecules gain enough energy to escape from the surface and enter the vapor phase. At the same time, some vapor molecules return to the liquid. A balance is eventually reached where the rate of evaporation equals the rate of condensation. The pressure exerted by the vapor at this equilibrium stage is called the vapor pressure of the liquid.

Vapor pressure is a key concept in understanding evaporation, boiling, phase changes, and the behaviour of liquids at different temperatures. It helps explain everyday processes such as drying of clothes, boiling of water, and volatility of perfumes and fuels.

How Vapor Pressure Is Formed

When a liquid is sealed in a container:

• Molecules at the surface escape into the air (evaporation).
• These molecules form vapor above the liquid.
• The vapor molecules collide with the container walls and exert pressure.
• Some vapor molecules re-enter the liquid (condensation).
• After some time, evaporation and condensation become equal.

The pressure of the vapor at this point is the vapor pressure.

This equilibrium condition is essential. Vapor pressure can be measured only when evaporation and condensation rates match.

Factors Affecting Vapor Pressure

Several factors influence vapor pressure, the most important being:

1. Temperature

Temperature has a direct and strong effect on vapor pressure.

• As temperature increases, more molecules gain energy to escape into vapor.
• This increases evaporation and vapor pressure.
• As temperature decreases, fewer molecules evaporate, lowering vapor pressure.

This explains why water boils at higher temperatures when vapor pressure equals external pressure.

2. Nature of the Liquid

Liquids differ in their intermolecular forces:

• Weak intermolecular forces → High vapor pressure
  Example: Ether, alcohol, petrol
• Strong intermolecular forces → Low vapor pressure
  Example: Water, glycerin, honey

Liquids with weak forces allow molecules to escape more easily.

3. Surface Area

Surface area affects the rate of evaporation, not the vapor pressure.
But indirectly, faster evaporation can help reach equilibrium quickly.

4. External Pressure

External pressure does not change vapor pressure, but it changes the boiling point.

Relation Between Vapor Pressure and Boiling

A liquid boils when its vapor pressure becomes equal to the external (atmospheric) pressure.

For example:

• At sea level, the atmospheric pressure is high, so water boils at 100°C.
• At high altitudes, atmospheric pressure is low, so water boils at a lower temperature.

This is because vapor pressure reaches atmospheric pressure earlier at high altitudes.

Vapor Pressure and Volatility

Volatility means how easily a liquid evaporates.

• High vapor pressure → High volatility
• Low vapor pressure → Low volatility

Perfumes, alcohol, and nail polish removers are volatile because they have high vapor pressure.

Importance of Vapor Pressure

Vapor pressure is important in many fields:

1. Weather and Climate

Evaporation of water from oceans depends on vapor pressure.
Humidity also depends on water vapor pressure in the air.

2. Cooking and Food Industry

Boiling point changes with vapor pressure.
Pressure cookers increase pressure and raise boiling point to cook food faster.

3. Medicine and Health

Inhalers and aerosol medicines depend on vapor pressure for spray action.

4. Chemical Industries

Distillation, purification, and solvent selection depend on vapor pressure.

5. Fuel Handling

Gasoline has high vapor pressure, which helps in engine combustion but also increases evaporation losses.

Examples of Vapor Pressure in Daily Life

• Clothes drying: Higher temperature increases vapor pressure of water, speeding up drying.
• Perfume spreading: High vapor pressure liquids evaporate quickly, releasing scent.
• Pressure cooker working: Increased pressure raises boiling point by changing vapor pressure balance.
• Cooling by sweating: Sweat evaporates when vapor pressure of water on skin becomes higher than air.

Understanding Vapor Pressure Through Molecular Behaviour

The concept of vapor pressure is deeply connected to molecular motion:

• Molecules with higher kinetic energy escape easily.
• More escaping molecules mean stronger vapor pressure.
• Attractive forces resist evaporation.
• Weak forces allow molecules to evaporate quickly, increasing vapor pressure.

This shows how vapor pressure depends on both molecular movement and intermolecular forces.

Conclusion

Vapor pressure is the pressure exerted by vapor molecules when a liquid and its vapor are in equilibrium in a closed container. It depends mainly on temperature and intermolecular forces. Higher temperature and weaker forces lead to higher vapor pressure. Understanding vapor pressure is essential for explaining boiling, evaporation, weather patterns, industrial processes, and everyday observations involving liquids.