Vapour Pressure: Meaning, Factors, Importance & Applications
Introduction to Vapour Pressure
Vapour pressure is one of the most fundamental concepts in physical chemistry and plays a crucial role in understanding liquids, solutions, boiling point, volatility, and colligative properties. It frequently appears in JEE, NEET, and board exams, often linked with terms like volatile liquids, aqueous tension, and Raoult’s law.
In simple words, vapour pressure tells us how easily a liquid evaporates.
What is Vapour Pressure?
Vapour pressure is the pressure exerted by the vapour of a liquid when the liquid and its vapour are in dynamic equilibrium at a given temperature in a closed container.
When a liquid is kept in a closed vessel:
- Some molecules escape from the liquid surface and enter the vapour phase (evaporation).
- At the same time, vapour molecules return to the liquid (condensation).
When the rate of evaporation equals the rate of condensation, equilibrium is established, and the pressure exerted by the vapour is called saturated vapour pressure.
Saturated Vapour Pressure
The saturated vapour pressure of a liquid is the maximum vapour pressure it can exert at a given temperature.
Key points:
- It depends only on temperature, not on the amount of liquid.
- For a pure liquid, saturated vapour pressure is fixed at a given temperature.
- As temperature increases, saturated vapour pressure increases.
👉 Higher saturated vapour pressure = greater tendency to evaporate.
Volatile and Non-Volatile Liquids
Volatile Liquids
Liquids that evaporate easily at room temperature are called volatile liquids.
Examples:
These liquids have high vapour pressure.
Non-Volatile Liquids
Liquids that evaporate very slowly have low vapour pressure.
Examples:
Factors Affecting Vapour Pressure
Understanding the factors affecting vapour pressure is extremely important for competitive exams.
1. Nature of the Liquid
- Liquids with weak intermolecular forces (like hydrogen bonding or van der Waals forces) have higher vapour pressure.
- Strong intermolecular attraction → low vapour pressure.
Example: Ether > Alcohol > Water (vapour pressure order)
2. Temperature
- Vapour pressure increases with increase in temperature.
- At higher temperature, more molecules gain enough kinetic energy to escape the liquid surface.
This is why liquids evaporate faster on hot days.
3. Presence of Non-Volatile Solute
- Adding a non-volatile solute (like sugar or salt) lowers the vapour pressure of the solvent.
- This concept forms the basis of Raoult’s Law and colligative properties.
4. Surface Area (Indirect Effect)
- While surface area does not change the final vapour pressure at equilibrium, it affects the rate of evaporation.
Aqueous Tension
When a gas is collected over water, the gas obtained is not pure. It contains water vapour.
The pressure exerted by water vapour at a given temperature is called aqueous tension.
Relation:
{Pressure of dry gas} = {Total pressure} - {Aqueous tension}
Aqueous tension is essentially the vapour pressure of water at that temperature.
Vapour Pressure and Boiling Point
A liquid boils when:
{Vapour pressure} = {External pressure}
- Liquids with high vapour pressure have low boiling points.
- Liquids with low vapour pressure have high boiling points.
Example: Ether boils at a much lower temperature than water because it has a higher vapour pressure.
Importance of Vapour Pressure
The importance of vapour pressure can be seen in both theory and real-life applications:
1. Understanding Volatility
Helps classify liquids as volatile or non-volatile.
2. Basis of Colligative Properties
Vapour pressure lowering leads to:
3. Weather and Climate
Evaporation and humidity are directly related to vapour pressure.
4. Industrial Applications
Used in:
5. Medical and Biological Importance
- Body cooling through sweating occurs due to evaporation, which depends on vapour pressure.
Vapour Pressure Curve
A plot of vapour pressure vs temperature gives a rising curve, showing:
- Vapour pressure increases exponentially with temperature.
- Each liquid has its own characteristic vapour pressure curve.
Common Exam-Oriented Points (JEE/NEET)
- Vapour pressure depends only on temperature and nature of liquid
- Independent of surface area and volume at equilibrium
- Higher vapour pressure → higher volatility → lower boiling point
- Aqueous tension is vapour pressure of water
Conclusion
Vapour pressure is a core concept that links evaporation, volatility, boiling point, solutions, and colligative properties. Mastery of this topic builds a strong foundation in physical chemistry and helps solve multiple exam problems with ease.
At Chemca, we believe that when concepts are understood clearly, chemistry becomes simple and enjoyable.
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Easy explanation. Thank you sir 🙏
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