Vapor pressure is a key idea in IB Chemistry Topic 1 (States of Matter) and Topic 5 (Energetics), especially when discussing phase changes, boiling points, and intermolecular forces. Understanding vapor pressure helps explain why liquids evaporate, why volatility varies between substances, and what determines boiling behavior. This article breaks the concept down clearly in simple IB-friendly terms.
What Is Vapor Pressure?
Vapor pressure is the pressure exerted by vapor molecules when a liquid (or solid) is in equilibrium with its vapor at a given temperature.
In other words:
- Molecules at the surface of a liquid escape into the gas phase
- Some return to the liquid
- Eventually, a balance is reached
- The vapor above the liquid exerts a measurable pressure
This pressure is called the vapor pressure.
It applies to:
- Liquids in closed containers
- Solids that can sublime (like iodine or dry ice)
- Boiling and evaporation behavior
Why Does Vapor Pressure Exist?
Even in cold liquids, molecules have kinetic energy.
Some have enough energy to escape into the gas phase.
When these escaping molecules collide with the container walls, they exert pressure.
This is the vapor pressure.
At equilibrium:
- Rate of evaporation = rate of condensation
- Vapor pressure becomes constant
This constant value depends only on temperature and intermolecular forces.
How Temperature Affects Vapor Pressure
Vapor pressure increases with temperature.
Why?
- Higher temperature → more kinetic energy
- More molecules escape the surface
- More vapor accumulates
- More pressure is exerted
This is why warm liquids evaporate faster and boil sooner.
A graphical representation (often shown in IB textbooks) is an exponential curve of vapor pressure vs. temperature.
Vapor Pressure and Boiling Point
Boiling occurs when:
Vapor pressure = external pressure
At this moment:
- Bubbles of vapor can form inside the liquid
- The liquid boils
Examples:
- At sea level (100 kPa), water boils at 100°C
- At high altitude (lower external pressure), water boils at a lower temperature
- In a pressure cooker (higher pressure), boiling point increases
This explains why cooking times vary with altitude.
Intermolecular Forces and Vapor Pressure
Vapor pressure depends strongly on how tightly molecules attract each other.
Strong intermolecular forces → low vapor pressure
Example:
- Water (strong hydrogen bonding)
- Low vapor pressure
- High boiling point
Weak intermolecular forces → high vapor pressure
Example:
- Ether, acetone, gasoline
- Evaporate quickly
- Highly volatile
This explains why some substances have distinct smells—they vaporize easily.
Vapor Pressure and Volatility
Volatility refers to how easily a liquid evaporates.
- High vapor pressure → high volatility
- Low vapor pressure → low volatility
Examples:
- Perfumes contain volatile liquids (high vapor pressure)
- Cooking oil has low vapor pressure and evaporates slowly
Volatility connects directly to intermolecular forces and molecular mass.
Vapor Pressure in IB Chemistry Graphs
The IB often uses:
- Vapor pressure curves
- Phase diagrams
- Clausius–Clapeyron relationships (HL)
Common exam tasks include:
- Determining boiling point from a vapor pressure curve
- Explaining why a curve is exponential
- Predicting how pressure changes affect boiling
Understanding vapor pressure helps you read these graphs confidently.
Vapor Pressure and Closed Systems
In a closed container:
- The liquid evaporates until equilibrium is reached
- Vapor pressure becomes constant
- The amount of liquid may not change, but evaporation is still happening
- At equilibrium, dynamic exchange occurs
IB examiners love questions about dynamic equilibrium in phase changes.
FAQs
Does a liquid with high vapor pressure always boil faster?
Yes—high vapor pressure means it reaches atmospheric pressure at a lower temperature, so it boils sooner.
Why does water evaporate even when it’s cold?
Some molecules always have enough kinetic energy to escape. Temperature affects how many, but evaporation never stops unless the system reaches equilibrium.
Does vapor pressure depend on volume?
No. Vapor pressure depends only on temperature and intermolecular forces, not volume.
Conclusion
Vapor pressure is the pressure exerted by vapor in equilibrium with its liquid (or solid) phase. It increases with temperature and decreases with stronger intermolecular forces. Vapor pressure determines volatility, boiling point, and the behavior of substances in closed and open systems. Mastering this concept helps you understand phase changes, energetics, and equilibrium in IB Chemistry.
