Vapor Pressure Explained for IB Chemistry

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.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.