Vaporization is a fundamental concept in IB Chemistry Topic 1 (Stoichiometry) and Topic 5 (Energetics). It explains how a liquid transforms into a gas and why this phase change requires energy. Vaporization is closely tied to intermolecular forces, boiling points, enthalpy changes, and kinetic theory. Understanding vaporization helps students predict physical behavior, estimate energy requirements, and interpret heating curves in exam questions.
What Is Vaporization?
Vaporization is the process by which a liquid changes into a gas.
This phase change requires energy because:
- Molecules in a liquid are held together by intermolecular forces
- To enter the gas phase, molecules must overcome these attractive forces
- This energy input is known as enthalpy of vaporization (ΔHvap)
Vaporization can occur in two forms:
- Evaporation — at the surface, below boiling point
- Boiling — throughout the liquid, at boiling point
Evaporation vs Boiling
Evaporation
- Happens at any temperature
- Only surface molecules escape
- Slow process
- Cooling effect (endothermic)
Boiling
- Occurs at the boiling point
- Vapor forms both at the surface and within the liquid
- Rapid, energetic process
- Requires atmospheric pressure to be overcome
Both processes involve vaporization but under different conditions.
Why Vaporization Requires Energy
To vaporize, molecules must:
- Break free from intermolecular forces
- Move further apart
- Increase kinetic energy
- Expand into the gas phase
Because bond breaking and separation require energy, vaporization is endothermic.
The required energy is absorbed from surroundings, which is why boiling water cools if heat is not continuously supplied.
Intermolecular Forces and Vaporization
The strength of intermolecular forces determines how easily a liquid vaporizes.
Weak IMFs → easier vaporization
Substances like hexane or ether evaporate quickly because they have weak London dispersion forces.
Strong IMFs → harder vaporization
Water, alcohols, and carboxylic acids vaporize slowly due to hydrogen bonding.
This explains:
- Why water has a high boiling point
- Why volatile liquids evaporate rapidly
- Why viscosity correlates with IMF strength
IB exam questions often link vaporization with intermolecular forces.
Boiling Point and Vaporization
The boiling point is the temperature at which the vapor pressure of a liquid matches the external pressure.
Higher external pressure → higher boiling point
Example: pressure cookers.
Lower external pressure → lower boiling point
Example: water boils at a lower temperature on a mountain top.
Boiling is simply rapid vaporization occurring throughout the entire liquid.
Enthalpy of Vaporization (ΔHvap)
Enthalpy of vaporization is the amount of energy required to vaporize one mole of a liquid at its boiling point.
Typical values:
- Water: 40.7 kJ mol⁻¹
- Ethanol: 38.6 kJ mol⁻¹
- Diethyl ether: 27.0 kJ mol⁻¹
Higher ΔHvap → stronger intermolecular forces.
This quantity is used in energy calculations, heating curves, and phase diagrams.
Vaporization in Heating Curves
During vaporization:
- Temperature remains constant
- Heat energy goes into breaking intermolecular forces, not heating the liquid
- Plateau appears at the boiling point on a heating curve
This plateau represents ΔHvap.
Real-World Applications of Vaporization
Vaporization plays a crucial role in:
- Distillation (separating liquids by boiling point)
- Refrigeration and air conditioning
- Evaporative cooling (sweat, cooling towers)
- Steam turbines and power generation
- Perfume volatilization
Understanding vaporization explains many everyday and industrial phenomena.
Common IB Misunderstandings
“Temperature increases during boiling.”
Incorrect—temperature remains constant while vaporization occurs.
“Vaporization breaks covalent bonds.”
No—only intermolecular forces are broken, not chemical bonds.
“Evaporation happens only at high temperatures.”
Evaporation can occur at any temperature.
“A liquid will boil at the same temperature everywhere.”
Boiling point depends on pressure.
FAQs
Why does vaporization cause cooling?
Because it absorbs energy from the surroundings, lowering temperature.
Why do some liquids evaporate faster than others?
Weaker intermolecular forces allow molecules to escape more easily.
Is vaporization reversible?
Yes—condensation is the reverse process.
Conclusion
Vaporization is the process by which a liquid becomes a gas. It requires energy to overcome intermolecular forces and occurs through evaporation or boiling, depending on temperature and pressure. Vaporization plays a major role in physical chemistry, energetics, and real-world systems. Mastering it is essential for understanding phase changes and energy transitions in IB Chemistry.
