When a substance undergoes a change of state, the heat energy absorbed or released does not cause a change in temperature; instead, it is used to transform the substance from one phase to another. This energy is known as latent heat.
Understanding Heat During Phase Changes
During a phase change, such as melting, boiling, freezing, or condensation, the temperature of the system remains constant. The added or removed heat energy is entirely dedicated to altering the intermolecular bonds and arrangement of particles within the substance, rather than increasing or decreasing its kinetic energy (which would manifest as a temperature change).
What is Latent Heat?
Latent heat (from the Latin latere, meaning "to lie hidden") refers to the heat energy absorbed or released by a substance during a change of state at constant temperature. There are two primary types of latent heat:
- Latent Heat of Fusion: The energy required to change a substance from a solid to a liquid (melting) or from a liquid to a solid (freezing) at its melting/freezing point.
- Example: When ice melts at 0°C, the added heat doesn't raise its temperature above 0°C. Instead, it breaks the rigid bonds holding water molecules in a crystalline ice structure, allowing them to flow as liquid water.
- Latent Heat of Vaporization: The energy required to change a substance from a liquid to a gas (boiling/evaporation) or from a gas to a liquid (condensation) at its boiling/condensation point.
- Example: When water boils at 100°C, the incoming heat provides the energy needed for liquid water molecules to overcome the attractive forces between them and escape into the gaseous state as steam, without the water's temperature rising above 100°C.
The Role of Energy in Phase Transitions
The energy input (or output) during a phase change is used to:
- Break intermolecular bonds: For processes like melting and vaporization, heat energy is absorbed to overcome the attractive forces between molecules, allowing them to move more freely. This increases the potential energy of the molecules.
- Form intermolecular bonds: For processes like freezing and condensation, heat energy is released as molecules form stronger bonds and settle into a more ordered arrangement, decreasing their potential energy.
| Phase Change | Energy Status | Temperature Change | Example |
|---|---|---|---|
| Melting | Absorbed | None | Ice to Water at 0°C |
| Freezing | Released | None | Water to Ice at 0°C |
| Boiling | Absorbed | None | Water to Steam at 100°C |
| Condensation | Released | None | Steam to Water at 100°C |
| Sublimation | Absorbed | None | Dry Ice to CO2 Gas |
| Deposition | Released | None | Water Vapor to Frost |
Special Considerations: Superheated Liquids
While the temperature generally remains constant during a phase change, there are interesting exceptions. A superheated liquid can exist temporarily as a liquid with a temperature above its normal boiling point. This occurs when a liquid is heated in a very smooth container, free of nucleation sites (like tiny scratches or impurities) where bubbles can easily form. Without these sites, the liquid can exceed its boiling point without vaporizing. However, once a nucleation site or disturbance is introduced, the liquid can rapidly and explosively flash into gas.
Practical Implications
The concept of latent heat is crucial in many natural phenomena and technological applications:
- Weather Patterns: Latent heat plays a significant role in weather systems. When water vapor condenses in the atmosphere to form clouds and rain, it releases a large amount of latent heat, which can fuel storms.
- Refrigeration and Air Conditioning: These systems work by exploiting the latent heat of vaporization. A refrigerant absorbs heat from the surrounding area as it evaporates, cooling the space.
- Cooking: Boiling water remains at a constant temperature, which is why boiling is a consistent cooking method regardless of how intensely the heat source is applied (beyond simply reaching the boiling point).
In summary, during a change of state, heat is converted into or from the potential energy stored in the intermolecular bonds of a substance, rather than increasing or decreasing its kinetic energy, thereby maintaining a constant temperature until the phase transition is complete.
