Table of Contents
What Is Latent Heat?
Latent heat is the energy absorbed or released by a substance during a phase change (solid to liquid, liquid to gas, or vice versa) at constant temperature. Unlike sensible heat, which changes the temperature of a substance, latent heat changes the internal energy associated with molecular bonding without a temperature change. The word "latent" means hidden, referring to the fact that the energy is stored or released without an observable temperature change.
This concept is fundamental to understanding weather systems (evaporation and condensation drive hurricanes), industrial processes (refrigeration, distillation), and everyday cooking (boiling water absorbs energy at 100 degrees C).
Latent Heat Formula
Where Q is the heat energy (Joules), m is the mass (kg), and L is the specific latent heat (J/kg). For fusion, L = Lf; for vaporization, L = Lv.
Types of Latent Heat
- Latent heat of fusion (Lf): Energy to change from solid to liquid (or vice versa). For water: 334 kJ/kg.
- Latent heat of vaporization (Lv): Energy to change from liquid to gas (or vice versa). For water: 2,260 kJ/kg.
- Latent heat of sublimation: Energy to change directly from solid to gas. Approximately equals Lf + Lv.
Common Latent Heat Values
| Substance | Lf (kJ/kg) | Lv (kJ/kg) |
|---|---|---|
| Water | 334 | 2,260 |
| Ethanol | 209 | 855 |
| Aluminum | 397 | 10,900 |
| Iron | 247 | 6,090 |
| Nitrogen | 11.7 | 166 |
Frequently Asked Questions
Why does the temperature stay constant during a phase change?
During a phase change, all the energy goes into breaking or forming intermolecular bonds rather than increasing kinetic energy of molecules. Since temperature is a measure of average kinetic energy, it remains constant until the phase transition is complete.
Why is latent heat of vaporization much larger than fusion?
Vaporization requires completely separating molecules against all intermolecular forces, while melting only partially disrupts the ordered structure. Gas molecules are essentially independent, requiring far more energy to achieve than the semi-ordered liquid state.
How does latent heat affect weather?
When water evaporates from oceans, it absorbs latent heat. When this vapor condenses in clouds, it releases that energy, warming the surrounding air and driving convection currents that power storms and hurricanes. This energy transfer is a major driver of global weather patterns.