Latent Heat
Latent heat is the energy absorbed or released by a substance during a phase change that occurs at a constant temperature. Unlike sensible heat, which changes the temperature of a substance, latent heat is “hidden” energy that changes the physical state by altering the arrangement of molecular bonds. During this process, the substance’s temperature remains constant even though energy is being transferred.
There are two main types of latent heat:
Latent Heat of Fusion (Lₓ): This is the energy required to change a unit mass of a substance from solid to liquid or vice versa, at its melting/freezing point. For example, ice melting to water absorbs latent heat without changing temperature.
Latent Heat of Vaporization (Lᵥ): This is the energy required to change a unit mass of a substance from liquid to gas or vice versa, at its boiling/condensation point. For example, water boiling into steam absorbs latent heat, but the temperature remains 100°C at standard pressure.
The formula to calculate the energy associated with a phase change is:
Q = mL
Where:
Q = Heat energy absorbed or released
m = Mass of the substance
L = Latent heat (fusion or vaporization, depending on the process)
Latent heat is an important concept in thermodynamics, meteorology, and engineering. For instance, the high latent heat of water makes it an effective coolant and plays a vital role in regulating Earth’s climate through evaporation and condensation cycles.
In summary, latent heat enables phase transformations without temperature change, representing energy stored or released in the molecular structure of matter.
Phase Changes and Latent Heat
During heating or cooling, a substance can change its physical state. Each phase change involves latent heat, energy absorbed or released without a change in temperature. Here’s an organized overview of the main types of phase changes:
| Phase Change | Description | Heat Absorbed or Released | Symbol/Example |
|---|---|---|---|
| Fusion (Melting) | Solid → Liquid | Heat absorbed (latent heat of fusion) | Ice → Water |
| Solidification (Freezing) | Liquid → Solid | Heat released (latent heat of fusion) | Water → Ice |
| Vaporization (Boiling/Evaporation) | Liquid → Gas | Heat absorbed (latent heat of vaporization) | Water → Steam |
| Condensation | Gas → Liquid | Heat released (latent heat of vaporization) | Steam → Water |
| Sublimation | Solid → Gas directly | Heat absorbed (latent heat of sublimation) | Dry ice (CO₂ solid) → Gas |
| Deposition (Desublimation) | Gas → Solid directly | Heat released (latent heat of sublimation) | Frost formation from water vapor |
Key Points:
Latent heat is the “hidden” energy used to rearrange molecular bonds during a phase change.
Temperature remains constant during the process until the phase change is complete.
The formula to calculate latent heat remains:
Q = mL
Where Q = heat energy, m = mass, L = latent heat for the specific phase change.
Examples of Latent Heat Values:
| Substance | Latent Heat of Fusion (J/g) | Latent Heat of Vaporization (J/g) |
|---|---|---|
| Water | 334 | 2260 |
| Ice | 334 | — |
| Steam | — | 2260 |
This table helps visualize how energy is absorbed or released in each phase change.