Specific latent heat (L) is defined as the amount of thermal energy (heat, Q) that is absorbed or released when a body undergoes a constant-temperature process. The equation for specific latent heat is:
L = Q / m
where:
- L is the specific latent heat
- Q is the heat absorbed or released
- m is the mass of a substance
Common units for specific latent heat include joules per gram (J/g) and kilojoules per kilogram (kJ/kg).
Specific Latent Heat
- Specific latent heat is an intensive property of matter, maintaining a consistent value regardless of sample size or the location within a substance from which the sample is taken.
- Phase changes, like melting, freezing, vaporization, or condensation, represent the most prevalent forms of constant-temperature processes.
- The energy associated with these processes is termed “latent” because it remains concealed within the molecules until the actual phase change occurs.
- This energy is termed “specific” as it is measured in relation to energy per unit mass.
- Common units for specific latent heat include joules per gram (J/g) and kilojoules per kilogram (kJ/kg).
Types of Latent Heat Transfer
| Type of Heat | Description | Daily Life Examples |
|---|---|---|
| Latent Heat of Fusion | The heat absorbed or released when matter undergoes a phase change from solid to liquid (melting) at a constant temperature. | Melting ice, freezing food, turning chocolate from a solid to liquid. |
| Latent Heat of Vaporization | The heat absorbed or released when matter undergoes a phase change from liquid to gas (vaporization) at a constant temperature. | Boiling water, steam burns, evaporation of sweat during exercise. |
| Sensible Heat | Heat transfer between matter and its surroundings that does not involve a phase change and occurs at varying temperatures. Can be “sensed” as a change in the object’s temperature. | Touching a hot cup of coffee, feeling the warmth of the sun, cooling down with a fan. |
Examples
This is a table of specific latent heat (SLH) of fusion and vaporization for common materials.
| Material | Phase Change | Latent Heat (J/g) |
|---|---|---|
| Water | Fusion (Melting) | 334 |
| Water | Vaporization (Boiling) | 2260 |
| Ice (at 0°C) | Fusion (Melting) | 334 |
| Steam (at 100°C) | Vaporization (Boiling) | 2260 |
| Chocolate (fat) | Fusion (Melting) | Approximately 50 |
| Ethanol | Fusion (Melting) | ~110 |
| Ethanol | Vaporization (Boiling) | ~840 |
| Acetone | Fusion (Melting) | ~120 |
| Acetone | Vaporization (Boiling) | ~510 |
| Oxygen (O2) | Fusion (Sublimation) | 0.444 |
| Nitrogen (N2) | Fusion (Sublimation) | 0.250 |
| Carbon Dioxide (CO2) | Sublimation | ~571 |
