What Is Latent Heat?
Latent heat represents the thermal energy absorbed or released when a substance undergoes a phase change at constant temperature. Unlike sensible heat, which raises or lowers temperature, latent heat restructures molecular bonds without altering how hot the material feels.
Consider ice melting at 0 °C. You can transfer heat continuously, yet the temperature remains at 0 °C until all ice becomes liquid water. Only after the phase transition completes does further heating raise the temperature. This energy input during the unchanging-temperature transition is the latent heat.
Two key latent heat values matter:
- Fusion: Energy for solid-to-liquid transition (melting or freezing)
- Vaporization: Energy for liquid-to-gas transition (boiling or condensing)
Vaporization typically demands far more energy than fusion. Converting 1 kg of water to steam requires roughly nine times more energy than melting 1 kg of ice.
Latent Heat Formula
The relationship between latent heat, mass, and specific latent heat is straightforward. Multiply the substance's mass by its specific latent heat value to obtain the total thermal energy involved in the phase change.
Q = m × L
Q— Total latent heat energy (kilojoules, kJ)m— Mass of the substance (kilograms, kg)L— Specific latent heat (kilojoules per kilogram, kJ/kg)
Specific Latent Heat Values
Specific latent heat is an intensive property—it depends only on the material and the type of transition, not on quantity. Water, for instance, has a latent heat of fusion around 334 kJ/kg and a latent heat of vaporization around 2264 kJ/kg.
Common values include:
- Ice melting: 334 kJ/kg
- Water boiling: 2264 kJ/kg
- Ammonia boiling: 1371 kJ/kg
- Ethanol boiling: 846 kJ/kg
Different substances have dramatically different latent heat requirements due to varying molecular bond strengths and intermolecular forces. Metals often have lower latent heats than polar liquids like water.
Practical Example
Suppose you need to melt 2.5 kg of ice at 0 °C into liquid water, still at 0 °C. Using the specific latent heat of fusion for ice (334 kJ/kg):
Q = 2.5 kg × 334 kJ/kg = 835 kJ
You must supply 835 kilojoules of thermal energy to complete the melting process. If instead you vaporize the same 2.5 kg of water at 100 °C, the energy requirement becomes:
Q = 2.5 kg × 2264 kJ/kg = 5660 kJ
Notice that vaporization demands nearly seven times more energy than fusion for the same mass. This explains why boiling water away takes considerably longer than melting ice.
Key Considerations
Avoid these common pitfalls when working with latent heat calculations:
- Confuse latent and sensible heat — Latent heat drives phase changes at constant temperature, while sensible heat changes the temperature without phase transition. A thermometer stays at 0 °C while ice melts, even though you're adding heat.
- Mix up fusion and vaporization values — Different phase transitions require different energy inputs. Melting ice and boiling water are both latent heat processes, but their specific latent heat values differ dramatically. Always select the correct transition type.
- Forget unit consistency — Most tables list specific latent heat in kJ/kg, but your mass might be in grams. Convert grams to kilograms first by dividing by 1000, or your result will be off by a factor of 1000.
- Assume direction independence — Melting absorbs latent heat (endothermic), while freezing releases it (exothermic). The magnitude stays the same; only the sign and energy flow direction change. Your calculator handles this automatically.