Latent Heat
Latent heat is the heat energy absorbed or released by a substance during a phase change (such as melting, freezing, boiling, or condensing) at constant temperature and pressure. The word "latent" means hidden — the energy goes into breaking or forming intermolecular bonds rather than raising the temperature, so the temperature remains constant throughout the transition. There are two principal types: latent heat of fusion (solid ↔ liquid) and latent heat of vaporisation (liquid ↔ gas), and water's exceptionally high latent heat of vaporisation (2.26 MJ kg⁻¹) is critical to weather systems and evaporative cooling.
Key Formula
Q = m × L
LaTeX: Q = mL
| Symbol | Meaning | Unit |
|---|---|---|
| Q | Heat energy absorbed or released | J |
| m | Mass of substance undergoing phase change | kg |
| L | Specific latent heat | J kg⁻¹ |
Worked Example
Problem
How much heat energy is needed to completely melt 500 g of ice at 0 °C? (Specific latent heat of fusion of water = 334 000 J kg⁻¹)
Solution
Step 1: Convert mass to kg: m = 500 g = 0.5 kg. Step 2: Apply Q = mL. Q = 0.5 × 334 000 = 167 000 J.
Answer
Q = 167 000 J = 167 kJ
Specific Latent Heats of Common Substances
| Substance | Phase Change | Specific Latent Heat (kJ kg⁻¹) | Temperature (°C) |
|---|---|---|---|
| Water | Fusion (melting) | 334 | 0 |
| Water | Vaporisation (boiling) | 2260 | 100 |
| Ethanol | Vaporisation | 841 | 78.4 |
| Nitrogen | Vaporisation | 199 | -196 |
| Lead | Fusion (melting) | 24.5 | 327.5 |
Interactive Tools
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Related Terms
Specific Heat Capacity
Specific heat capacity (symbol c) is the amount of heat energy required to raise the temperature of one kilogram of a substance by one degree Kelvin (or one degree Celsius). It is an intrinsic material property that reflects how strongly a substance resists temperature change when heat is added or removed. Water has an exceptionally high specific heat capacity (4186 J kg⁻¹ K⁻¹), which makes it an effective thermal buffer in climatic systems, industrial cooling, and biological organisms.
Heat (thermodynamics)
Heat is the transfer of thermal energy between two objects or systems due to a temperature difference; it always flows spontaneously from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached. Unlike temperature (a state property), heat is a process quantity — it only exists as energy in transit, not stored within a body. Heat transfer occurs via three mechanisms: conduction (direct molecular contact), convection (fluid movement), and radiation (electromagnetic waves), and it is measured in joules (J) in the SI system.
Thermal Energy
Thermal energy is the total internal kinetic energy associated with the random translational, rotational, and vibrational motion of all particles (atoms and molecules) within a substance. It is a state property stored within a system, proportional to both the temperature and the number of particles present. Thermal energy is the source of heat flow when a temperature difference exists, and it underpins all thermodynamic processes including phase changes, chemical reactions, and the operation of heat engines.
From Latin "latere" meaning "to lie hidden", coined by the Scottish chemist Joseph Black around 1761. Black observed that heat added to melting ice did not raise the temperature, concluding the heat was "hidden" or latent within the substance during the transformation.