What Is Latent Heat?
The latent heat calculator helps you compute the energy released or absorbed during a phase transition — such as melting, vaporizing, or sublimating a substance. Unlike sensible heat (which raises temperature), latent heat is "hidden" energy: the temperature stays constant while the substance changes its physical state.
Specific Latent Heat
Specific latent heat (L) is the amount of energy absorbed or released per unit mass of a substance during a phase transition. Its unit is kJ/kg (metric) or BTU/lb (American/imperial).
A classic example: when you heat ice to 0 °C, it starts melting. The temperature stays at 0 °C until all the ice has turned to water — all that energy goes into breaking the crystal structure of ice. This is the latent heat of fusion. Similarly, at 100 °C water boils but stays at 100 °C until fully vaporized — that is the latent heat of vaporization.
Latent Heat Formula
The formula is straightforward:
Where:
- m — mass of the substance (kg or lb)
- L — specific latent heat of the substance (kJ/kg or BTU/lb)
- Q — heat energy absorbed or released (kJ or BTU)
Common Specific Latent Heat Values
| Substance | Fusion L (kJ/kg) | Vaporization L (kJ/kg) | Fusion L (BTU/lb) | Vaporization L (BTU/lb) |
|---|---|---|---|---|
| Water | 334.0 | 2 257.0 | 143.6 | 970.3 |
| Ethanol | 104.6 | 841.8 | 45.0 | 361.9 |
| Aluminum | 396.9 | 10 500 | 170.6 | 4 514 |
| Copper | 205.4 | 4 726 | 88.3 | 2 032 |
| Gold | 65.6 | 1 645 | 28.2 | 707.2 |
| Iron | 272.2 | 6 264 | 117.0 | 2 694 |
| Lead | 22.9 | 856.0 | 9.85 | 368.0 |
| Mercury | 11.44 | 296.1 | 4.92 | 127.3 |
| Nitrogen | 25.7 | 198.6 | 11.0 | 85.4 |
| Oxygen | 13.9 | 213.1 | 5.98 | 91.6 |
| Silver | 111.3 | 2 323.7 | 47.9 | 999.0 |
Worked Example
How much energy is needed to melt 20 g of ice?
- Identify the transition: melting → fusion.
- Specific latent heat of water (fusion): L = 334 kJ/kg.
- Convert mass: 20 g = 0.02 kg.
- Apply the formula: Q = 0.02 kg × 334 kJ/kg = 6.68 kJ = 6 680 J.
To vaporize the same 20 g of water (at 100 °C): Q = 0.02 kg × 2 257 kJ/kg = 45.14 kJ = 45 140 J. This is almost seven times more energy than melting!
Types of Phase Transition
- Fusion (Melting / Freezing) — solid ↔ liquid. E.g., ice melting, water freezing.
- Vaporization / Condensation — liquid ↔ gas. E.g., water boiling, steam condensing.
- Sublimation — solid → gas directly. E.g., dry ice (CO₂), snow sublimating in cold dry air.
The reverse processes (freezing, condensation) release the same amount of energy.
FAQs
What is the difference between latent heat and sensible heat?
Sensible heat changes the temperature of a substance (calculated with Q = m × c × ΔT). Latent heat changes the phase of a substance at constant temperature (Q = m × L).
Why does water have such a high latent heat of vaporization?
Water molecules form strong hydrogen bonds. Breaking those bonds during vaporization requires a lot of energy — hence L = 2 257 kJ/kg, one of the highest values among common liquids. This is why sweating is such an effective cooling mechanism.
Can I use this calculator for freezing/condensation?
Yes. The energy values are the same — freezing releases the same energy that melting absorbs, and condensation releases the same energy that vaporization absorbs. The formula Q = m × L applies in both directions.
What units are supported?
Metric: mass in kg / g / mg; specific latent heat in kJ/kg or J/kg; result in J, kJ, MJ, GJ, cal, kcal, BTU, kWh.
American/Imperial: mass in lb / oz; specific latent heat in BTU/lb; result in all the same units.