Heat of Vaporization/Fusion Calculator

Phase changes (melting and vaporization) require energy without changing temperature. This energy is called latent heat:

Q = mL

Where:

• Q = Heat absorbed or released (joules)
• m = Mass of the substance (kg)
• L = Latent heat (J/kg), specific to each substance and phase change type

Types of latent heat:

• Heat of fusion (L_f): Energy to melt solid → liquid (or released when freezing)
• Heat of vaporization (L_v): Energy to boil liquid → gas (or released when condensing)

Latent heat reference values:

Why L_v is so much bigger than L_f: Melting requires only separating molecules enough to move past each other. Vaporization requires completely breaking all intermolecular bonds and then expanding the gas against atmospheric pressure, which takes far more energy. For water the ratio is nearly 7 to 1.

Why water has such a high L_v: Water molecules form strong hydrogen bonds, and boiling means breaking all of them. That is why sweating cools the body so effectively: each gram of sweat evaporated carries away roughly 2,400 J from your skin.

Note that 2,400 rather than 2,260. The 2,260 kJ/kg figure everyone quotes is measured at 100°C. Latent heat rises as temperature falls, so at skin temperature it is nearer 2,400 kJ/kg. The difference is only 6%, but it is the reason evaporative cooling is even more effective than the textbook number suggests.

Getting there is a separate calculation

This page handles the phase change alone. Turning a kilogram of ice at −20°C into steam at 120°C is five steps, not one: warm the ice (mcΔT), melt it (mL_f), warm the water (mcΔT), boil it (mL_v), then warm the steam (mcΔT). The two latent-heat steps dominate. Melting costs 334 kJ and boiling costs 2,260 kJ, while heating the water across its entire 100 degree liquid range costs only 418 kJ.

That is why a pan of water sits stubbornly at boiling point instead of racing past it, and why steam burns are so much worse than hot-water burns: condensing steam dumps 2,260 kJ per kilogram into your skin before the temperature starts falling at all.