Last updated: 2026-06-15

Latent Heat

Heat does not always raise temperature. When ice melts or water boils, energy pours in while the thermometer holds still — that energy is going into the change of phase itself. Latent heat is the amount of energy involved, and this calculator works it out from $Q = mL$, solving for the heat, the mass or the specific latent heat depending on what you know.

Where the formula comes from

Changing phase means rearranging how molecules are bound together: melting loosens a rigid solid into a liquid, boiling tears a liquid apart into a gas. Each kilogram of substance needs a fixed amount of energy to make that change, and that fixed amount is the specific latent heat $L$. The total energy is simply that per-kilogram figure times the mass, $Q = mL$.

Formula

Use the latent heat of fusion for melting and freezing, and the latent heat of vaporisation for boiling and condensing. The energy released when a substance freezes or condenses equals the energy absorbed to melt or boil it.

Worked example

Melting $0.5\ \text{kg}$ of ice at $0\ ^\circ\text{C}$, with the latent heat of fusion of water $L = 334\ \text{kJ/kg}$:

That 167 kJ is absorbed at a steady 0 °C; only once all the ice has melted does adding more heat start to warm the water.

Why phase changes dominate energy budgets

Latent heats are large compared with the heat to change temperature. Warming water by 1 °C takes about 4.2 kJ/kg, but boiling it away takes 2256 kJ/kg — over five hundred times as much. This is why sweating cools the body so effectively, why steam burns are far worse than hot-water burns, and why melting snow keeps the ground near freezing during a thaw.

Limitations

The formula assumes the substance is already at its melting or boiling point, so the heat goes entirely into the phase change. To find the total energy of a process that also changes temperature — heating ice, melting it, then warming the water — you add a sensible-heat term $mc\Delta T$ for each temperature span and a latent-heat term $mL$ for each phase change. Latent heat values also depend slightly on pressure; the figures here are for standard atmospheric pressure.