MotionLab

Latent Heat Calculator

Why ice stays at 0°C while melting — the energy goes into breaking molecular bonds, not raising temperature.

Mass (kg)

Latent heat (J/kg)

Heat energy (J)

334,000

Q = mL

What you entered

Mass (kg): 1Latent heat (J/kg): 334000

1
Q = mL
1 × 334000= 334000 J

Result

Heat energy (J): 334,000

A phase change of 1kg needs 334,000 J of energy.

Energy for a phase change, with no temperature change

Latent heat is mass times the specific latent heat. Water's heat of fusion is 334 kJ/kg and of vaporisation 2,260 kJ/kg.

Mass (kg)	Latent heat (J/kg)	Energy (J)	Change
0.1	334000	33400.00	Melting 100 g of ice
0.5	334000	167000.00	Melting 500 g of ice
1	334000	334000.00	Melting 1 kg of ice
1	2260000	2260000.00	Boiling 1 kg of water
2	2260000	4520000.00	Boiling 2 kg of water

Boiling water takes 2,260 kJ per kilogram against 334 kJ to melt it - nearly seven times more - and that gap is why a pan takes minutes to reach the boil and much longer to boil dry. Compare against specific heat: melting a kilogram of ice costs the same energy as heating a kilogram of liquid water by 80 degrees, all of it absorbed with the temperature stuck at 0 C. That is what makes ice such an effective coolant, and why steam burns are far worse than hot water burns - condensing steam releases all 2,260 kJ per kilogram onto the skin.

Why temperature 'plateaus' during a phase change

During melting or boiling, all added energy goes into breaking intermolecular bonds rather than increasing kinetic energy (temperature) — this is why a pot of boiling water stays at exactly 100°C (at sea level) no matter how high you turn up the heat, until all the water has boiled away.

Latent heat of fusion vs. vaporization

Melting/freezing (fusion) and boiling/condensing (vaporization) each have their own latent heat value for a given substance, and vaporization almost always requires much more energy than fusion — which is why steam burns are often worse than boiling-water burns at the same temperature.

Frequently asked questions

How much energy does it take to melt 1 kg of ice at 0°C?

Q = mL = 1 × 334,000 = 334,000 J = 334 kJ. That's the same energy needed to heat 1 kg of water from 0°C to 80°C — melting requires a surprising amount of energy with no temperature change at all.

Why does sweating cool you down?

Evaporating sweat absorbs latent heat of vaporization (2,260 kJ/kg for water) from your skin. Evaporating just 100 mL of sweat removes 226 kJ — enough to cool a 70 kg body by about 0.8°C. This is why humid environments feel hotter: sweat can't evaporate as efficiently.

How is latent heat different from specific heat?

Specific heat (Q = mcΔT) governs temperature changes within a phase. Latent heat (Q = mL) governs phase changes at constant temperature. Heating ice from -20°C to steam at 120°C requires both types of calculation in sequence. See the specific heat calculator.

Why does it take so much more energy to boil water than to melt ice?

Latent heat of vaporization for water (2,260 kJ/kg) is nearly 7× the latent heat of fusion (334 kJ/kg). Boiling completely breaks intermolecular bonds so molecules can fly freely; melting only loosens them enough for flowing. Breaking bonds entirely costs much more energy.