Heat of fusion is the amount of energy needed to change a substance from a solid to a liquid at its melting point. For water, this value is 334 joules per gram, meaning it takes that much energy just to melt ice at 0°C into liquid water at 0°C. The key detail that surprises most people: all that energy goes into breaking apart the solid structure, not into raising the temperature.
Why the Temperature Stays the Same
If you heat a block of ice from below, its temperature rises steadily until it reaches 0°C. Then something unexpected happens. You keep adding heat, but the temperature stops climbing. It stays locked at 0°C until every bit of ice has melted. Only after the last crystal dissolves does the temperature start rising again.
This plateau exists because the incoming energy is doing a different job. Instead of making molecules move faster (which is what raises temperature), the energy is pulling molecules apart from their rigid positions in the solid. Think of a solid as a tightly packed crowd standing shoulder to shoulder. Melting doesn’t make people run faster. It gives them enough room to move around freely. That transition from locked-in-place to free-flowing is what the heat of fusion pays for.
This is also why the word “latent” appears in older terminology. “Latent” means hidden. The energy is absorbed, but you can’t detect it with a thermometer because the temperature doesn’t change.
What Happens at the Molecular Level
In a solid, molecules are held in a fixed arrangement by intermolecular forces, essentially the attractions between neighboring particles. These forces are what give solids their rigid shape. When you supply heat energy equal to the heat of fusion, you’re giving molecules enough energy to overcome those attractions and start sliding past one another.
The strength of these forces varies widely between substances. Materials with strong intermolecular attractions need more energy to melt, which means they have a higher heat of fusion and typically a higher melting point. Materials with weak attractions melt easily and have a low heat of fusion. This is why metals, with their strong bonding networks, require far more energy to melt than something like wax or butter.
The Formula
The basic equation is straightforward:
Q = m × L
- Q is the total energy required (in joules)
- m is the mass of the substance (in grams)
- L is the specific heat of fusion for that substance (in joules per gram)
For example, to melt 500 grams of ice at 0°C, you’d calculate: 500 g × 334 J/g = 167,000 joules, or about 167 kilojoules. That’s a substantial amount of energy, and it only gets you from solid ice to liquid water at the same temperature. You’d then need additional energy to warm that water above 0°C.
In chemistry courses, you’ll often see the molar version instead. Water’s molar heat of fusion is 6.02 kilojoules per mole. The formula works the same way, just substituting moles for grams: Q = n × ΔH_fus, where n is the number of moles.
Heat of Fusion vs. Heat of Vaporization
Heat of fusion covers the solid-to-liquid transition. Heat of vaporization covers liquid-to-gas. The energy difference between these two is dramatic. For water, the heat of vaporization is 40.7 kilojoules per mole, nearly seven times larger than the heat of fusion at 6.02 kilojoules per mole.
This makes intuitive sense. When ice melts, molecules only need to loosen from their fixed positions. They still stay close together and maintain significant attraction to their neighbors. When water boils, molecules must completely escape from one another and fly off as gas. That requires far more energy. The same pattern holds for virtually every substance: boiling always costs more energy than melting.
The Process Works in Reverse
Heat of fusion applies in both directions. When water freezes, it releases exactly 334 joules per gram back into the surroundings. This is why frost on crops can actually protect them in a counterintuitive way. As water on plant surfaces freezes, the released energy slightly warms the immediate area around the plant.
This symmetry is a core principle. The same amount of energy that melts a substance must be removed to freeze it again. The temperature holds steady during freezing too, just as it does during melting, staying at the freezing point until the entire liquid has solidified.
Why Water’s Heat of Fusion Matters
Water has an unusually high heat of fusion compared to most common substances, at 334 joules per gram. This is a direct result of hydrogen bonding, the relatively strong attractions between water molecules. That high value has real consequences you encounter regularly.
Ice in a drink keeps it cold for a long time precisely because melting absorbs so much energy from the surrounding liquid. A handful of ice cubes pulls thousands of joules out of your beverage before the ice fully melts. This same property is why ice packs are so effective for injuries and why frozen foods take a long time to thaw, even at room temperature. The phase change itself acts as an energy buffer, absorbing heat without letting the temperature rise.
Practical Uses of Stored Heat
Engineers take advantage of heat of fusion for thermal energy storage. The idea is simple: melt a material when excess energy is available (from solar panels during the day, for example), then let it solidify later to release that stored heat when it’s needed. Materials like molten salts are used in concentrated solar power plants to store thermal energy collected during peak sunlight for use after dark.
This approach also shows up in building design. Some modern construction materials contain tiny capsules of wax-like substances that melt as rooms warm during the day, absorbing excess heat, then resolidify at night and release it. Industrial facilities use similar systems to capture waste heat from manufacturing processes and redistribute it, reducing overall energy consumption.