How does the heat of fusion change ice into liquid water?

The heat of fusion is the energy required to change a substance from a solid to a liquid at a constant temperature. In the case of ice, this process involves overcoming the forces that maintain its rigid, ordered crystalline solid structure.1

In ice, water molecules are held in a fixed lattice by intermolecular forces. As heat is added at the melting point of 0 °C (32 °F), the temperature of the ice does not rise; instead, the energy is absorbed as latent heat to break these lattice bonds. This transition allows the water molecules to move out of their fixed positions and into the more disordered, cohesive state of liquid water.1

For water, the heat of fusion is approximately 334 joules per gram. Because this energy is used entirely to change the physical state rather than to increase the kinetic energy of the molecules, the resulting liquid water remains at 0 °C until the entire mass of ice has melted.1