Why does melting ice remain at 0 °C while it changes into liquid water?

Melting ice remains at 0 °C because the energy being added to the system is consumed as latent heat, specifically the heat of fusion, rather than being used to increase the temperature of the substance.12

In a crystalline solid like ice, molecules are held in a fixed lattice by attractive forces. As heat is applied, the temperature rises until it reaches the melting point. At this stage, the added energy is required to perform the work necessary to overcome these attractive forces and break the rigid structure, allowing the molecules to transition into the more disordered liquid state. Because all incoming energy is utilized for this phase change, the temperature of the ice-water mixture remains constant at 0 °C until the entire solid has melted.132