When heat is transferred to a solid object, such as an ice cube, the temperature of the ice rises. When the temperature of the ice reaches a critical value known as the melting point, 0ºC (32ºF), it can change from a solid state to a liquid state. In order to melt, the ice at the surface of the cube must receive enough heat to bring its temperature up to 0ºC. The ice must then receive additional heat, called the heat of fusion, to give the water molecules the energy they need to break away from the ice structure and move about more freely in the liquid state. During this transition, ice and water coexist. The temperature of the water remains at 0ºC until all of the ice is melted. After that, the temperature of the water will begin to rise if additional heat is transferred to it.
