Heat of fusion, also called enthalpy of fusion, is the amount of energy required to transform a substance from a solid to a liquid. Once a solid has reached the temperature at which it melts, its temperature does not continue to rise while it is melting, even if it is exposed to the same heat source. While it is melting, a solid continues to absorb energy from its heat source, which allows the molecular change needed for melting to occur.
When a solid is heated, its temperature climbs until it reaches its melting point. Once this temperature is reached, additional energy must be supplied to the solid in order to transform it into a liquid. The heat of fusion refers to the energy needed once the melting temperature is reached, but not the energy required to heat the solid to its melting point.
The process of transforming a solid to a liquid involves more than just the phase transformation observable to the human eye. On a microscopic level, the molecules in a solid attract one another, which is what allows them to remain in a relatively stable formation. To melt a solid, the molecules must be separated from one another, which means the substance must receive additional energy. The energy supplied during melting is stored by the molecules as potential energy, rather than kinetic energy, as the constant temperature during melting means the movement of the molecules does not increase or decrease at this time.
After the substance has been entirely transformed into a liquid, its temperature again begins to rise. It does this until the boiling point is reached, at which time the temperature again will remain constant while the liquid transforms into a gas. For this transformation, the substance again requires additional energy — this time referred to as enthalpy of vaporization. Temperature always remains constant during changes between the states of matter: solid, liquid and gas.
The heat of fusion required to melt a solid depends largely on the strength of the molecular bond, so different substances require different amounts of heat of fusion in order to transform into liquids. The amount of energy needed to melt lead, for example, is less than the amount required to melt ice into liquid water. This is because the heat of fusion does not take into account the temperature necessary to bring the substance up to its melting point but is only measured as the amount of heat needed to transform the substance entirely into a liquid once it reaches its melting point.