The heat of fusion is the energy needed to transform a solid into a liquid. The temperature remains constant during this process, and the energy is stored as potential energy. Different substances require different amounts of heat of fusion, depending on molecular bonds.
The heat of fusion, also called the enthalpy of fusion, is the amount of energy required to transform a substance from solid to liquid. Once a solid has reached the temperature at which it melts, its temperature does not continue to rise while it is melting, even when exposed to the same heat source. While it is melting, a solid continues to absorb energy from its heat source, which allows for the molecular change necessary for fusion.
When a solid is heated, its temperature rises until it reaches its melting point. Once this temperature is reached, additional energy must be supplied to the solid to transform it into a liquid. The heat of fusion refers to the energy required 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 into a liquid involves more than just the phase transformation observable to the human eye. At a microscopic level, the molecules in a solid attract each other, which allows them to remain in a relatively stable formation. To melt a solid, the molecules must be separated from each other, which means that the substance must receive additional energy. The energy supplied during fusion is stored by the molecules as potential energy, rather than kinetic energy, as the constant temperature during fusion means that the motion of the molecules does not increase or decrease at this time.
After the substance has been completely transformed into a liquid, its temperature begins to rise again. It does this until the boiling point is reached, at which point the temperature will again remain constant as the liquid turns into a gas. For this transformation, the substance again requires additional energy, this time called the enthalpy of vaporization. The temperature always remains constant during the 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 to turn into liquids. The amount of energy required to melt lead, for example, is less than the amount required to melt ice in liquid water. This is because the heat of fusion does not take into account the temperature required to bring the substance to its melting point but is only measured as the amount of heat required to completely transform the substance into a liquid once its melting temperature has been reached.
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