The amount of heat necessary to change a liquid to a solid at a constant temperature is called the heat of fusion (also known as the enthalpy of fusion).

Here's a breakdown:

* Heat of fusion: This is the amount of energy required to change a substance from a solid to a liquid state at its melting point.

* Constant temperature: The temperature remains constant during the phase change because the energy input is used to break the bonds holding the molecules in a fixed structure (solid), not to increase the kinetic energy of the molecules (which would increase the temperature).

Key points:

* The heat of fusion is a specific value for each substance. For example, the heat of fusion for water is 334 J/g.

* The process of changing from a liquid to a solid is called freezing or solidification, and it releases the same amount of heat as the heat of fusion.

Let me know if you have any other questions!