Compare the amount of thermal energy needed to melt a solid and the amount of thermal energy needed to freeze the same liquid.
The amount of thermal energy needed to melt a solid and the amount of thermal energy needed to freeze the same liquid are equal in magnitude, but they have opposite signs.
When melting a solid, thermal energy is added to the solid to break the intermolecular forces holding the solid together and convert it into a liquid. This process is endothermic, meaning it requires energy input. The energy input required is known as the heat of fusion.
On the other hand, when freezing a liquid, thermal energy is removed from the liquid to slow down the molecular motion, allowing the intermolecular forces to reform and convert the liquid into a solid. This process is exothermic, meaning it releases energy. The energy released is equal in magnitude but opposite in sign to the heat of fusion.
Overall, the amount of thermal energy needed to melt a solid and the amount of thermal energy released to freeze the same liquid are the same, but their effects on the system are different.