Here's why:

* Melting is a phase change: The energy being added to the naphthalene is used to break the bonds between the molecules in the solid state, allowing them to move more freely and transition into the liquid state.

* Constant temperature during phase change: This energy input does not increase the kinetic energy of the molecules, which is directly related to temperature. Instead, it's used to overcome the intermolecular forces holding the solid structure together.

Therefore, even though you're adding heat, the temperature stays at the melting point (around 80.2 °C for naphthalene) until all of the solid has melted.