Here's why:

* Melting point is the temperature at which a solid substance transitions into a liquid state.

* When you heat naphthalene, its temperature increases until it reaches its melting point.

* At 80 degrees Celsius, naphthalene will start to melt, and the temperature will remain constant until all of the solid has transformed into liquid.

Therefore, you know that 80 degrees Celsius is the melting point of naphthalene.