* Physical Changes: Involve changes in the state or appearance of a substance but not its chemical composition. The substance remains the same at a molecular level. Examples include melting, freezing, boiling, condensing, sublimation, and deposition.

* Chemical Changes: Involve the formation of new substances with different chemical compositions. This often results in changes in properties like color, odor, or reactivity. Examples include burning, rusting, and cooking.

In the case of sodium melting:

* The sodium atoms are still sodium atoms, just arranged differently in the liquid state.

* There's no change in the chemical composition of the sodium.

* It's simply a change in the physical state of the substance.

Therefore, melting sodium is a physical change, not a chemical change.