Melting butter is a physical change because the butter when melted simply changes from a solid state to a liquid and can be solid again when it's cooled down.
The chemical composition of the melted butter remains the same when the butter is melted.
Butter melting including on bread is not a chemical change and is instead a physical change.
The butter melting including on bread is a physical change and not a chemical change because the butter is simply changing from the state of being a solid to the state of being a liquid.
And the chemical composition of the melted butter still remains the same.
This means that the butter that is melted can be reversed by cooling the butter back down to a solid state.
Toasting bread causes a chemical change known as the Maillard reaction and spreading and melting butter on warm toast is a physical change that is related to state transition.
When butter is heated up, the emulsion then separates and the fat also separates from the water which forms 2 layers.
The separated fat layer is also commonly known as ghee which is clarified butter.
Ghee and butter have different properties though and you cannot extract butter from ghee so that is a chemical change.
Melting anything including butter is a physical change as it changes the state of matter from a solid to a liquid.
And during the melting process the chemical composition of the substance doesn't change.
When an ice cube for example melts, it's still water and the chemical composition of the water is H2O.