The melting of gold is a physical change. why?

Because when a matter melts (or generally speaking, changes phases) its chemical structure does not change. By change of chemical structure I mean, reaction. When a matter melts it does not react with another one. It does not, give or take electrons to another matter or it does not share with them.

Melting gold can be reversed. Physical changes are ones that can be reversed, like bending a paperclip or getting whacked with a hammer and getting a dent. Chemical changes can't be reversed: like burning wood, or baking bread.