The melting of gold is a physical change. why?

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.

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.