text: why is heat needed to cause the zinc coating to diffuse into the copper penny? a penny was placed in a zinc copper solution. the penny then turned silver because of the zinc coating. the penny was passed over a flame to turn it into brass. if you coat a copper penny with zinc at room temperature, nothing interesting happens.   the zinc just lies around on the copper substrate.   if you heat it, the atoms start vibrating and some of the zinc and copper change places, and you get a solid solution of copper and zinc, which is the alloy brass.