How/why does the density of an object change when heated?

Because the atoms are separating bc of the heat. A solid has really compact atoms so if it was melted its atoms would become looser and form a liquid

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.