Why is it necessary to classify element

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.

13) a 14) a15) b16) i don't know sorry17) a (maybe)