What must models and theories do in order to remain valid

There must not be credible contrary evidence to the ideas suggested by the theory.

Engineers have contribute to society in many ways like making the technology we use today or any type of power energy material

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.