Which of the following is not an oxidation-reduction reaction?
(pictured below)

Answer is: the third reaction.
Elements in this chemical reaction do not change their oxidation number. Hydrogen has oxydation number +1, sulfur oxidation number is +6, oxygen has oxidation number -2 and barium has +2 on both sides of chemical reaction. In other reactions elements change their oxidation numbers.

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.