How many moles are in 20 grams of O₂ gas?

20 g O2 x 1 mol O2/32 g O = 0.625 mol O2

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.