Consider the following balanced equation:
2 H2 + O2 --> 2 H20
What mass of water (H20) will be collected if 20.0 grams of hydrogen
gas (H2) are consumed?

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.