Look at this equation: 2 H2 + O2 --> 2 H20. Why isn't there any coefficient in front of oxygen?

U whay do you need with?

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.

They can measure  the amount of energy gained or lost