If the acetylene tank contains 37.0 moles of c2h2 an oxygen tank contains 81.0 moles of o2 what is the limiting reactant for this reaction

2C2H2 + 5O2 ---> 4CO2 + 2H2O 
mole ratio of O2:C2H2 is 2.5:1 
for 37 mol C2H2 you need 2.5 x 37 mol = 92.5 moles O2. 
Thus, O2 is limiting.

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.