Rust forming on a piece of iron is an example of which of the following: A: A physical change because the process can be reversed
B: The natural process of decay as metallic ions age
C: A chemical change because oxygen atoms from the atmosphere bonded with the iron atoms
D: A violation of the laws of physics because new mass has been created

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .

particle   p is an electron and particle q is a neutron