In this unbalanced reaction, which atom is reduced? h2o + cl2 + so2 → hcl + h2so4

The balanced equation for the above reaction is 
H₂O + Cl₂ + SO₂ ---> HCl + H₂SO₄
redox reactions are made of reduction and oxidation reactions.
reduction is when the oxidation state of the species reduces and in oxidation the oxidation state increases in the species.
in the above reaction,
oxidation state of Cl in Cl₂ is 0
and oxidation state of Cl in HCl is -1
theres a reduction in oxidation number of Cl from 0 to -1.
therefore Cl is reduced in the reaction 

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.

The formula is (nh4)2co3