I was trying to solve the following exercise: Choose the option that has the pair of substances whose chemical reaction is easily perceptiple a)Br2(aq) + NaCl(aq) b) Cl2(aq) + NaI(aq) c) H2(g) + MgSO4(aq) d) Ag(c) + ZnSO4(aq) e) HCl(aq) + Cu(c) I would say if the salt was an alkali salt, a would discolor the solution (Br2 is brown/red), as Br2 + H2O -> HBrO + HBr, and the equilibrum tends to form the products when the solution is basic. But the salt is NaCl, so nothing would happen The second one I would say no change would be noticed The third I would say some of the SO4-- could reduce to SO2 or some of the Mg++ to Mg, oxydating the H2 to H2O, but in a very low scale, so nothing would be noticed too For the third, as ZnSO4 forms a acid solution, I would say some of the silver would oxydate to Ag+, even silver being a noble metal. This will occur with a low velocity but I would say it could be noticed. For the last one, as Cu isn't a noble metal, I would say it will fastly oxidate to Cu2+, and the solution will color to blue, easily noticed I chose e as the answer, but by book is saying it is b. Why is that?