Could I2 be used instead of Cl2 to test for Br? Why?

looking at the periodic table, Cl is above Br while I is below. this means that Cl is more reactive than Br and will replace it in a reaction releasing Br as a gas or as a precipitate. I, on the other hand, is less reactive than Br and will not displace Br from any compound. so you will not release Br in any form and the reaction will not proceed.