A more reactive halogen will displace a less reactive halogen from a solution of its halide. This is a displacement reaction. If chlorine water is added to potassium bromide solution, the solution turns orange as bromine is made: Cl2 + 2KBr → 2KCl + Br2. Chlorine added to potassium iodide gives a brown solution of iodine: Cl2 + 2KI → 2KCl + I2. Bromine added to potassium iodide also gives brown iodine.
Bromine does not displace chlorine from potassium chloride, and iodine displaces neither chlorine nor bromine, so there is no reaction in those cases. This shows the order of reactivity: chlorine is more reactive than bromine, which is more reactive than iodine. Reactivity decreases down group 7.
Using this pattern, astatine is the least reactive halogen. It would not displace chloride, bromide or iodide ions, and all the other halogens would displace astatide ions from solution.