Some reactions of the halide ions — common mistakes

Be precise with your descriptions. Use 'white', 'cream', and 'yellow' for the precipitates. When discussing ammonia, always specify whether it is 'dilute' or 'concentrated' as it is crucial for distinguishing AgCl from AgBr.

Remember the two key trends. Oxidising power of the HALOGENS ($X_2$) decreases down the group. Reducing power of the HALIDE IONS ($X^−$) increases down the group. Don't mix them up!

Because concentrated sulfuric acid is a strong enough oxidising agent to oxidise the HBr and HI formed into bromine (Br₂) and iodine (I₂) respectively. This results in an impure sample of the hydrogen halide. A weaker, non-oxidising acid like concentrated phosphoric(V) acid is used instead.

It's due to atomic structure. As you go down the group, the ions get larger (increasing ionic radius) and have more inner shells of electrons (increased shielding). This means the outermost electron is further from the nucleus and feels a weaker electrostatic pull. Consequently, this electron is lost more easily, making the ion a stronger reducing agent.

Yes, in principle, fluorine is the most powerful oxidising agent and would readily displace chloride ions. However, this reaction is not performed in a typical school laboratory because fluorine is extremely reactive and reacts violently with water, the solvent, producing oxygen and HF.