Group 7: The Halogens and Their Reactions
Halogen trends are the opposite of Group 2 in many ways. Electronegativity decreases down the group as atoms get bigger, boiling points increase due to stronger van der Waals forces, and oxidising ability decreases (fluorine is the most powerful oxidising agent).
Displacement reactions show these trends perfectly - chlorine displaces bromine and iodine from their compounds, bromine displaces iodine, but you can't test fluorine in water because it reacts too violently.
The reactions with concentrated sulfuric acid reveal reducing power: fluorine and chlorine just have acid-base reactions, bromine produces orange fumes and SO₂, whilst iodine goes further - producing purple iodine vapour, yellow sulfur, and even smelly hydrogen sulfide gas.
Testing halides uses silver nitrate: white precipitate for chloride, cream for bromide, pale yellow for iodide. If colours look similar, try ammonia - AgCl dissolves in dilute NH₃, AgBr needs concentrated NH₃, and AgI won't dissolve at all.
Lab Safety Note: These halogen reactions produce toxic gases - always work in a fume cupboard and know your observation colours for exams!