Group VII: An Overview
The halogens — fluorine, chlorine, bromine and iodine — make up Group VII, each with seven valence electrons.
Physical properties: low melting and boiling points, increasing down the group (fluorine has the lowest); at room temperature, fluorine and chlorine are gases, bromine a liquid, and iodine a solid; colour deepens down the group — fluorine pale yellow, chlorine greenish-yellow, bromine orange, iodine dark blue-black.
Chemical properties: halogens gain one electron to form singly-charged negative ions (e.g. Cl⁻), forming ionic compounds with metals (e.g. MgCl₂) and covalent bonds with non-metals or each other. When a halogen forms a simple compound, its “-ine” ending changes to “-ide” — fluorine → fluoride, chlorine → chloride, iodine → iodide (e.g. NaCl is sodium chloride, CaBr₂ is calcium bromide, KF is potassium fluoride).
Chlorine
Laboratory preparation: reacting potassium permanganate with concentrated hydrochloric acid: 2KMnO₄(aq) + 16HCl(aq) → 2MnCl₂(aq) + 2KCl(aq) + 8H₂O(l) + 5Cl₂(g).
Properties: a greenish-yellow, poisonous gas with an unpleasant, choking smell; about 2.5 times denser than air; can be liquefied under roughly 6 atmospheres of pressure; bleaches moist litmus paper; dissolves readily in water to form “chlorine water” (a mixture of HCl and chloric(I) acid, HOCl), which acts as a bleaching agent by oxidising dyes to colourless compounds: Cl₂(g) + H₂O(l) → HCl(aq) + HOCl(aq). (Anything bleached with chlorine water should be rinsed well afterwards, since the leftover HCl can otherwise attack it.)
Hydrogen Chloride
Preparation: reacting concentrated sulphuric acid with sodium chloride, collecting the gas by downward delivery (since it’s denser than air): H₂SO₄(aq) + NaCl(aq) → NaHSO₄(aq) + HCl(g).
Properties: a colourless gas with a pungent, choking smell; slightly denser than air; does not support combustion; turns moist blue litmus red; dissolves readily in water to form hydrochloric acid; reacts with ammonia gas to form dense white fumes of ammonium chloride: HCl(g) + NH₃(g) → NH₄Cl(s).
Displacement Reactions
A more reactive halogen will displace a less reactive one from its salt solution. The order of displacing power is F > Cl > Br > I, so, for example: Cl₂ + 2KBr → 2KCl + Br₂ (greenish-yellow chlorine displaces orange bromine), and Br₂ + 2KI → 2KBr + I₂ (orange bromine displaces dark, blue-black iodine). These reactions don’t run in reverse — iodine cannot displace chlorine from KCl.
Testing for Halide Ions
Add excess dilute nitric acid to the sample, then a few drops of silver nitrate solution. A precipitate confirms a halide is present, and its colour identifies which one:
- Ag⁺(aq) + Cl⁻(aq) → AgCl(s) — white precipitate
- Ag⁺(aq) + Br⁻(aq) → AgBr(s) — pale yellow (cream) precipitate
- Ag⁺(aq) + I⁻(aq) → AgI(s) — yellow precipitate
Uses of Halogens
Chlorine sterilises water; chlorine and fluorine are used in manufacturing pesticides and durable plastics; iodine sterilises wounds; fluorine is used in toothpaste manufacture.
