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K326 / K328 / 2027
Periodicity and Metals

Lesson 4 of 8

Group 17: identify displacement from the evidence

A more reactive halogen oxidises a less reactive halide.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

  • I can predict group properties.Syllabus

    Syllabus K326 / K328, 8.1(f). Predict group properties

  • I can compare halogens.Syllabus

    Syllabus K326 / K328, 8.2(b). Compare halogens

Group 17: identify displacement from the evidence

A more reactive halogen oxidises a less reactive halide.

Make a guess. You are not marked.

Chlorine water is added to colourless aqueous potassium bromide. What do you see?

  1. The solution turns orange-brown as bromine forms
  2. No change, because both of them are halogens
  3. A white precipitate of potassium chloride forms
Show the answer

The solution turns orange-brown as bromine forms

Chlorine is more reactive than bromine, so it displaces bromine from the bromide. The orange/brown colour is the bromine formed.

Halogens at room temperature
ElementForm, state and appearanceRelative reactivity
ChlorineCl2, pale green gasMost reactive of these three
BromineBr2, red-brown liquidIntermediate
IodineI2, grey-black solid; purple vapour on heatingLeast reactive of these three

Down the group, colour becomes darker and melting/boiling points rise, giving the gas-liquid-solid sequence. All three are diatomic non-metals with seven outer electrons. Their displacement reactivity decreases down the group.

Halogen added to a halide solution
Added halogenChloride solutionBromide solutionIodide solution
ChlorineNo displacementBromine formsIodine forms
BromineNo displacementNo displacementIodine forms
IodineNo displacementNo displacementNo displacement

Cl2(aq) + 2Br-(aq) -> 2Cl-(aq) + Br2(aq). The orange/brown bromine colour is evidence of displacement. Chlorine or bromine added to iodide can produce brown aqueous iodine; distinguish solution colours from the colours of pure elements.

Check your understanding

Halogen X displaces bromine from a bromide, but bromine cannot displace X from its halide. What follows?

  1. X is less reactive than bromine, so of Cl, Br and I it fits iodine.
  2. X and bromine are equally reactive, so neither reaction should take place.
  3. X is more reactive than bromine, so of Cl, Br and I it fits chlorine.
Show the answer

X is more reactive than bromine, so of Cl, Br and I it fits chlorine.

A more reactive halogen displaces a less reactive one from its halide. X displaces bromine, and not the other way round, so X is above bromine in Group 17.

Check your understanding

Astatine is below iodine in Group 17. Which prediction is most likely to be correct?

  1. A pale yellow gas that displaces iodine from potassium iodide solution
  2. A dark solid that does not displace iodine from potassium iodide solution
  3. A red-brown liquid that is more reactive than chlorine in displacement
  4. A dark solid made of single At atoms, since it is below iodine
Show the answer

A dark solid that does not displace iodine from potassium iodide solution

Down Group 17, colour darkens and melting and boiling points rise, so astatine should be a dark solid. Reactivity falls, so it cannot displace iodine.