Topic 3 of 7
Group 17: identify displacement from the evidence
A more reactive halogen oxidises a less reactive halide.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Group 17: identify displacement from the evidence
A more reactive halogen oxidises a less reactive halide.
| Element | Form, state and appearance | Relative reactivity |
|---|---|---|
| Chlorine | Cl2, pale green gas | Most reactive of these three |
| Bromine | Br2, red-brown liquid | Intermediate |
| Iodine | I2, grey-black solid; purple vapour on heating | Least 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.
| Added halogen | Chloride solution | Bromide solution | Iodide solution |
|---|---|---|---|
| Chlorine | No displacement | Bromine forms | Iodine forms |
| Bromine | No displacement | No displacement | Iodine forms |
| Iodine | No displacement | No displacement | No 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 understandingHalogen X displaces bromine from bromide but bromine cannot displace X from its halide. What follows?Think it through, then reveal the answer
X is more reactive than bromine. Among chlorine, bromine and iodine, X fits chlorine. The direction of displacement determines the order.