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Learn7405Paper 1 · Paper 1: Physical and inorganic chemistryGroup 7 (17), the halogensHalogens and halides
Group 7 (17), the halogens · Subtopic 1 of 1

Halogens and Halides

Down group 17, halogens become darker and less volatile (stronger London forces), electronegativity and oxidising power decrease, while halide ions become stronger reducing agents (larger ions lose electrons more easily). A more reactive halogen displaces a less reactive halide. Silver nitrate test (acidified with HNO₃): Cl⁻ white (dissolves in dilute NH₃), Br⁻ cream (concentrated NH₃), I⁻ yellow (insoluble). With concentrated H₂SO₄, NaCl gives only HCl; NaBr gives Br₂ and SO₂; NaI gives I₂, S and H₂S. Chlorine disproportionates in water and in cold or hot NaOH.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 1≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe trends in volatility, colour and oxidising power.
  • Explain the reducing power of halide ions.
  • Describe tests for halide ions and their reactions with concentrated sulfuric acid.
  • Describe reactions of chlorine with water and sodium hydroxide.
Pause and think

Why can iodide ions reduce sulfuric acid all the way to H₂S, while chloride ions can't reduce it at all?

Section 2 of 14

Core idea

Start hereFrom poison gas to pool cleaner

Chlorine was used as a weapon in World War I; today it keeps billions of people's drinking water safe. Group 17 chemistry is all about electron transfer.

The big idea

Halogens are oxidising agents, weaker down the group. Halide ions are reducing agents, stronger down the group.

1. Halides with sulfuric acid

How far can each halide reduce sulfuric acid?

Stronger reducing agents go further.

Worked exampleIdentifying a halide

A solution gives a cream precipitate with acidified AgNO₃ that dissolves in concentrated but not dilute NH₃.

Goal
Halide.
Strategy
Colour and solubility in ammonia.
  1. cream⇒AgBr\text{cream}\Rightarrow\mathrm{AgBr}
    Colour.
  2. soluble in conc. NH3\text{soluble in conc. NH}_3
    Confirms bromide.
AnswerBromide, Br⁻.
✓ CheckAgCl would dissolve in dilute ammonia ✓.
Key takeawayDisproportionation Cold NaOH: Cl₂ → Cl⁻ + ClO⁻ (bleach). Hot NaOH: Cl₂ → Cl⁻ + ClO₃⁻. Water: Cl₂ → HCl + HClO.
Pause and think

Why is chlorine added to water even though it's toxic?

Summary

  • Oxidising power ↓ down.
  • Reducing power of X⁻ ↑ down.
  • AgX colours; conc. H₂SO₄ products.
Key words
Halide
Negative ion of a halogen.
Oxidising agent
Accepts electrons and is reduced.
Chlorate(I)
ClO⁻, the active ingredient in bleach.
HistoryBleach. Sodium chlorate(I), made from chlorine and cold sodium hydroxide, was first sold as "Eau de Javel" in France in 1789 to bleach cloth.
Fun factPhotographic film. Traditional black-and-white film used silver bromide crystals that darken where light hits them, the same reaction you can see with silver halide precipitates.
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