Group 17 Elements: The Halogens
Learn Group 17 halogen colours and states, the reactivity trend, displacement predictions, balanced ionic equations and precise observations.
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Chlorine can turn a colourless bromide solution orange-brown by producing bromine. Reversing the pairing does not produce chlorine. Why does one direction work? Compare the halogens’ reactivity, then track the atoms and ions that change.
Halogen molecules and halide ions
The halogens are Group 17 elements. Chlorine, bromine and iodine are non-metals that exist as diatomic molecules: Cl₂, Br₂ and I₂.
A halide ion forms when a halogen atom gains one electron. For example, Cl + e⁻ → Cl⁻. Chloride, bromide and iodide ions have charge -1. Keep Br₂ (a neutral bromine molecule) distinct from Br⁻ (a bromide ion).
A halogen displacement converts a less reactive halogen’s halide ions into that halogen. The added, more reactive halogen becomes halide ions instead.
State the physical state as well as the colour
| Pure halogen | Formula | State at room temperature | Colour |
|---|---|---|---|
| Chlorine | Cl₂ | Gas | Yellow-green |
| Bromine | Br₂ | Liquid | Red-brown |
| Iodine | I₂ | Solid | Grey-black; its vapour is purple |
These are the pure elements’ appearances. In the displacement experiments below, the halogens are dissolved in water. Bromine water is orange to orange-brown; aqueous iodine is brown. The depth of colour depends on concentration. Dissolved halide salts such as potassium bromide are colourless.
Compare reactivity before predicting a reaction
Reactivity decreases down Group 17. For the three halogens compared here:
Cl₂ > Br₂ > I₂
A halogen displaces another from its halide solution only if the added halogen is more reactive. Chlorine displaces bromide and iodide; bromine displaces iodide. Iodine displaces neither chloride nor bromide. A halogen also does not displace its own halide ions: no new element would be formed.
For chlorine added to potassium bromide solution:
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Potassium ions remain unchanged, so the ionic equation is:
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Two bromide ions lose two electrons in total. One chlorine molecule gains those two electrons, forming two chloride ions. Bromide is oxidised; chlorine is reduced. This is a redox reaction. Both sides contain two Cl atoms, two Br atoms and total charge 2-.
Link the product to the observation
The table describes small additions of dilute halogen solutions to excess halide solution. Name the halogen formed as well as the colour you observe.
| Added halogen | Halide solution | Displacement? | Halogen formed | Observation in water |
|---|---|---|---|---|
| Chlorine | Potassium bromide | Yes | Bromine | Orange to orange-brown solution forms. |
| Chlorine | Potassium iodide | Yes | Iodine | Brown solution forms. |
| Bromine | Potassium iodide | Yes | Iodine | Brown solution forms. |
| Bromine | Potassium chloride | No | None | The added bromine’s orange colour remains, possibly diluted. |
| Iodine | Potassium chloride or potassium bromide | No | None | The added iodine’s brown colour remains, possibly diluted. |
The starting halide solution is colourless. Do not confuse its colour with the colour of the halogen solution being added. A coloured mixture alone does not prove displacement: its colour might come from the added halogen.
A different question: which halide ion is present?
Displacement compares halogen reactivity. The silver nitrate test identifies halide ions through precipitates. See the separate procedure and observations in Qualitative Analysis.
Chlorine and bromine vapours are toxic and irritating. These comparisons use dilute solutions in a supervised laboratory, with a fume cupboard where needed to avoid inhalation.
Check the species, direction and evidence
- Bromine, Br₂, has an aqueous colour; bromide, Br⁻, is a colourless ion in these salt solutions.
- The more reactive halogen displaces the less reactive one. The order runs chlorine > bromine > iodine.
- Use two halide ions per diatomic halogen molecule when balancing the ionic equation. Check total charge as well as atoms.
- No displacement establishes that the added halogen is less reactive only when the two halogens are known to be different. An element also fails to displace its own halide ions.
Explain an outcome in three steps
Compare the halogens’ reactivity, decide whether displacement occurs, then name the product and its aqueous colour. For an unfamiliar set of results, convert each successful displacement into a reactivity comparison before combining them.
Worked examples
Modelled example 1
Predict displacement and products
Problem
Chlorine water is added to potassium iodide solution. Predict whether a reaction occurs and write the ionic equation.
Study the worked solution
Compare reactivity
Method
Place chlorine above iodine in the Group 17 reactivity order.Reason
A more reactive halogen can displace a less reactive halogen from its halide ions.Working
Cl₂ > I₂ in reactivity.Predict the species change
Method
Convert iodide ions into iodine and chlorine into chloride ions.Reason
Chlorine gains electrons while iodide loses them.Working
Cl₂ → Cl⁻ and I⁻ → I₂.Balance charge and atoms
Method
Use two halide ions for each diatomic halogen molecule.Reason
This balances both atoms and electron transfer.Working
Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)
Guided practice 2
Observation (don’t be vague)
Problem
A small amount of dilute chlorine water is added to excess potassium bromide solution. State a precise observation and name the product responsible.
Pair the colour change with a named halogen
Hints
Hint 1: apply the displacement order
Chlorine is more reactive than bromine.
Hint 2: name before colouring
The displaced halide becomes its halogen; then recall that halogen’s aqueous colour.
View solution step by step
Identify the reaction
Method
Displace bromide ions with chlorine.Reason
Chlorine is more reactive than bromine.Working
Bromine, Br₂(aq), is formed.Report the observation
Method
State the initial and final colours.Reason
The product bromine gives the resulting aqueous colour.
Working
The colourless potassium bromide solution becomes orange to orange-brown as bromine forms.
Common misconception 3
No displacement case
Learner response
Bromine water is added to potassium chloride solution. A student predicts chlorine will form because “bromine displaces chloride.” Correct the prediction and state the observation.
Check the direction before naming a product
View solution step by step
Locate the reversed rule
Method
Compare bromine with chlorine.Reason
Reactivity decreases down Group 17, so bromine is less reactive than chlorine.
Working
Cl₂ > Br₂.Apply the displacement condition
Method
Require the added halogen to be more reactive than the halogen in the halide.
Reason
Bromine does not meet that condition for chloride ions.
Working
No displacement occurs.State the observation
Working
The solution remains orange, possibly paler through dilution, because bromine remains present and no chlorine forms.
Examiner practice 4
Redox language (short)
Examination question
Track one reactant into its product
View solution step by step
Track bromide
1 markMethod
Follow Br⁻ from reactant to Br₂ product.Reason
Bromide ions are the species whose oxidation state changes from -1 to 0.Working
2Br⁻ → Br₂.Use electron loss
1 markMethod
Write the electron change.Reason
Oxidation is loss of electrons.Working
2Br⁻ → Br₂ + 2e⁻, so Br⁻ is oxidised.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the species and electron-loss justification separately.
Challenge 5
Deduce an order from evidence
Reverse evidence transfer
Turn each result into one inequality
Hints
Hint 1: translate success
If X displaces Y-, X must be more reactive than Y.
Hint 2: translate failure
Because X and Z are different halogens, failure to displace Z- places Z above X.
View solution step by step
Use the successful displacement
Method
Place X above Y.Reason
Only a more reactive halogen displaces a less reactive halogen’s ions.Working
X > Y.Use the failed displacement
Method
Place Z above X.Reason
X and Z are different halogens; X cannot displace Z⁻, so X is less reactive than Z.Working
Z > X.Combine both constraints
Method
Join the two inequalities without discarding either result.Reason
The only supported chain places X between Z and Y.Working
Z > X > Y.
Try these independently
Mind stretcher 1: Two-step reasoningExtension
Question: You add chlorine water to a mixture of KCl(aq) and KI(aq). Which halogen will be produced and why?
Show Answer
Iodine, I₂, is produced because chlorine is more reactive than iodine and displaces iodide ions. Chloride ions are already the halide of chlorine, so they are not displaced by chlorine.
Mind stretcher 2: Use results to predictExtension
Question: An unknown halogen is one of chlorine, bromine or iodine. It displaces bromide ions but does not displace chloride ions. Identify it. Why does the failure with chloride not prove that it is less reactive than chlorine?
Show Answer
It is chlorine: only chlorine among these three can displace bromide. Chlorine does not displace its own chloride ions because no different halogen would be produced. A failed displacement means “less reactive” only when the halogens being compared are known to be different.
Practise and check
Test colours and states, reactivity trends, displacement predictions, ionic equations and evidence-based deductions.
Open the Periodic Table topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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