The halogens exist as diatomic molecules and change from a green gas to a brown liquid to a black solid down the group. A more reactive halogen pushes a less reactive one out of its compounds, and the colour change shows it happening.
The Group VII halogens are diatomic non-metals: chlorine (Cl2) is a pale yellow-green gas, bromine (Br2) a red-brown liquid and iodine (I2) a grey-black solid. Down the group the colour gets darker, the density increases and the reactivity decreases. A more reactive halogen displaces a less reactive halogen from a solution of its halide: Cl2 + 2KBr → 2KCl + Br2.
Fig. 1: chlorine, bromine and iodine at room temperature
Section 01
Colours and states.
Warm each halogen in the simulator and compare the colours and how easily each turns to vapour.
Diatomic: each molecule has two atoms joined by a covalent bond, Cl–Cl.
States at room temperature: chlorine gas, bromine liquid, iodine solid, so melting and boiling points increase down the group.
Predicting astatine: below iodine, so a dark solid, denser than iodine and even less reactive.
Safety: chlorine and bromine are toxic; use them in a fume cupboard.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
What is the appearance of bromine at room temperature?
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How does the reactivity of the halogens change down the group?
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Why is chlorine written as Cl2?
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Chlorine water is added to potassium iodide solution. What is seen?
Section 02
Displacement reactions.
Add each halogen solution to each halide solution in the simulator. A reaction happens only when the halogen is more reactive than the one in the halide.
Halogen added
potassium chloride
potassium bromide
potassium iodide
chlorine
no reaction
orange (bromine formed)
brown (iodine formed)
bromine
no reaction
no reaction
brown (iodine formed)
iodine
no reaction
no reaction
no reaction
Equation: Br2 + 2KI → 2KBr + I2.
Extended, ionic equation: Cl2 + 2Br− → 2Cl− + Br2. This is redox: chlorine gains electrons (reduced) and bromide ions lose them (oxidised).
Examiner trap
In a displacement reaction the colour change comes from the halogen that is formed, not the one added. Aqueous bromine is orange and iodine solution is brown, so "the solution turns brown" with potassium iodide means iodine has been made.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which mixture produces no reaction?
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(Extended) Which is the ionic equation for the reaction of chlorine with potassium bromide solution?
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Astatine is below iodine in Group VII. Which prediction is most likely correct?
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(Extended) In Cl2 + 2I− → 2Cl− + I2, which species is oxidised?
Skill unlocked
Group VII, mastered.
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