Group VII: The Halogens
Group VII halogens for IGCSE Chemistry 0620: colours and states at r.t.p., reactivity decreasing down the group, displacement and electron-gain ideas.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
Halogen questions in 0620 are won on three recall items and one logic step: the colours and states at r.t.p., the direction of the reactivity trend, and whether a given displacement reaction actually happens. Examiner reports note the same error every year: students state the trend correctly, then predict a displacement that contradicts it. This page covers Group VII as it sits inside the Periodic Table topic.
The halogens at room temperature and pressure
Group VII elements are diatomic non-metals (Cl2, Br2, I2), and the colour-and-state table is pure recall, tested almost every series:
| Halogen | State at r.t.p. | Colour |
|---|---|---|
| Chlorine (Cl2) | Gas | Pale yellow-green |
| Bromine (Br2) | Liquid | Red-brown |
| Iodine (I2) | Solid | Grey-black |
Two trends run down the group: melting and boiling points increase (gas → liquid → solid), and the colours darken. Given astatine, you predict a dark-coloured solid with a higher melting point than iodine. Note the contrast with Group I, where melting points fall down the group; confusing the two directions is a classic Paper 1 error.
Each halogen has seven outer-shell electrons, gains one electron in reactions, and forms a 1− halide ion (Cl−, Br−, I−).
Reactivity decreases down the group
Chlorine > bromine > iodine. This is the reverse of Group I, and the exam expects you to know both directions cold.
Displacement reactions are the evidence. A more reactive halogen displaces a less reactive halide from a solution of its salt:
- Cl2 + 2KBr → 2KCl + Br2: the colourless solution turns orange (bromine formed)
- Cl2 + 2KI → 2KCl + I2: the solution turns brown (iodine formed)
- Br2 + 2KI → 2KBr + I2: the solution turns brown
Run it backwards and nothing happens: bromine cannot displace chloride, iodine cannot displace bromide or chloride. “No reaction” is a legitimate, mark-earning answer; students are reluctant to write it, and lose the mark by inventing products.
The observation marks come from the colour change of the solution, so link each product to its colour: bromine in solution is orange, iodine in solution is brown (and iodine with starch gives blue-black, which appears in chemical tests).
Why reactivity decreases down the group (Supplement)
Halogens react by gaining one electron. Down the group the atoms get larger, so:
- The outer shell is further from the nucleus.
- The attraction between the nucleus and an incoming electron is weaker, so the electron is gained less easily.
Harder electron gain = lower reactivity. This is the mirror image of the Group I argument: alkali metals lose an electron more easily as atoms get bigger, halogens gain one less easily. Writing the Supplement ionic equation for a displacement also scores: Cl2 + 2Br− → 2Cl− + Br2. Chlorine is reduced (gains electrons); bromide ions are oxidised (lose electrons), so halogen displacement is a redox reaction.
Worked exam question
Aqueous chlorine is added to separate solutions of potassium bromide and potassium iodide. (a) Describe what is seen in each case and explain why a reaction occurs. [3] (b) Write the equation for the reaction with potassium bromide. [2] (c) Supplement: explain, in terms of electrons, why chlorine is more reactive than bromine. [2]
Model answer: (a) With potassium bromide the solution turns orange (1); with potassium iodide it turns brown (1); chlorine is more reactive than bromine and iodine, so it displaces them from solution (1). (b) Cl2 + 2KBr → 2KCl + Br2: formulae (1), balancing (1). (c) Chlorine atoms are smaller, so the incoming electron is closer to the nucleus (1) and is attracted more strongly / gained more easily (1).
Mark-by-mark: in (a) the colours are the observations; “a reaction happens” earns nothing. The explanation mark needs the comparative “more reactive” plus the word displaces. In (c), both the size/distance idea and the stronger-attraction idea are required for both marks.
The mistakes that cost marks
- Trend direction reversed. Group VII reactivity decreases down the group. Students who carry the Group I direction across lose every prediction mark that follows.
- Predicting impossible displacements. Iodine added to sodium chloride solution: no reaction. Writing products anyway is the single most common halogen error. Check the order first.
- Wrong colours for the dissolved halogens. Bromine in solution is orange; iodine in solution is brown. Quoting the element colours (red-brown liquid, grey-black solid) for the solution costs the observation mark.
- Forgetting halogens are diatomic. Equations with Cl instead of Cl2 fail the formula mark instantly.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”Chlorine kicks out the bromine" | "Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution" |
| "The liquid goes a different colour" | "The solution turns orange because bromine is formed" |
| "Chlorine wants electrons more" | "Chlorine atoms are smaller, so the incoming electron is more strongly attracted to the nucleus and gained more easily" |
| "Nothing much happens" | "No reaction, because bromine is less reactive than chlorine” |
The whole subtopic runs on one ordered list (Cl > Br > I) plus the colours. Learn those, and every displacement question becomes a lookup. If displacement logic still trips you under timed conditions, book a free trial lesson and we will drill it with real past-paper questions.
Test yourself
Check the order Cl > Br > I before each answer, then click to mark each one.
Q1 (2 marks). State the colour and physical state of bromine at r.t.p., and predict the physical state of astatine at r.t.p.
Show answer
• bromine is a red-brown liquid [1] • astatine is predicted to be a solid, because melting and boiling points increase down the group [1]
Q2 (3 marks). Bromine water is added to separate solutions of potassium iodide and potassium chloride. Describe what is seen in each case and explain the results.
Show answer
• with potassium iodide the solution turns brown, because iodine is displaced [1] • bromine is more reactive than iodine, so it displaces iodide from solution [1] • with potassium chloride there is no reaction, because bromine is less reactive than chlorine [1]
Q3 (2 marks). (Extended) Write the ionic equation for the reaction between chlorine and aqueous iodide ions, and state which species is oxidised.
Show answer
• Cl2 + 2I− → 2Cl− + I2 [1] • the iodide ions are oxidised: they lose electrons (oxidation number −1 → 0) [1]
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Frequently asked questions
What colours and states do I need for the halogens at r.t.p.?
Chlorine: pale yellow-green gas. Bromine: red-brown liquid. Iodine: grey-black solid. These three, at room temperature and pressure, are tested almost every series.
Which way does reactivity go in Group VII?
Reactivity decreases down the group, the opposite of Group I. Chlorine is more reactive than bromine, which is more reactive than iodine. Displacement reactions prove the order.
How do I know if a displacement reaction happens?
A more reactive halogen displaces a less reactive halide from solution. Chlorine added to potassium bromide turns the solution orange because bromine is formed. Iodine added to potassium chloride does nothing: no reaction.