Halogen colours and states at RTP
The halogens are diatomic molecules (, , ). At RTP: chlorine is a pale yellow-green gas; bromine a red-brown liquid (brown vapour); iodine a dark grey/black solid that sublimes to a purple vapour. Down the group colour darkens and boiling points rise, so the state goes gas → liquid → solid. Chlorine is a gas (not a "green liquid"); iodine is a solid (the purple is its vapour).
Reactivity decreases down Group 7
A halogen reacts by gaining one electron to form a 1− halide ion (). Reactivity decreases down the group (): atoms have more shells, so the outer shell is further from the nucleus and more shielded, the attraction for an incoming electron is weaker, so it gains an electron less easily (spec 2.8C). This mirrors Group 1, where losing an electron gets easier down the group.
More reactive halogen displaces less reactive
A more reactive halogen displaces a less reactive one from a solution of its halide. Since : chlorine displaces bromine and iodine; bromine only iodine; iodine neither. E.g. — colourless turns orange. Chlorine is reduced (), bromide oxidised ().
Drawn from real examiner reports.
Colours the wrong way round
Marks are lost by stating a displacement colour change the wrong way round (writing the product colour as the start), or by confusing bromine water (orange) with titration indicator colours. Learn the exact colours, and always state the colour before and after, in the right order.
June 2024 Paper 2C Q5(b)(ii): a wide range of colours seen; some lost marks for the colour change the wrong way round, with "brown to colourless" seen — this may confuse bromine water with titration indicators.
It is the bromide ion, not bromine
Oxidation is the loss of electrons. It is the bromide ion () that is oxidised, losing electrons to form bromine: . Writing "bromine loses electrons" scores 0 — is the neutral product, not the reactant. "Electrons gained" is the wrong direction for oxidation.
June 2024 Paper 2C Q6(d)(ii): "bromine lost electrons" was not accepted — the mark needed the bromide ion losing electrons; a few wrongly said electrons were gained.
Chloride ion is not chlorine
A chloride ion () is NOT the same species as chlorine (). Only chlorine gas — a chlorine molecule — bleaches litmus; chloride ions do not, and neither does hydrogen chloride (). Treating chloride as chlorine, or writing "turns red then bleached white", loses the mark.
June 2024 Paper 2C Q7(b): poorly answered; many wrongly thought chloride ions or hydrogen chloride bleach. Examiner stressed chloride ions are not the same as chlorine.
Bleaching needs DAMP litmus + chlorine
Bleaching of litmus works only with the chlorine molecule AND the litmus must be damp — a dry test, or the wrong species, scores nothing. State both conditions: damp litmus paper and chlorine gas. (The "turns red then bleached" answer also fails because chloride ions cannot bleach in the first place.)
June 2024 Paper 2C Q7(b): bleaching needs damp litmus and a chlorine molecule; candidates wrongly said the solution "turned red then was bleached white".
Iodine is a solid, not a purple liquid
At RTP iodine is a dark grey/black solid, not a "purple liquid". The purple is the vapour formed when iodine sublimes on heating; in aqueous solution iodine appears brown. Give iodine's room-temperature state as a solid.
(common Edexcel chemistry error)
Reactivity DECREASES down Group 7
Group 7 reactivity decreases down the group — the opposite of Group 1 metals. Do not reverse it: chlorine is more reactive than iodine, because a halogen reacts by gaining an electron and that gets harder with more shells. Getting the direction backwards loses the explanation marks.
(common Edexcel chemistry error)
The displacement routine
For a Group 7 displacement: (1) order ; (2) if the added halogen is more reactive than the halide present, it reacts, else "no change"; (3) state the colour change; (4) write the balanced equation with the metal as a spectator ion.
Identify oxidation and reduction
Split a displacement into half-equations: the halide ion is oxidised (, loses electrons); the halogen molecule is reduced (, gains electrons). Name the ion, not the element.
Predicting other halogens
To predict another halogen (spec 2.6), extend the trends. Astatine (below iodine) would be a darker solid at RTP and less reactive than iodine — more shells, so it gains an electron even less easily. Give colour, state and reactivity, each justified by the trend.
The halogens are the elements in Group 7 of the periodic table. For 4CH1 you study chlorine, bromine and iodine. They all exist as diatomic molecules (, , ) and they all react by gaining one electron to form 1− halide ions.
| Term | Mark-scheme definition (two elements) |
|---|---|
| Halogen | A non-metal element in Group 7; exists as diatomic molecules and forms 1− ions |
| Halide ion | The 1− ion formed when a halogen atom gains one electron (e.g. , , ) |
| Displacement reaction | A reaction in which a more reactive halogen takes the place of a less reactive halogen in a solution of its halide |
| Oxidation | Loss of electrons (also gain of oxygen / loss of hydrogen) |
| Reduction | Gain of electrons (also loss of oxygen / gain of hydrogen) |
OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons) is a memory aid — not an acceptable definition on its own.
Define a halogen.
Chlorine displaces bromine from potassium bromide solution. Balance the equation:
