GCSE-readyHalogens: properties, reactions and redox
The halogens react with metals and with hydrogen, and a more reactive halogen pushes a less reactive one out of its compounds.
Colours, states and trends
| Halogen | State at room temperature | Colour |
|---|---|---|
| Fluorine, F2 | Gas | Pale yellow |
| Chlorine, Cl2 | Gas | Pale green (yellow-green) |
| Bromine, Br2 | Liquid | Red-brown (orange vapour) |
| Iodine, I2 | Solid | Dark grey (purple vapour) |
Reactions with metals and hydrogen
sodium + chlorine → sodium chloride: 2Na + Cl2 → 2NaCl
Halogens react with hydrogen to make hydrogen halides: H2 + Cl2 → 2HCl
Hydrogen halides are gases that dissolve in water to form acidic solutions. Hydrogen chloride dissolves to make hydrochloric acid.
Down the group, melting and boiling points increase and the colour gets darker. Astatine, below iodine, would be expected to be a dark solid and the least reactive.
Displacement as redox (Higher)
chlorine + potassium bromide → potassium chloride + bromine
Ionic equation: Cl2 + 2Br− → 2Cl− + Br2
Chlorine gains electrons: it is reduced. Bromide ions lose electrons: they are oxidised.
Reactivity decreases down the group: atoms are bigger, so the outer shell is further from the nucleus and more shielded, and it is harder to gain an electron.
Chlorine water is added to sodium iodide solution. Predict what you would see and write the word equation.
- Chlorine is more reactive than iodine.
- So chlorine displaces iodine.
- Iodine in solution is brown.
Answer: The solution turns brown as iodine forms: chlorine + sodium iodide → sodium chloride + iodine.
Fluorine and chlorine are gases, bromine is a red-brown liquid and iodine a dark grey solid. Halogens react with metals to form halides and with hydrogen to form hydrogen halides, which dissolve to give acids. A more reactive halogen displaces a less reactive one; the halogen is reduced and the halide ion oxidised.
- Halogen
- An element in Group 7, such as chlorine, bromine and iodine. Halogens are diatomic and get less reactive down the group.
- Ion
- An atom or group of atoms with a charge, because it has lost or gained electrons.
- Acid
- A substance that releases hydrogen ions, H^{+}, in water (a proton donor). Its solution has a pH below 7.
- Group
- A vertical column of the periodic table. Elements in a group have the same number of outer electrons and similar reactions.
- Solution
- A mixture formed when a solute dissolves in a solvent.
- Salt
- The compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.
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Which halogen is a pale yellow-green gas at room temperature?
- Iodine
- Chlorine
- Astatine
- Bromine
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Answer: Chlorine
Chlorine is a yellow-green gas.
Hydrogen chloride dissolves in methylbenzene. Why does the solution NOT behave as an acid?
- Methylbenzene removes chlorine
- Hydrogen chloride reacts to form water
- Hydrogen chloride does not split into H+ ions in methylbenzene
- Methylbenzene is an alkali
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Answer: Hydrogen chloride does not split into H+ ions in methylbenzene
Acidity needs H+ ions, which form in water.
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Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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