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Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · 6.10 Check the specification (PDF) (opens in a new tab)
Chlorine, bromine and iodine are Group 7 elements called . Each exists as molecules containing two atoms, written Cl₂, Br₂ and I₂. They react with hydrogen, H₂, to form : hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI). Each product contains one hydrogen atom bonded to one halogen atom, so its general formula is HX.
For chlorine, the balanced equation is:
The same formula pattern gives . Hydrogen and iodine also form hydrogen iodide, although that reaction is reversible: .
Hydrogen halides dissolve in water to form acidic solutions. For example, dissolving hydrogen chloride gas, HCl(g), produces hydrochloric acid, HCl(aq). The formula still contains HCl, but the state symbol matters: identifies the gas, while means it is dissolved in water. In the solution, hydrogen ions are responsible for its acidity. Hydrogen bromide and hydrogen iodide likewise give acidic solutions when dissolved.
Use the shared Group 7 pattern rather than memorising each reaction separately: hydrogen reacts with a halogen to form a hydrogen halide, and that hydrogen halide forms an acidic solution in water. Fluorine is another halogen, so predict . The product is hydrogen fluoride; when it dissolves in water, the solution is acidic. This prediction does not mean that all hydrogen halide solutions have the same acid strength.
Hydrogen + halogen → hydrogen halide. For chlorine, bromine and iodine the products are HCl, HBr and HI. In general, , where X is a halogen; the iodine reaction is reversible.
Hydrogen halides dissolve to form acidic solutions. Distinguish HCl(g), hydrogen chloride gas, from HCl(aq), hydrochloric acid. Apply the same pattern to predict that fluorine forms HF, which gives an acidic solution in water.
Distinguish hydrogen chloride gas, HCl(g), from hydrochloric acid, HCl(aq): the acidic solution forms when the gas dissolves in water.
When writing an equation, remember that hydrogen and the halogens are diatomic: use H₂ and Cl₂, Br₂ or I₂.
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Halogen
An element in Group 7 of the periodic table, such as chlorine, bromine or iodine.
Hydrogen halide
A compound of hydrogen and a halogen, with the general formula HX, where X represents a halogen.
Acidic solution
A solution that contains hydrogen ions, H⁺, and has a pH below 7.
Put your knowledge into practice — try past paper questions for Chemistry
Halogen
An element in Group 7 of the periodic table, such as chlorine, bromine or iodine.
Hydrogen halide
A compound of hydrogen and a halogen, with the general formula HX, where X represents a halogen.
Acidic solution
A solution that contains hydrogen ions, H⁺, and has a pH below 7.