Test for halogen Test method Test observations Test chemistry and comments
Chlorine gas Cl2
A pungent green gas. (i) Apply damp blue litmus. (Can use red litmus and just see bleaching effect.)
(ii) A drop silver nitrate on the end of a glass rod into the gas. (i) litmus turns red and then is bleached white.
(ii) White precipitate. (i) Non-metal, is acid in aqueous solution and a powerful oxidising agent
(ii) It forms a small amount of chloride ion in water, so gives a positive result for the chloride test.
Bromine Br2 (l or aq)
A dark red liquid - orange-brown fumes, yellow-orange aqueous solution. The other common orange-brown gas is nitrogen dioxide (i) Shake with a liquid alkene.
(ii) Mix with silver nitrate solution. (ii) Decolourised. See alkene test.
(ii) Cream ppt. of silver bromide as in the test for bromide. (i) Forms a colourless organic dibromo-compound
>C=C< + Br2 ==> >CBr-CBr<
(ii) Ag+(aq) + Br-(aq) ==> AgBr(s) Any soluble bromide gives a silver bromide precipitate.
Iodine (i) solid or (ii) solution
A very dark solid (i) Gently heat the dark coloured solid.
(ii) Test aqueous solution or solid with starch solution. (i) Gives brilliant purple vapour.
(ii) A blue black colour. (i) Iodine forms a distinctive coloured vapour.
(ii) Forms a blue-black complex with starch and in biology the test is used to detect starch with iodine solution.
Tests for Halide Ions
In test (i) the silver nitrate is acidified with dilute nitric acid to prevent the precipitation of other non-halide silver salts.
Test for halide ion Test method Observations Test chemistry and comments
Fluoride Ion F-
Fluoride and hydrogen fluoride gas are harmful, irritating and corrosive substances. (i) If the suspected fluoride is soluble add dilute nitric acid and silver nitrate solution.
(ii) You can warm a solid fluoride with conc. sulphuric acid and hold in the fumes (ONLY!) a glass rod with a drop of water on the end.