Halide Tests

I have made detailed notes and revision sheets on each area of the specification, which I share with my students. I go through each past paper from January 2000 to compile these notes, making sure every possible question is addressed before a student sits an examination. Here is an example for the halide tests (note equation arrows, diagrams and photographs do not show up in this format) :

(h) describe the use of the precipitation reactions in (g) as a test for different halide ions. 

Test for chloride ions – add aqueous silver nitrate, AgNO₃ (aq) and a white precipitate should form:

Ag⁺ (aq) + Cl^- (aq)              AgCl (s)   

Test for bromide ions – add aqueous silver nitrate, AgNO₃ (aq) and a cream precipitate should form:

Ag⁺ (aq) + Br^- (aq)              AgBr (s)   

Test for iodide ions – add aqueous silver nitrate, AgNO₃ (aq) and a yellow precipitate should form:

Ag⁺ (aq) + I^- (aq)               AgI (s)   

It is sometimes hard to distinguish between the white, cream and yellow precipitates of chloride, bromide and iodide ions respectively. Aqueous ammonia can be used to make the distinction:

Silver chloride is soluble in NH₃ (aq) so it dissolves

Silver bromide is partially soluble in NH₃ (aq) (it will dissolve in concentrated NH₃ (aq))

Silver iodide is insoluble in NH₃ (aq) so it does not dissolve

NB1 in the aqueous solutions used to test the halides, it is important that distilled water is used. This is because tap water can contain chloride ions, which can interfere with the tests!

NB2 this test cannot be used for chlorine-containing organic compounds e.g. chloro-alkanes. This is because the C-Cl bond is covalent, not ionic, so no chloride ions are present.