Test for ammonium ions NH4+(aq) in solution
Add NaOH(aq)
Gently heat
Test fumes with damp red litmus paper
If present paper turns red to blue
NH4+(aq) + OH-(aq) → NH3(aq) + H2O(aq)
Heat to (g)
Then ammonia reacts with water on damp litmus to reform ammonium and hydroxide ions, which turns litmus blue.
NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)
Test for halide ions
Add nitric acid, HNO3(aq) to remove impurities such as CO32- which would also form a white precipitate. H+ react to make CO2 and H2O.
Add silver nitrate solution, AgNO3(aq)
If present insoluble silver halide precipitates are formed, AgX (s)
AgCl - White, soluble in dilute NH3(aq).
AgBr - Cream, insoluble in dilute NH3(aq), soluble in concentrated.
AgI - Yellow, insoluble in dilute and concentrated NH3(aq).
Test for CO32-(aq)
Add dilute acid e.g HCl(aq)
If present efference viewed and CO2 produced
CO32-(aq) + 2H+ → CO2(g) + H2O(l)
Bubbled gas through limewater which turns cloudy
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Test for SO42-
Add HCl, to remove impurities such as CO32- which would also form a white precipitate. H+ react to make CO2 and H2O.
Do not add H2SO4 as this would add SO42- and give a positive result
Add BaCl2 If present white precipitate forms, BaSO4(s)
Testing for unsaturation
Bromine water
If it contains a C=C then it will open up and Br2 will be added across the double bond by electrophilic addition, making a colourless solution.
Orange/Brown to colourless
Test for alcohol oxidation products
Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent.
When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu2+ ions are reduced to Cu+ ions.
Colour change blue solution to red Cu2O precipitate.
Ketones cannot be oxidised and therefore give a negative test.
Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution.
When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag+ ions are reduced to Ag atoms, forming a silver mirror.