Chemical test
NON-METALLIC IONS (anions) can be identified based on the identification of the gases produced when the compounds are heated or reacted with acids. The colour of precipitates formed on reaction with barium and silver ions, and the solubility of the ions in ammonia (for the halide ions), are also used.
The anions we are concerned with are CO32-, SO42-, SO32-, NO3-, Br-, Cl- and I-.
A simply way to identify anions is to heat the compound and identify the gas produced. Remember, most gases are formed from non-metallic compounds. Therefore, CO2 gas is produced from the carbonate ion (CO32-), NO2 gas is produced from the nitrate ion (NO3-), and SO2 is formed from the sulphite ion (SO32-).
CO3 2- (heat) produces the gas CO2 NO3- (heat) gives off O2 and/or NO2 SO32- (heat) gives off SO2
These gases can then be identified by their characteristic tests. All acidic gases can be tested with damp blue litmus paper, which should change to red. An alkaline gas should be tested with damp red litmus, which should change to blue.
CO2 is weakly acidic and turns lime water (Ca(OH)2) milky. O2 is neutral and relights a glowing splint.
NO2 is a red-brown/yellow-brown acidic gas.
SO2 is an acidic gas which turns acidified potassium manganate (VII) solution from purple to colourless and changes acidified potassium dichromate (VI) from orange to green.
NH3 is an alkaline gas which forms dense white fumes with hydrogen chloride gas.
Cl2 will bleach damp litmus paper.
Some anions react with dilute and concentrated acids (if they are added to the solid).
CO3 reacts with dilute and concentrated acid to release CO . 2-
2 SO32- forms SO2 gas on reaction with dilute and concentrated acid.
When concentrated H2SO4 acid is added to a solid containing the halide ions (I-, Cl-, and Br-), the following gases can be formed: HI(g), which decomposes to I2, HCl(g), and HBr, which decomposes to Br2. In the case of iodide ions, the sulphuric acid will further break down to produce hydrogen sulphide gas, H2S, which smells like rotten eggs.
The halide ions, in solution, can be identified differently based on their reaction with silver nitrate solution (AgNO3) and the solubility of the precipitate formed in ammonia.
Cl- forms a white precipitate with AgNO3 solution, which is soluble in aq NH3.
Equation: Ag+ (aq) + Cl- (aq) AgCl (s) white ppt.
Br- forms a cream precipitate with AgNO3, which is partially soluble in aq NH3.
Equation: Ag+ (aq) + Br- (aq) AgBr (s) cream ppt.
I- forms a light-yellow precipitate, which is insoluble in aqueous NH3.
Equation: Ag+ (aq) + I- (aq) AgI (s) yellow ppt.
The halide ions can also be differentiated by reaction with aqueous Pb2+ ions.
Pb2+ (aq) + 2Cl- (aq) PbCl2 (s) white ppt.
Pb2+ (aq) + 2Br- (aq) PbBr2 (s) cream ppt.
Pb2+ (aq) + 2I- (aq) PbI (s) yellow ppt. 2
Nitrate test - To test for nitrate (NO3-) ions, dilute sodium hydroxide is first added, after which aluminium is added and the mixture heated. The presence of ammonia gas is tested by using damp red litmus, which changes to blue.
Finally, some anions form precipitates with a solution of Ba2+ ions and Pb2+ ions.
CO32- forms a white precipitate, which dissolves on addition of acid to form CO2.
Ba2+ (aq) + CO32- (aq) BaCO3 (s)
SO32- forms a white precipitate, which dissolves on addition of acid to give SO2.
Ba2+ (aq) + SO (aq) BaSO (s)
2- 3 3
SO42- forms a white precipitate, which is insoluble in dilute acid. Ba2+ (aq) + SO (aq) BaSO (s) white ppt.
2- 4 4