3.1.4 Qualitative analysis
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how do you test for halides
• add solution containing aqueous silver ions (silver nitrate solution) • if halide present, an insoluble silver halide precipitate forms (they have diff colours)
what is the general equation for the reaction of halides when testing for them
Ag+(aq) + X-(aq) —> AgX(s)
when doing halide tests, what colour precipitates are formed for Cl2, Br2 and I2
• Cl2 = white • Br2 = cream • I2 = yellow
how can you tell the different precipitate colours apart in halide tests
• add aqueous ammonia to test for solubility of precipitate • chloride + bromide are soluble in dilute NH3 • Iodide is insoluble in conc NH3
how do you test for carbonate ions
• add nitric acid • effervescence observed as CO2 produced • to prove gas is CO2, bubble it through limewater (Ca(OH)2) which will turn cloudy
give the equation for CO2 with limewater
CO2(g) + Ca(OH)2(aq) —> CaCO3(s) + H2O(l)
how do you tests for sulfate ions
• add solution containing aqueous barium ions (barium nitrate solution) • if sulfate ions present, insoluble barium sulfate forms as a white precipitate
give the ionic equation for the sulfate test
SO4 2- (aq) + Ba2+ (aq) —> BaSO4 (s)
if you need to analyse an unknown organic compound, you need to carry out the tests for the anions in the correct order which is:
• Carbonate, CO3 2- • Sulfate, SO4 2- • Halides, Cl-, Br-, I-
how do you test for ammonium ions
• when heated together, ammonium ions + aqueous hydroxide ions react to form ammonia gas • add aqueous hydroxide • warm gently - ammonia gas will be released (alkali) • damp red litmus paper will turn blue in presence of ammonia gas