[CHEM] Chapter 1.3 - Qualitative Analysis

1 DARRELL ER (COPYRIGHTED) © TOPIC 1.3: QUALITATIVE ANALYSIS DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Heavy memorising • 3 key concepts • Always tested • Typical format is a flowchart question which involves backtracking to identify ions • Medium overall weightage • Constitute to 3.5% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 QUALITATIVE ANALYSIS TEST FOR CATIONS TEST FOR ANIONS TEST FOR GASES KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 TEST FOR CATIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) © Cation Reaction with NaOH (aq) (strong alkaline) Reaction with NH3 (aq) (weak alkaline) Copper(II) Cu2+ Forming blue precipitate of Cu(OH)2 Precipitate is insoluble in excess NaOH Forming blue precipitate of Cu(OH)2 Precipitate dissolves in excess NH3 to give dark blue complex ion Iron(II) Fe2+ Forming green precipitate of Fe(OH)2 Precipitate is insoluble in excess NaOH Forming green precipitate of Fe(OH)2 Precipitate is insoluble in excess NH3 Iron(III) Fe3+ Forming reddish-brown precipitate of Fe(OH)3 Precipitate is insoluble in excess NaOH Forming reddish-brown precipitate of Fe(OH)3 Precipitate is insoluble in excess NH3 Calcium Ca2+ Forming white precipitate of Ca(OH)2 Precipitate is insoluble in excess NaOH No observable reaction Aluminium Al3+ Forming white precipitate of Al(OH)3 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Al(OH)3 Precipitate is insoluble in excess NH3 Lead(II) Pb2+ Forming white precipitate of Pb(OH)2 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Pb(OH)2 Precipitate is insoluble in excess NH3 Zinc Zn2+ Forming white precipitate of Zn(OH)2 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Zn(OH)2 Precipitate dissolves in excess NH3 to give colourless solution Ammonium NH4+ No precipitate formed Warming the solution produces pungent ammonia gas which turns moist red litmus blue No observable reaction PPT formed is amphoteric oxide and is able to react with alkaline to produce soluble salt. Gives exactly same result for both reaction. Add potassium iodide, lead (II) ions will give a yellow ppt (insoluble lead iodide). BLUE GREEN REDDISH BROWN WHITE

5 TEST FOR ANIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) © Anions Test Observation Carbonate CO32- Add dilute H2SO4 and bubble gas produced through limewater. (Any other suitable dilute acid can be used.) 2H+ (aq) + CO32- (aq)  CO2 (g) + H2O (l) Bubbles of CO2 gas produced which gives a white precipitate of CaCO3 in limewater. Sulfate SO42- Add dilute HNO3, followed by adding Ba(NO3)2 solution. Ba2+ (aq) + SO42- (aq)  BaSO4 (s) A white precipitate of BaSO4 is formed. Chloride Cl- Add dilute HNO3, followed by adding AgNO3 solution. Ag+ (aq) + Cl-(aq)  AgCl