RGS 2025 Y4 QA Notes 2

1 QA Notes (2) – Tests for Cations, Anions & Gases 2.1 Tests for Cations Cations can be distinguished based on their reactions with aqueous sodium hydroxide and aqueous ammonia. To test for cations, aqueous sodium hydroxide or aqueous ammonia is added to the unknown solution dropwise until it is in excess. The colour of the precipitate formed and its solubility in excess alkali can be used to identify the cation present in the solution. Cations in syllabus: Zn2+, Al3+, Pb2+, Ca2+, Cu2+, Fe2+, Fe3+, NH4+ 2.1.1 Sodium hydroxide  An aqueous solution of sodium hydroxide consists of sodium ions, hydroxide ions and water molecules. NaOH (aq)  Na+ (aq) + OH- (aq)  Upon addition of aqueous sodium hydroxide to a solution of an unknown cation, the hydroxide ions in sodium hydroxide solution may combine with the cations to form metal hydroxides.  The formation of insoluble metal hydroxides is a precipitation reaction.  Adding NaOH (aq) to salt solutions containing transition metal ions (Cu2+, Fe2+ and Fe3+) usually form coloured precipitates while adding NaOH (aq) to salt solutions containing other metal ions such as Ca2+, Zn2+, Pb2+, Al3+ (except Group I and ammonium) form white precipitates.  Metal hydroxides as Al(OH)3, Zn(OH) 2, and Pb(OH) 2 are amphoteric in nature i.e. they can react with both acids and bases. In this case, they react with excess sodium hydroxide to form complex salts that are soluble to give colourless solutions. RAFFLES GIRLS’ SCHOOL (SECONDARY) RAFFLES PROGRAMME YEAR 4 CHEMISTRY Name : _________________________ ( ) Class : _______ Date : _____________ Think: Which hydroxides are soluble and which are insoluble? NaOH, KOH (Group I metal hydroxides) are soluble and no precipitate will form. All other metal ions react with hydroxide ions in aqueous sodium hydroxide to form insoluble hydroxides. Established in 1879

2 Table 2.1: Test and observations for identifying cations using sodium hydroxide solution  denotes advanced content for enrichment Cation Sodium hydroxide solution, NaOH (aq) Test On adding a few drops On adding excess Zn2+ Observations White precipitate is formed. White precipitate dissolves in excess NaOH (aq) to form a colourless solution. Theory On adding a few drops of NaOH (aq): Zn2+ (aq) + 2OH- (aq)  Zn(OH)2 (s) On adding excess of NaOH (aq):  [Ionic equation] Zn(OH)2 (s) + 2OH- (aq) ⇌ Zn(OH)42- (aq)  [Chemical equation] Zn(OH)2 (s) + 2NaOH(aq) ⇌ Na2Zn(OH)4 (aq) Al3+ Observations White precipitate is formed. White precipitate dissolves in excess NaOH (aq) to form a colourless solution. Theory On adding a few drops of NaOH (aq): Al3+ (aq) + 3OH- (aq)  Al(OH)3 (s) On adding excess of NaOH (aq):  [Ionic equation] Al(OH)3 (s) + OH- (aq) ⇌ Al(OH)4- (aq)  [Chemical equation] Al(OH)3 (s) + NaOH(aq) ⇌ NaAl(OH)4 (aq) Pb2+ Observations White precipitate is formed. White precipitate