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 dissolves in excess NaOH (aq) to form a colourless solution. Theory On adding a few drops of NaOH (aq) : Pb2+ (aq) + 2OH- (aq)  Pb(OH)2 (s) On adding excess of NaOH (aq):  [Ionic equation] Pb(OH)2 (s) + 2OH- (aq) ⇌ Pb(OH)42- (aq)  [Chemical equation] Pb(OH)2 (s) + 2NaOH(aq) ⇌ Na2Pb(OH)4 (aq) white ppt colourless solution colourless solution white ppt white ppt white ppt white ppt white ppt colourless solution colourless solution white ppt white ppt colourless solution white ppt colourless solution

3 Ca2+ Observations White precipitate is formed. White precipitate insoluble in excess NaOH (aq). Theory On adding a few drops of NaOH (aq): Ca2+ (aq) + 2OH- (aq)  Ca(OH)2 (s) No reaction takes place when excess of NaOH (aq) is added. Cu2+ Observations Light blue precipitate is formed. Light blue precipitate is insoluble in excess NaOH (aq). Theory On adding a few drops of NaOH (aq): Cu2+ (aq) + 2OH- (aq)  Cu(OH)2 (s) No reaction takes place when excess of NaOH (aq) is added. Fe2+ Observations Green precipitate is formed. Green precipitate is insoluble in excess NaOH (aq). Theory On adding a few drops of NaOH (aq): Fe2+ (aq) + 2OH- (aq)  Fe(OH)2 (s) No reaction takes place when excess of NaOH (aq) is added. Fe3+ Observations Reddish-brown precipitate is formed. Reddish-brown precipitate is insoluble in excess NaOH (aq). Theory On adding a few drops of NaOH (aq): Fe3+ (aq) + 3OH- (aq)  Fe(OH)3 (s) No reaction takes place when excess of NaOH (aq) is added. NH4+ Observations No precipitate is formed. On h eating, colourless and pungent gas produced turns moist red litmus paper blue. Gas is NH3 gas. No change is observed. 2.1.2 Aqueous ammonia Like sodium hydroxide, aqueous ammonia also precipitates insoluble hydroxides from salt solutions. As aqueous ammonia is a weak alkali, it dissociates partially in water as follows: NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq) white ppt light blue ppt green ppt reddish brown ppt

4 Table 2.2: Test and observations for identifying cations using aqueous ammonia Cation Aqueous ammonia, NH3 (aq) Test On adding a few drops On adding excess Zn2+ Observations White precipitate is formed. White precipitate dissolves in excess NH3 (aq) to form a colourless solution. Theory On adding a few drops of NH3 (aq) : Zn2+ (aq) + 2OH- (aq)  Zn(OH)2 (s) On adding excess of NH3 (aq):  Zn(OH)2 (s) + 4NH3 (aq) ⇌ Zn(NH3)42+ (aq) + 2OH- (aq) Al3+ Observations White precipitate is formed. White precipitate insoluble in excess NH3 (aq). Theory On adding a few drops of NH3 (aq) : Al3+ (aq) + 3OH- (aq)  Al(OH)3 (s) No reaction takes place when excess of NH3 (aq) is added. Pb2+ Observations White precipitate is formed. White precipitate insoluble in excess NH3 (aq). Theory On adding a few drops of NH3 (aq) : Pb2+ (aq) + 2OH- (aq)  Pb(OH)2 (s) No reaction takes place when excess of NH3 (aq) is added. Ca2+ Observations No precipitate is formed. No precipitate is formed. Theory Recall that Ca(OH)2 is sparingly soluble. Hence, since NH3 (aq) is a weak base, the concentration of OH- ions contributed by NH3 (aq) to the solution will be very low which will result in a very low concentration of Ca(OH)2 to be formed. The small quantity of Ca(OH)2 formed allows it remain soluble. Cu2+ Observations Light blue precipitate is formed. Light blue precipitate dissolves in excess NH3 (aq) to form a deep blue solution. Theory On adding a few drops of NH3 (aq): Cu2+ (aq) + 2OH- (aq)  Cu(OH)2 (s) On adding excess of NH3 (aq):  Cu(OH)2 (s) + 4NH3 (aq) ⇌ Cu(NH3)42+ (aq) + 2OH- (aq) white ppt white ppt white ppt white ppt colourless solution blue ppt blue ppt deep blue solution

5 Fe2+ Observations Green precipitate is formed. Green precipitate is insoluble in excess NH3 (aq). Theory On adding a few drops of NH3 (aq): Fe2+ (aq) + 2OH- (aq)  Fe(OH)2 (s) No reaction takes place when excess of NH3 (aq) is added. Fe3+ Observations Reddish-brown precipitate is formed. Reddish-brown precipi tate is insoluble in excess NH3 (aq). Theory On adding a few drops of NH3 (aq): Fe3+ (aq) + 3OH- (aq)  Fe(OH)3 (s) No reaction takes place when excess of NH3 (aq) is added. Think: What do you observe if an acid is added to the insoluble hydroxide formed? Explain. Observations: White (coloured) precipitate is observed to dissolve and form a colourless (coloured) solution. Theory: Acid-base neutralisation takes place to form a salt and water. If dilute nitric acid is added, it reacts with the insoluble metal hydroxide to form a soluble salt and water. For example, Chemical Equation: Pb(OH)2 (s) + 2HNO3 (aq)  Pb(NO3)2 (aq)+ 2H2O (l) Base Acid Salt Water Ionic Equation: Pb(OH)2(s) + 2H+(aq)  Pb2+(aq) + 2 H2O (l) green ppt reddish-brown ppt white ppt colourless solution

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