5.3.2 Qualitative Analysis

Question 1

what cations can you use qualitative analysis to test for

Answer 1

• ammonium ions NH4+
• transition metals

Question 2

how do you test for the presence of NH4+ ions

Answer 2

• heat gently with NaOH(aq)
• ammonia will evolve
• will turn damp red litmus paper blue
• NH4+ (aq) + OH- (aq) → NH3(g) + H2O(l)

Question 3

how do you test for transition metals

Answer 3

• react with aqueous NaOH(aq)
• will all produce different coloured precipitates

Question 4

how do you test for Cu2+ ions

Answer 4

Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)

• blue solution to blue precipitate
• insoluble in excess NaOH

Question 5

how do you test for Fe2+ ions

Answer 5

Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) → Fe(OH)3(s)

• pale green solution
• turns into green precipitate
• insoluble in excess NaOH
• but turns brown standing on air
• forms orange-brown precipitate

Question 6

how do you test for Fe3+ ions

Answer 6

Fe3+(aq) + 3OH- → Fe(OH)3(s)

• pale yellow solution
• to orange-brown precipitate
• insoluble in excess NaOH

Question 7

how do you test for Mn2+ ions

Answer 7

Mn2+(aq) + 2OH- → Mn(OH)2(s)

• pale pink solution
• to light brown precipitate
• insoluble in excess NaOH
• but darkens on standing in air

Question 8

how do you test for Cr3+ ions

Answer 8

Cr3+ (aq) + 3OH- → Cr(OH)3(s) + 3OH-(aq) → [Cr(OH)6]3+ (aq)

• violet solution
• to grey-green precipitate
• soluble in excess NaOH
• forms a dark green solution

Question 9

how do you test for carbonate CO3 2- ions

Answer 9

• add dilute nitric acid HNO3(aq)
• effervescence as CO2 is evolved
• CO3 2- + 2H+ → CO2(g) + H2O

Question 10

how do you test for the sulfate SO4 2- ion

Answer 10

• add Ba2+ ions (aq)
• forms a white precipitate
• as BaSO4 forms
• Ba 2+ + SO4 2- → BaSO4 (s)

Question 11

how do you test for halide ions

Answer 11

• add Ag+ ions (AgNO3)
• forms silver halides, which are precipitates