Tests for ions

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  • Tests for ions
    • Negative ions
      • Halides
        • Dissolve the halide in water if not already aqueous, add some dilute nitric acid and aqueous silver nitrate 
          • Coloured precipitate will form, if hard to distinguish add dilute NH3 then concentrated NH3, note the solubility 
        • Cl-
          • With AgNO3(aq) Cl- = white precipitate 
            • Solubility of precipitate = soluble in dilute NH3 
            • Ag+(aq) + Cl- (aq) --> AgCl(s)
        • Br-
          • With AgNO3(aq) = cream precipitate 
            • Ag+(aq) + Br- (aq) --> AgBr(s)
            • Solubility of precipitate = soluble in concentrated NH3(aq) only
        • I-
          • With AgNO3(aq) = yellow precipitate
            • Ag+(aq) + I- (aq) --> AgI(s)
            • Solubility of precipitate = insoluble in both dilute and concentrated NH3(aq)
      • Sulfate ions, (SO4)2-
        • Half fill a test tube with the sample, add dilute HCl(aq) followed by BaCl2(aq)
          • White precipitate if (SO4)2- is present 
            • Ba2+(aq) + (SO4)2-(aq) - > BaSO4(s) 
      • Carbonate ions, (CO3)2-
        • Half fill a test tube with sample, add dilute HCl(aq) 
          • collect bubbles and pass it through limewater 
            • If gas turns limewater cloudy, (CO3)2- is present
          • (CO3)2-(s) + 2H+(aq) -- > H2O(l) + CO2(g) 
      • Hydroxides, OH-
        • Will turn damp red litmus paper blue
      • Nitrate(V) ions, NO3-(aq)
        • Warm solution with NaOH(aq) and either aluminium foil or Devardas's alloy
          • Aluminium reduces the NO3-(aq) to produce ammonia gas, NH3(g)
            • Ammonia turns damp red litmus paper blue
          • 3NO3-(aq) + 8Al(s) + 5OH-(aq) +18H2O(l) --> 3NH3(g) + 8[Al(OH)4]-(aq)
    • Positive ions
      • Ammonium, NH4+(aq)
        • Fill half of a test tube, add NaOH(aq) and warm gently 
          • If ammonia gas, NH3(g), given out then ammonium ions are present
            • NH3(g) turns damp red litmus paper blue
          • NH4+(aq) + OH-(aq) -- > NH3(g) + H2O(l)
      • Metal ions
        • Flame tests
          • Dip dichrome wire loop in conc HCl(aq). Dip wire loop in sample. Hold loop in clear blue part of flame and observe colour change.
          • Li+ = crimson
          • Na+ = yellow
          • K+ = lilac
          • Ca2+ = Brick red
          • Ba2+ = green
          • Cu2+ = blue
        • Metal hydroxides
          • Add NaOH(aq) drop by drop. An insoluble metal hydroxide will form
          • Ag+ =brown
            • Ag+(aq) +2OH-(aq) --> Ag2O(s) +H2O(l)
          • Ca2+ = white
            • Ca2+(aq) + 2OH-(aq) --> Ca(OH)2(s)
          • Cu2+ = blue
            • Cu2+(aq) + 2OH-(aq) --> Cu(OH)2(s)
          • Pb2+ =white
            • Pb2+(aq) + 2OH-(aq) --> Pb(OH)2(s)
          • Fe2+ = green
            • Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s)
          • Fe3+ = brown
            • Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s)
          • Zn2+ = white then redissolves in excess NaOH(aq) to form colourless solution
            • Zn2+(aq) + 2OH-(aq) --> Zn(OH)2(s) then Zn(OH)2(s) +OH-(aq) -->Zn(OH)3-(aq)
          • Al3+ = white then redissolves in excess NaOH(aq) to form colourless solution
            • Al3+(aq) + 3OH-(aq) --> Al(OH)3(s) then Al(OH)3(s) +OH-(aq) -->Al(OH)4-(aq)

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