Identification of Cations | Chemistry for Grade 11 (IGCSE) PDF Download

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Identification of Cations

• Metal cations dissolved in water can be distinguished by the hue of the solid they produce when sodium hydroxide or ammonia is introduced.
• Typically, with a small quantity of NaOH, the metal hydroxide solidifies and separates from the solution.
• In cases of excessive NaOH, some of the solids formed may dissolve again.
• Initially, a few drops of NaOH are introduced, and any color alterations or solid formations are recorded.
• Subsequently, excess NaOH is added, and the reaction is observed once more.
• This procedure is repeated for the test using ammonia solution.

Analysing results

• The results for each cation in the syllabus are detailed below.
• If a precipitate forms with NaOH or aqueous ammonia, it indicates the hydroxide's insolubility in water.
• Example with Zinc chloride:
  • ZnCl2(aq) + 2NaOH(aq) → Zn(OH)2(s) + 2NaCl(aq)
• Calcium ions (Ca²⁺) can be differentiated from Zinc (Zn²⁺) and Aluminum (Al³⁺) ions because calcium hydroxide precipitate remains insoluble in excess NaOH, unlike zinc and aluminum hydroxide.
• Zinc ions (Zn²⁺) can be distinguished from Aluminum ions (Al³⁺) as Zn(OH)₂ dissolves in excess aqueous ammonia while Al(OH)₃ does not.
• Most transition metals form hydroxides with distinct colors.
  
• The flame test is employed to recognize metal cations based on the hue of the flame they generate.
• Different metal ions produce distinct colors.
• Immerse a loop made of a non-reactive metal wire like nichrome or platinum in concentrated acid, then expose it to the blue flame of a Bunsen burner until no color change occurs.
• This step is crucial to ensure the presence of only one type of ion, as multiple ions can blend colors, leading to inaccurate identification.
• Cleansing the wire loop in this manner helps prevent contamination.
• Apply a small amount of the compound onto a non-reactive metal wire loop like nichrome or platinum.
• Submerge the loop into the solid sample or solution, then position it at the edge of the blue Bunsen flame.
• Be cautious not to overheat the wire to the point where it emits a red glow, which could cause confusion with flame coloration.
• Diagram showing the technique for carrying out a flame test:
  
• The colour of the flame is observed and used to identify the metal ion present: