Testing for unknown Cations and Anions in a salt


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  • Unknown Chemical
    • Flame Tests
      • Use a platinum or nichrome wire dipped into HCl (to cleanse the wire of any impurities) and then into the salt you want to test. Salt will stick on the end
        • Bright Red
          • Lithium (Li+)
        • Golden Yellow
          • Sodium (Na+)
        • Brick Red
          • Calcium (Ca+)
        • Lilac
          • Potassium (K+)
        • Blue-green
          • Copper (II) (Cu+)
    • Sulphate Test
      • Add dilute HCl acid (you add acid to the solution the react with and remove other anions which might also create white precipitates when barium chloride solution is added)
        • Then, add barium chloride (BaCl)
          • White precipitate
            • Barium sulphate
              • Ba2+ + SO42- --> BaSO4
        • For example, if you didn't add acid, you would also obtain a white precipitate if there were carbonate ions present
          • This is because barium carbonate is also white and insoluble (refer to solubility hand)
            • The acid reacts with the carbonate ions and removes them. IMPORTANT: Add acid before barium chloride solution
              • You must NEVER acidify the solution with sulphuric acid because it contains sulphur ions. If you add those, a precipitate of barium sulphate is bound to form due to the sulphur ions
    • Halide Tests
      • Add dilute nitric acid (HNO3) and silver nitrate (AgNO3)
        • White precipitate
          • Chloride (Cl-)
            • Cl- + Ag+ --> AgCl
        • Cream Precipitate
          • Bromide (Br-)
            • Br- + Ag+ --> AgBr
        • Pale Yellow Precipitate
          • Iodide (I-)
            • I- + Ag+ --> Ag1
        • The acid is added to react with the and remove other anions which might also produce precipitates with silver nitrate solution for example carbonate and hydroxide ions
          • You must NEVER use hydrochloric acid (or hydrobromic acid or hydriodic acid) as you will be adding halide ions, resulting in a definite production of precipitate
    • Ammonium Test
      • Sodium hydroxide solution reacts with ammonium salts (either solid or liquid) to produce ammonia gas
        • To test for the gas produced efficiently, hold in the palm of your hand for at least 30 sec for it to warm up enough to be able to smell the gas
          • Test the gas coming out using damp red litmus paper. Ammonia is an alkaline and turns litmus paper blue
      • NH4+ + OH- ---->NH3 + H2O
        • This is a reversible, equilibrium reaction as well
        • No precipitate is produced, so the test works on a solid or solution
    • Carbonate Test
      • Add dilute HCl to the sample of salt and look for fizzing. This indicates a gas being given off
        • Test the gas by bubbling it through limewater to show it is CO2
      • ZnCO3 + 2HNO3 ---> Zn(NO3)2 + CO2 + H20
        • Tested using zinc carbonate and dilute nitric acid
          • IONIC EQUATION: CO32- + 2H+ ---> CO2+ H20
    • Add Sodium Hydroxide solution (NaOH)
      • Dissolve the salt in distilled (pure) water, and add about the same volume of dilute NaOH solution
        • Any precipitate formed is known as gelatinous precipitate
      • Light Blue
        • Copper (II) (Cu2+)
          • Full equation: CuSO4 + 2NaOH ---> Cu(OH)2 + Na2SO4
          • Ionic Equation: Cu2+ + 2OH- ---> Cu(OH)2
      • Sludgy Green
        • Iron (II) (Fe2+)
          • Full Equation: FeSO4 + 2NaOH ---> Fe(OH)2 + Na2SO4
            • The green precipitate darkens on standing and turns orange brown at the top of the tube. This is due to the iron (II) hydroxide being oxidized to the iron (III) hydroxide by the air
          • Ionic Equation: Fe2+ + 2OH- ---> Fe(OH)2
      • Reddish-Brown
        • Iron (III) (Fe3+)
          • Full Equation: FeCl3 + 3NaOH ---> Fe(OH)3 + 3NaCl
          • Ionic Equation: Fe3+ + 3OH- ---> Fe(OH)3

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