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**Qualitative Analysis** ------------------------ ### **Background** Unlike covalent compounds, which can be identified using physical properties like boiling point and refractive index, ionic compounds are more appropriately identified with their chemical properties. In the qualitative analysis p...

**Qualitative Analysis** ------------------------ ### **Background** Unlike covalent compounds, which can be identified using physical properties like boiling point and refractive index, ionic compounds are more appropriately identified with their chemical properties. In the qualitative analysis procedure, the chemical properties of an unknown substance are determined by systematically reacting the unknown with a number of different reagents. By predetermining what the particular reaction will produce if a specific ion is present, the ions that actually are in the solution can be identified. For example, if a reaction is known to produce a precipitate if ion A is present and a precipitate is formed when the reaction is run, then ion A may be present in solution (there may be, and usually are, other ions that will also precipitate with a particular reagent). If no precipitate is formed when the reaction is run, then ion A is clearly not present in the unknown solution and a different reaction will have to be run to determine what ions are present. There are two general situations in which qualitative analysis is used - in the identification of a simple salt, or the identification of multiple cations in a solution. ### **Identifying a Simple Salt** The basic testing procedure for identifying a salt is as follows. 1. **Appearance of compound\ **The compound will most likely be in solid form. Note the color and shape of the crystals. Ionic compounds formed from the representative elements tend to be white or colorless, while ions of transition elements tend to be colored. The following is a table of the colors of metal ions in solution with NO~3~^-^. -- -- -- -- 2. 3. **Flame test\ **Solutions of ions, when mixed with concentrated HCl and heated on a nickel/chromium wire in a flame, cause the flame to change to a color characteristic of the atom.Visible colors occur with the following ions: -- -- -- -- 4. - - - - - - - 5. 6. **Reaction with sodium hydroxide\ **Add NaOH dropwise to the solution, stir or shake the solution, and observe any reaction (if the compound was dissolved in nitric acid, the first several drops will neutralize the acid so be sure to check the pH with litmus paper). Look for a precipitate (refer to the solubility rules for hydroxides). If a precipitate forms, continue adding NaOH. Some metal hydroxides are amphoteric and will form a complex ion and redissolve. See Figures 1, 2, and 3 for an example of this reaction. The following ions are amphoteric: -- -- -- -- -- -- -- -- 7. 8. 9. 10. 11. 12. 13. **Reaction with ammonia\ **Add NH~3~ dropwise to the solution, stir or shake the solution, and observe any reaction. If a metal hydroxide precipitate forms, continue adding ammonia. Some metal hydroxides fom a complex ion and redissolve. See Figures 4, 5, and 6 for an example of this reaction. The following ions form ammonia complexes: -- -- -- -- -- -- -- -- 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. - - - - - - - - 1. 2. 3. 4. 25. - - - - - - 5. 6. 7. 26. - - - - - - - - - - - 8. 9. 10. 11. 12. 13. 14. 27. - 15. - - - 16. - - - 17. - - - 18. - - - - 19. - - - 20. - - - - - - - - - - - -