Flame tests identify alkali metal ions in compounds. Transition metals form different coloured precipitates with sodium hydroxide solution. Gases are identified by simple tests. 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 NO3-.
Heating a compound can cause a liquid to condense on the inside of the test tube. This is probably water, indicating that the compound is a hydrate. If a gas is given off, note the color and odor of the gas. The nitrate, carbonate, and sulfite ions may decompose, as illustrated by the reactions: 2 Pb(NO3)2(s) + heat → 2 PbO(s) + O2(g) + 4 NO2(g, brown) Some bromides and iodides decompose to give Br2(g, orange-brown) and I2(g, purple). 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:
Place one small spatula of the compound in 1 mL of water. If the compound is soluble this amount will dissolve after considerable stirring. If the compound is moderately soluble, some of this amount will dissolve. If the compound is insoluble, even a very small amount will not dissolve. General solubility rules:
Add nitric acid to the compound and observe any reaction that occurs. If the compound dissolved in water, it should dissolve in nitric acid. If it did not dissolve in water, but appears to be dissolving in nitric acid, it is undergoing a chemical reaction. In general, compounds that contain anions that are the conjugate bases of weak acids will react (unless the compounds are very insoluble). For example: CaCO3(s) + 2 H+(aq) → Ca2+(aq) + H2O(l) + CO2(g, colorless, odorless) The remaining tests must be perfomed on a solution of the compound. 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:
Figure 1. Al(NO3)3 in solution.
Figure 2. Al(OH)3 precipitates with the addition of NaOH.
Figure 3. When excess NaOH is added, the precipitate redissolves as the Al(OH)4- complex ion is formed. Add H2SO4 dropwise until solution is acidic and observe any reaction. A precipitate will form with any cation that forms an insoluble sulfate (refer to the solubility rules). For example: Ba2+ + SO42- → BaSO4(s) Add HNO3 dropwise until solution is acidic (unless of course it was dissolved in nitric acid), then add a few drops of AgNO3 and observe any reaction. A precipitate will form with certain cations that form insoluble silver compounds, but because of the acidic environment, some insoluble silver salts (e.g. salts containing CO32-, S2-, and PO43- ions) are "destroyed." Cl-, Br-, and I- form insoluble compounds, while SO42- forms a moderately insoluble compound. Ag+ + Cl- → AgCl(s) Add HNO3 dropwise until solution is acidic, boil the solution for two minutes, then test with litmus paper. Continue adding and boiling until solution remains acidic after boiling. Cool the solution and add a few drops of Ba(NO3)2 and observe any reaction. A precipitate will form with anions that form an insoluble barium compound (except the ones destroyed by acid as in the above test). Sometimes the above tests can not definitively confirm the presence of a specific ion. In these cases, it is necessary to do specific tests for a particular ion. Example Unknown Salts Sample 1 had the following characteristics:
Conclusion: Sample 1 is Pb(NO3)2 Sample 2 had the following characteristics:
Conclusion: Sample 2 is BaCO3 Sample 3 had the following characteristics:
Conclusion: Sample 3 is FeCl3 Specific Tests
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