SCH4C                     Lab #3 - Flame Colours and Electronic Energy Levels
Introduction:    When an atom, a molecule, or an ion is heated to a high temperature in a burner flame, one or more) of its electrons gains energy and is raised to a higher energy level.  Later, the electron returns to a lower energy level and gives off its excess energy in the form of light of a characteristic colour.  Since there are many atoms in any sample of a substance, all possible jumps from energy level to another can occur at the same time, each giving off its characteristic colour.  The colour we see is a mixture of all these colours, but the predominant colour is that which corresponds to the most frequently occurring transition.
Purpose:  To observe the colours produced when solutions of metal ions are heated to high temperatures, to explain the results in terms of the energy levels of the metal ions, and to use their characteristic colours to identify an unknown chemical sample.
Apparatus:   nichrome test wire,  cobalt glass squares,  Bunsen burner
Materials:
6 mol/L  hydrochloric acid, HCl             0.5 mol/L strontium nitrate, Sr(NO3)2
0.5 mol/L sodium nitrate, NaNO3          0.5 mol/L lithium nitrate, LiNO3
0.5 mol/L sodium chloride, NaCl           0.5 mol/L potassium nitrate, KNO3
0.5 mol/L barium nitrate, Na(NO3)2      0.5 mol/L copper(II) nitrate, Cu(NO3)2
0.5 mol/L calcium nitrate, Ca(NO3)2     solid sodium chloride, NaCl
Procedure:
1.  Light the burner and adjust it so the flame is almost colourless.  Hold a nichrome test wire momentarily in the flame.   Do not heat the holder that the wire is inserted into.
2. Wet the nichrome wire in a half-filled test tube of hydrochloric acid, and then place the wire in the colorless flame of your burner.  Repeat the process until the wire no longer imparts a colour to the flame.  (The wire will, or course, become red hot, but the flame should remain colourless).
3. Pour about 3 mL of sodium nitrate solution into a test tube. Dip the tip of the nichrome wire into the solution, and then hold the wire in the flame.
4. Using the procedure outlined in Step 2, clean the wire.  Pour about 3 mL of sodium chloride into another test tube.  Dip the clean wire into the sodium chloride solution , and hold the wire in the flame.
5. Clean the wire again.  Dip the hot wire into a little solid sodium chloride contained in a test tube, and hold the wire in the flame.
6. Using the procedures from Steps 2 and 3, test 3 mL  samples of nitrates of barium, calcium, strontium, lithium, potassium, and copper.  Clean the wire carefully after each test.
7. Again clean the wire, and retest the 3 mL sample of sodium nitrate solution, but this time observe the result through cobalt glass.  Then test the potassium nitrate solution in the same manner.
8. Now mix the solution of sodium nitrate and potassium nitrate.  Clean the nichrome wire, dip it into the mixture, and hold the wire in the flame.  Repeat, this time observing the result through the cobalt glass. 
9. Obtain an unknown solution from your teacher, and test it in the flame.  Observe it also using the cobalt glass.

Data Table
Substance     Flame Colour 
Sodium nitrate  
Sodium chloride solution  
Solid sodium chloride  
Barium nitrate solution  
Calcium nitrate solution  
Strontium nitrate solution  
Lithium nitrate solution  
Potassium nitrate solution  
Copper(II) nitrate solution  
Sodium nitrate solution with cobalt glass  
Sodium nitrate and potassium nitrate mixture with cobalt glass  
Unknown solution #1  
Unknown solution #2  

Concluding Questions
1. What is the purpose of the procedure in Step 2?
2. Is there any difference in the results you obtained in parts 3,4 and 5?
3. Is there any difference in the results for strontium and lithium?  If so, what is the difference?
4. Is there any difference in the results for barium and copper?  If so, what is the difference?
5. Is there any difference between the results obtained when the colour produced by the sodium nitrate is viewed directly and when it is viewed through cobalt glass?  If so, what is the difference?
6. Is there any difference between the results obtained when the colour produced by the mixture of sodium nitrate and potassium nitrate is viewed directly and when it is viewed through cobalt glass?  If so, what is the difference?
7. What is the purpose of the cobalt glass?
8.  What are the identities of your unknowns?
9. The energies of light quanta increase gradually as the colour of light changes in this sequence: red -> orange -> green -> blue -> indigo -> violet.  That is, a quantum of violet light contains much more energy that does a quantum of red light.  Which metal has electronic transitions involving the largest energy change?
10. Which metal has electronic transitions involving the smallest energy change?
11. What is the order of the metals using in this experiment, when they are arranged according to the increasing energies of their electronic transitions?
12. What do you think is the source of the colours observed in exploding fireworks and during the burning of many kinds of artificial logs?