FLAME TEST AND ATOMIC SPECTRA LAB


 

Introduction: What do fireworks, lasers, and neon signs have in common? In each case, we see the brilliant colors because the atoms and molecules are emitting energy in the form of visible light. The chemistry of an element strongly depends on the arrangement of the electrons. Electrons in an atom are normally found in the lowest energy level called the ground state. However, they can be "excited" to a higher energy level if given the right amount of energy, usually in the form of heat or electricity. Once the electron is excited to a higher energy level, it quickly loses the energy and "relaxes" back to a more stable, lower energy level. If the energy released is the same amount as the energy that makes up visible light, the element produces a color.


 

In Part I, compounds containing a metallic element (called salts) are heated using a Bunsen burner, producing different colored flame. By comparing the color given off by an unknown with the known metal salts, the identity of the metal salt can be determined. In Part II, electricity is passed through the gas discharge tube resulting in different colors. When the light is passed through a diffraction grating, it is broken into components, producing a line spectrum of the element. A comparison of the line spectrum with those of known gases will allow one to determine the identity of the gas.


 

Objective:

• Observe and record the flame colors of various metal salts

• Use the color of a flame to identity an unknown salt

• Observe and record the line spectrum of hydrogen, helium, and neon gas

• Determine the identity of an unknown gas

 

Safety: Be careful with using the Bunsen burners.


 

PART I. FLAME TEST

Procedure:

  1. Obtain a cobalt blue glass and 9 wooden splints (see table) that have been soaking in the salt solutions. Be sure to label each wooden splint with the names of the salts so they are not mixed up.

  2. Light the Bunsen burner and open the air vent to obtain a flame (nonluminous flame) with two blue cones. Be sure not to get a yellow flame.

  3. Carefully place the end of the wooden splint that was soaked in the salt solution at the top of the inner blue cone. Record the color and intensity (bright/faint) of the flame in the data table. The color given off by the salt is the initial color observed, not the yellow-orange color produced by the burning wood.

  4. Repeat with the other 8 salts. Be sure to record the colors as precisely as possible.

  5. For the sodium potassium mixture, observe the colors as before and then again by looking through the cobalt glass. The cobalt glass cuts out any yellow-orange color.

  6. If more observations are needed, dip the clean end of the wooden splint in the solutions for a few minutes and repeat. Otherwise, discard the wooden splints in the trashcan at the end of the experiment.

Data table 1

Metal found in the salt

Flame Color and Intensity (high/low)
Lithium
 
Barium
 
Strontium
 
Calcium
 
Copper
 
Sodium
 
Potassium
 
Sodium and Potassium

Without cobalt blue glass:

With cobalt blue glass:
Unknown
 


 


 


 


 


 

PART II. ATOMIC SPECTRA

Procedure:

  1. Go to one of the lab stations where a hydrogen gas discharge tube is set up. Turn on the electricity (for no more than 30 seconds) and observe the color given off by the gas. Record your observation in the data table. If more time for observation is needed, turn off the tube for ~15 sec and then turn on for another 30 sec. Do not leave the tube continuously turned on; otherwise, it will burn out the tube.

  2. Look through a diffraction grating and observe and record the colored lines (spectrum) produced by the gas. Record only one set of colors. Be sure to draw the color lines as seen.

  3. Go to the rest of the lab stations and repeat for each unknown.

  4. Turn off the gas discharge tube when not in use and return the diffraction grating.


 

Data Table 2


 

Color of the discharge tube

Sketch of the line spectrum

Hydrogen


 


 

Helium


 


 

Neon


 


 

Unknown


 


 


 

Analysis/Conclusion


 

Part I. Flame test

  1. List the colors observed in this lab from the lowest energy to the highest energy.

  2. List the colors observed in this lab from the lowest frequency to the highest frequency.

  3. List the colors observed in this lab from the longest wavelength to the shortest wavelength.

  4. What is the relationship between energy, frequency, and wavelength?

  5. Based on the results of your experiment, what metal is found in the unknown? Explain.

  6. What is the purpose of the cobalt blue glass? Why is only the purple color of the potassium seen through the cobalt glass?

  7. Do you think we can use the flame test to determine the identity of unknowns in a mixture? Why or why not?

  8. How are electrons “excited” in this part of the experiment? What does it mean the electrons are “excited”?

  9. Explain why we see distinct lines when metal salts are burned.

  10. How do fireworks give off color? (see p. 73 and 102).

  11. Give the short hand electron configuration of the following elements:

a) lithium b) barium c) copper d) strontium e) calcium f) sodium

g) potassium


 

Part II. Atomic Spectra

  1. Each line in the emission spectrum of the hydrogen corresponds to an electromagnetic radiation with a specific wavelength. Match the 4 observed colors with the following wavelengths: 410 nm, 434 nm, 486 nm, and 656 nm.

  2. How are electrons “excited” in this part of the lab? What happens when the electrons relax?

  3. What gas do you think is found in the unknown? Explain your reasoning.

  4. Each element has its own unique line emission spectrum, just like fingerprints. Explain how this technique can be used to determine the elemental composition of stars.


 

CONCLUSION

Summarize your understanding of how fireworks, neon signs, lasers, and street lamps emit light. Include your understanding of the internal structure of atoms and the relationship between electrons and light.