Lab 11: Flame Test Lab

Summary

In this lab students burned certain ionic compound solutions over a flame in order to observe the color of the flame produced. This lab was conducted in order to show students how flame color provided evidence of the presence of certain atoms withing the compound. In addition, two additional unknown substances were burned alongside the other given compounds; students were asked to identify the given substances using only observations of its flame.   

Photo: Combination Flame 

Pre-Lab Questions 

1. What is the difference between ground state and excited state? 

   The ground state of an atom is when all of the electrons of an atom are at the lowest energy levels available; excited state is when the electrons in an atom absorb so much energy that they can go from one energy level to a higher one. 

2. What does the word "emit" mean? 

   The word "emit" means "to radiate, to give off." For instance, in this experiment the atoms of the substance will emit light when they drop down energy levels. 

3. In this experiment, where are the atoms getting their excess energy from? 

  In this experiment the atoms will be getting their energy from the heat of the flame produced by the Bunsen burner. This energy will allow electrons to reach an excited state. 

4. Why do different atoms emit different colors of light? 

   When an excited electron drops down energy levels after absorbing energy, it will release its absorbed energy as it descends. Depending on the differences between energy levels, the color of the light emitted will vary; thus each atom should have a unique light colors it releases when electrons return to ground state. 

Analysis Questions 

2. What pattern do you notice in the groupings? 

 We noticed that a majority of the compounds that emitted a yellow light contained the metal sodium and that all of the substances that gave off blue/green light contained copper. We also noted that although nonmetals such as chlorine and carbon appeared in many compounds, they were not restricted to a certain color; we found this interesting. 

4. What evidence do you have that atoms of certain elements produce a flame of a specific color? 

Evidence that atoms of certain elements produce a flame of a specific color is that throughout all of the compounds, no metal produced two different colors, even with different nonmetal ions in the compound. Although there were overlaps (both sodium and calcium compounds gave off a yellow-orange glow) no metal appeared in two separate categories. In addition, all of the compounds that produced blue-green light had copper in the formula and two of the three compounds that emitted yellow/orange light contained sodium. 

5. Can a flame test be used to identify a metal atom in a compound? Why or why not? What about a nonmetal atom? 

Yes, a flame test can be used to identify a metal atom in a compound, based off of these results; the metal and color stayed consistent throughout the entire experiment (see copper and sodium). However, we do not believe that this can be applied to nonmetals, for there was not a consistent correlation between nonmetals and the color of the flame produced; chlorine, for instance, was present in may different color categories. 

6. Identify the two unknowns. What are they and how do you know? 

The two unknowns are lithium chloride (Unknown #1) and potassium chloride (Unknown #2). We determined this based off of their flame colors; both Unknown 1 and lithium chloride burned a hot pink color, and both Unknown 2 and potassium chloride had a lavender/light purple flame. 

7. Copper oxide, CuO, is a black solid. It doesn't look at all like the element copper. What color flame would it produce? 

Given that the substance contains copper, we can predict that the flame will be green-blue in color. Based off of our results, most copper compounds produce this color flame regardless of the color of the substance. As long as the element is present in the compound, the substance will burn the corresponding color.