Flame Tests for the Elements

EXPERIMENT 1 Flame Tests for the Elements

INTRODUCTION The colors that we perceive around us arise from electronic transitions in the chemicals that make up the objects that have those colors. In fact, the way that we perceive color is due to electronic transitions taking place in the molecule known as retinal, which is part of the sensory system in the retina of the eye. Color vision arises from some modifications of the environment of the retinal when it is attached to the protein, opsin, to form rhodopsin. The rhodopsins may maximally absorb blue, green or red-orange light in special cone cells in the retina. The combined stimulation of these 3 classes of cone cells is what allows our eyes to perceive the subtle variations in colors of the visible spectrum. In this lab session you will perform flame tests on several of the elements by dipping a wire into a solution of that element and heating in a burner flame. As the atoms of the elements get heated, the electrons are "excited" to higher energy states or electronic shells. When those excited electrons loose energy by dropping back to the "ground state" (a lower energy level for the electron), the energy difference between the excited state and ground state is lost in the form of a photon (a particle of light) with a color corresponding to the energy level difference. These flame tests have been used by chemists for centuries to identify elements in various substances.

Excited State

Energy heat in light out Difference

Ground State

up and down arrows represent electrons (2 per orbital)

This principle is used for making color in fireworks. Salts of various metals are mixed with the explosive powder in the fireworks and as they burn a color characteristic of the element is observed. Products have also been marketed for giving color to fires in a fireplace. One

1 sprinkles the salts on a wood fire and the salts will slowly ignite producing color in the flame.

MATERIALS NEEDED NiChrome wires about 5" long with small loop on one end and cork or other insulator for a handle on the other end (1 per student), a Bunsen burner and 10% solutions of the following salts: NaCl, Na2SO4, KCl, LiCl, CaCl2, BaCl2, SrCl2 and CuSO4, 8 small testtubes, test tube rack.

PROCEDURE Transfer a small amount (1-2 mL) of each test solution to a test tube, marking each test tube according to what solution you place in it. Set up the Bunsen burner, light it and adjust the flame to give a pale blue color (lots of air relative to gas). Dip the NiChrome wire loop in concentrated HCl and then heat it in the flame for several seconds to burn off any impurities on its surface. Dip the loop into one of the above solutions and place it in the flame for a few seconds to see what color that salt gives. The color that is characteristic of a given element will appear in the first few seconds; after that the wire will generally give a yellow flame. It may help to have a piece of white paper or white object behind the flame to allow you to see the colors more easily. Leave the wire in the flame long enough to burn off all the salt before dipping the wire in the next solution.

Record in your notebook the color of each salt solution and the color it produces in the flame test. You should set up a table listing the salt and the corresponding color it gives off in the flame test. You should compare the colors of each very carefully. If two salts give orange or yellow colors in the flame, see if you can detect any subtle differences in the color (hue). Your table should look like this: Salt Color of Solution Color in Flame NaCl Clear Yellow ......

Questions to answer in your notebook:

2 1. Two of the elements are present as sulfate salts, Na2SO4 and CuSO4, while the others are 2- - present as the chloride salts. What effect does the anion (SO4 or Cl ) component have on the color in the flame test? What evidence do you have to justify your answer? [Note: Sodium is present as both the sulfate and chloride salts. Do both sodium salts give the same color? Do all chlorides give the same color? Do all sulfates give the same color?]

2. Is there any relationship between the color of the solution and the color observed in the flame for each of these salts? Give some evidence to support your conclusion.

3.* Describe the physical (atomic) phenomenon that causes the colors to appear in the flame? Refer to the introduction regarding electronic structure of atoms and the relative energy of their orbitals and see the text book where it describes atoms and electrons and the section on quantum mechanics, matter and radiation. [Hint: Using the diagram for the schematic representation of the hydrogen and sodium atoms (Chapter 2), imagine one of the electrons of the sodium atom absorbing energy to go to the next higher orbital (further from the nucleus). After absorbing this energy, it returns to its normal level. As the electron drops from the higher energy level to a lower one, what happens to the energy? How do we perceive that energy?] Note: It is difficult for many people to imagine the behavior of electrons in atoms. Try imagining a child with a baseball bat and a ball poised on a tee for batting practice in the back yard. Sitting on the tee, the ball has little energy. As the child hits the ball, its energy suddenly increases and it crashes through a window and comes to rest nearby after hitting the window. The energy of the batted ball is transferred to the window and the ball returns to its low energy state (at rest or no longer moving). The shattered window is the evidence of energy transferred. If the ball had hit a tree or brick wall, the same amount of energy would have been transferred, it just wouldn't be so noticeable to an observer. However, electrons are in a different realm of the world, which we can't see, but we can still see evidence of their existence and changes in energy.