Description of Activity/Phenomena:
    Using a small wooden stick that has been soaked in water overnight, students will role the wooden rod in one of several salts (Cu(SO₄)·H₂O, BaCl₂, Ca(NO₃)₂, KNO₃, NaCl, Sr(NO₃)₂, LiNO₃)  so that the salt sticks to the rod.  The students then stick the wooden rod in a lit Bunsen burner and make observations.  They repeat the steps for each of the salts which they have samples of.  Once they have tested all of their salts, they then try to identify an unknown salt using similar methods.

Typical Observations:
    When the salt is placed in the flame, a new colored flame appears after a moment or two.
    Each compound produces a different colored flame.
    The colors are quite vivid and continue until all the compound is gone.
    A wooden rod by itself does not produce a new color of flame unless there is a salt on it.
    Ions in salts, specifically the cations, can be identified by the color of flame which it produces.

Explanation:
    Every atom has defined orbitals and energy levels for its electrons to be in.  When energy is added to an atom (in this case the flame is adding the energy), the electrons are excited to higher orbitals.  Electrons in a stable atom like to have the lowest energy they can and therefore are found in the lowest energy orbitals.  When energy is added to atom, it gives the electrons more energy and they have to jump up into a higher energy orbital.  Since they do not like to be at a higher energy than they have to be, the electrons give off their energy in the form of light and return to their lowest state.  When the salt is put in the flame, it excites the electrons to higher energy levels, the electrons fall back down to the lowest energy level and give off light, but because the flame is still present (the energy is still there), the cycle repeats itself.   Because each atom has orbitals with different energy values, they produce different colors of light.  The color of light is determined by the lights wavelength which is determined by the energy which the light is carrying.  Since atoms have different energy values for each of their orbitals, the energy difference between the orbitals will also be different.  It is this difference in energy which the electrons must give off in order to drop into a lower orbital.  Different energies released means different colors of light.  Since every atom has a different color which it produces in a flame, flame tests can be used to identify unknown samples (or at least identify one of the atoms in the sample).

    The light bulbs in your home use this idea to light a room.  The filament in light bulbs is often made from tungsten wire or another compound.  Using the energy from the electricity, the electrons in the tungsten wire are excited and the fall back down to lower orbitals.  Tungsten gives off white light instead of the colored light seen in this experiment so it is better for lighting rooms.  Eventually the filament will break because of to much use and energy being pumped through it-that is when it is time to replace the light bulb because it has "burned out."  We also have sodium lamps and neon lights that work in the same away, the element's electrons in the atoms are excited and then produce heat.  Many people realize that we have these kind of lights but do not know how they work or why they work.  (For more information about student's ideas on these type of lights, please click here.)

    Fireworks are another example of this phenomena put to work for us.  Fireworks are made with an oxidizer and fuel.  The firework is launched by lightly a fuse which ignites black powder, used as the propellant.  The firework also has a time-delayed fuse on it so that after is has been launched the oxidizer and fuel can be ignited.  When the oxidizer and fuel are ignite, the firework explodes, producing a brilliant flash and a loud noise.  To color the fireworks, salts, like those used in our experiment are added to the mixture of fuel and oxidizer.  Then when the fuel and oxidizer react, the energy from the reaction excites the atoms in salt.  Just like in our experiment, the excited atoms release energy in the form of visible light and produce the brilliant colors that we see.  So the next time you see a yellow fire work, think "Look at all that pretty sodium" or when you see a red one "Ah, what beautiful strontium."
 

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