EXP 13

• The flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic color the salt turns the flame of a bunsen burner.
• The heat of the flame excites the metals ions, causing them to emit visible light.
• The characteristic emission spectra can be used to differentiate between some elements.
• Visible light is the most familiar example of electromagnetic radiation.
• Differences in the wavelength of visible light are manifested as different colors. So, how does electromagnetic radiation relate to flame tests?
• When an atom (or ion) absorbs energy, its electrons can make transitions from lower energy levels to higher energy levels.
• The energy absorbed could be in the form of heat (as in flame test), or electrical energy, or electromagnetic radiation.
• However, when electrons subsequently return from higher energy levels to lower energy levels, energy is released predominantly in the form of “electromagnetic radiation”.
• If emitted photons are in the visible region of the spectrum, they may be perceived as lines of different colors (note that photons outside the visible spectrum may also be emitted, but cannot be seen).
• The result is called a “line emission spectrum”, and can serve as a “fingerprint” of the element to which the atoms belong.
• Unfortunately, techniques more sophisticated than those use in this laboratory are required to obtain such line spectra.
• To the naked eye, when an element is vaporized in a flame (or an electrical discharge) the emission spectrum will appear to be just one

  color.
• This one color results from a combination of all lines of the emission spectrum, in the proportion to their intensities.
• As many elements will still produce distinctive colors under such

  conditions, simple flame tests can be used to identify these elements. In fact, flame tests were used to identify elements long before the invention of modern techniques, such as emission spectroscopy.