Flame Lab Primer

The relationship between the wavelength, frequency and speed of an electromagnetic
wave is given by the equation: c=λν
c being the speed of light
λ being the wavelength
ν being frequency

Electromagnetic radiation also occurs as discrete packets of energy (or quanta) called
photons. Photons are also called light particles and are the fundamental unit of electromagnetic energy. The energy per photon (in Joules) is given by the equation: Ephoton=hν
h=Planck’s constant, which has a value of 6.626×10^-34J⋅s

Visible light is the most familiar example of electromagnetic radiation. Differences in
the wavelengths of visible light are manifested as different colors, shown in the Color
Spectrum below. Other examples of electromagnetic radiation include X-rays, ultraviolet
light, infrared light, microwaves and radio waves.

Visible light and all radiation on the electromagnetic spectrum moves at the speed of light (c). Light travels at 2.998x10⁸ m/s

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 (ground state) to higher energy levels (excited state). The energy absorbed could be in the form of heat (as in flame tests), or electrical energy, or electromagnetic radiation.

When electrons subsequently return from higher energy levels to lower energy levels, energy is released predominantly in the form of electromagnetic radiation (photons).

The spacing between energy levels in an atom determines the sizes of the transitions
that occur, and thus the energy and wavelengths of the collection of photons emitted:

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 by eye). The result is called a line emission spectrum, and can serve as a
‘fingerprint’ of the element to which the atoms belong. For example, the line spectra shown
below for the elements helium and carbon are clearly quite different.

Unfortunately, techniques more sophisticated than those used in this lab 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. For example,
helium gas when excited by an electrical discharge emits light that appears an orange-peach
color. This one color results from a combination of all lines of the emission spectrum, in
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.