To perform flame tests of metal cations in order to observe their characteristic colors, To perform calculations to determine the frequency and energy of the emitted photons. To relate these results to the types of electronic transitions occurring in these elements. To observe and understand line emission spectra of atoms using gas-discharge tubes. To practice writing electron configurations for these (and other) elements. Electromagnetic radiation is energy in the form of waves. Waves are characterized by their wavelength (\(\lambda\)) and frequency (\(\nu\)). Wavelength is defined as the distance between successive crests (or troughs) on a wave, and is measured in meters. Frequency is defined as the number of waves that pass a given point every second, and is measured in 1/seconds (1/s or s -1 ), or Hertz (Hz). All electromagnetic waves travel at the speed of light (\(c\)), or \(3.0 \times 10^8 m/s\). The relationship between the wavelength, frequency and speed of a wave is given by the equation: \[c= \lambda \times \nu\] Electromagnetic radiation also occurs as discreet “packets” called photons . The energy (\(E\)) of a photon (in Joules, J) is given by the equation: \[E_{photon}= h \times \nu\] Here, \(h\) is Planck’s constant , which has a value of \(6.63 \times 10^{-34} J \cdot s\). 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 (colors can be seen in the PDF document on-line). Other examples of electromagnetic radiation include X-rays, ultraviolet light, infrared light, microwaves and radio waves. So, how does electromagnetic radiation relate to flame tests? Well, 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 tests), 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 . 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). 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 (colors can be seen in the PDF document on-line). 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. Procedure Materials and Equipment Usually aqueous solutions of the following metal salts are sparyed into a Bunsen burner : \(\ce{LiCl}\) ( aq ), \(\ce{NaCl}\) ( aq ), \(\ce{KCl}\) ( aq ), \(\ce{CuCl2}\) ( aq ), \(\ce{BaCl2}\) ( aq ), \(\ce{CaCl2}\) ( aq ). Since this is a kitchen lab for online students no real wet experiment needs to be conducted , instead watch the following video to identify the flame color made by the above solutions and write the data in the following table. Since no real wet lab experiment is conducted, no safety measures are required in particular. Experimental Procedure - This will be shown as a pre-recorded video demonstration only. Part A: Flame Tests of Metal Cations Observe the color of the flame produced with each metal compound shown in the video. Students will record the dominant flame color observed. Analysis : For each metal cation flame test performed, determine the wavelength corresponding to the observed flame color from the table below. *Wavelength values here are given for the mid- range of the color indicated. Dominant Color Approximate Wavelength (in nm)* Red 701 Red-Orange 622 Orange 609 Orange-Yellow 597 Yellow 587 Yellow-Green 577 Green 535 Green-Blue 492 Blue 474 Blue-Violet 455 Violet 423 Part A: Flame Tests of Metal Cations Experimental Data and Observations Solution Dominant Flame Color Wavelength (nm) Flame Intensity (low, medium, high) Unnamed: 4 \(\ce{BaCl2}\) NaN NaN NaN NaN \(\ce{CaCl2}\) NaN NaN NaN NaN \(\ce{CuCl2}\) NaN NaN NaN NaN \(\ce{KCl}\) NaN NaN NaN NaN \(\ce{LiCl}\) NaN NaN NaN NaN \(\ce{NaCl}\) NaN NaN NaN NaN Part B: Line Emission Spectra-Research Do some research to find the line emission spectra of the following elements and copy paste them to your lab sheet and label. Copy paste the spectral lines for the indicated elements. Indicate names colors below each line drawn. Line Emission Spectrum for Helium colors: nm: Line Emission Spectrum for Mercury colors: nm: Line Emission Spectrum for Hydrogen colors: nm: Data Analysis Using the wavelengths recorded above, calculate the corresponding wavelengths, frequencies and photon energies for each compound tested. Record the values, in scientific notation, in the table below. Solution Wavelength (m) Frequency (s-1) Energy (J) \(\ce{BaCl2}\) NaN NaN NaN \(\ce{CaCl2}\) NaN NaN NaN \(\ce{CuCl2}\) NaN NaN NaN \(\ce{KCl}\) NaN NaN NaN \(\ce{LiCl}\) NaN NaN NaN \(\ce{NaCl}\) NaN NaN NaN Show a set of sample calculations for \(\ce{NaCl}\) only below. Clearly show any equations you have used. Wavelength (in m): Frequency (in s -1 ): Energy (in J): Questions Complete the following paragraph by circling the correct responses: In this experiment, the metal cations in the solutions were initially in the (ground, excited) state. When placed in the flame, the metals then (absorbed, emitted) energy as (electricity, heat, EM radiation). When this occurred, electrons made transitions from (low, high) energy levels to (low, high) energy levels. The metals were then in the (ground, excited) state. The electrons in these metals then made transitions from (low, high) energy levels to (low, high) energy levels, resulting in the (absorption, emission) of energy as (electricity, heat, EM radiation). What evidence is there that the colors observed in the flame tests are due to the metals, and not the nonmetals in the compounds tested? Which metal cation was observed to emit radiation with the longest wavelength ? Compared to the other metals studied, did the radiation emitted by this metal cation have • the highest or lowest frequency? • the highest or lowest energy? Why do the gas-discharge tubes need to be turned “on” before a color can be seen? You observed the spectral lines for different elements. What is happening within an atom that causes it to emit light in specific lines in a spectrum? Why did the different elements have different line emission spectra? The radio station JACK FM broadcasts at a frequency of 93.5 kHz. Calculate its wavelength (\(\lamda\)) in nanometers. (Hz = 1/s) For the same radio station, calculate the energy (E) in kilojoules, kJ. Contributors and Attributions The experiment is from Chem 10 Experiments shared under a CC BY-NC license and was authored, remixed, and/or curated by Santa Monica College . Manjusha Saraswathiamma, Minnesota State Community and Technical College, Moorhead, has modified this to fit o a homeschool environment with less hazardous and cost-effective lab supplies.