Lab: Flame Tests “At Home”

Objective

In this lab students will learn about atomic energy levels, emission spectroscopy, and flame tests for element identification.


Overview

Students will observe small samples of chloride salts of different metals. These will be placed into a flame in order to observe the colors produced. These colors come from the excitation of electrons which then resume their ground states by emitting light of very specific colors.


Atomic.Emission.Process

Background

The electrons in an atom occupy different energy levels. When all of the electrons are at the lowest possible energy level they are said to be in the ground state. In this state electrons are as close to the atom’s nucleus as they are allowed to be. When they are there, they have their lowest possible potential energy. Just as a ball sitting on the ground has its lowest potential energy, electrons which are close to the nucleus are similarly low in potential energy.

Electrons do not always stay in the ground state. Sometimes they can be promoted to an electron shell with a higher potential energy. This can happen in two ways. First, the electron can absorb a photon of just the right amount of energy to move it from one quantum shell to another. Second, when atoms are heated in a flame or energized with electricity their electrons can gain energy. This promotes them to the higher-energy shell. When an electron is in a higher-energy shell it is said to be in an excited state. Excited states are situation where one or more electrons within an atom are at a higher potential energy than they would be in the ground state. There are many possible excited states for atoms.

Electrons in excited states do not stay in them for very long. When electrons lose their energy they do so by emitting light. In this way, excess potential energy is transformed into the energy of light. Each time a single electron drops from higher to lower potential energy, a particle called a photon is produced. Photons are particles with energy but no mass. Their energy is inversely proportional to the wavelength of the light (this can be expressed in the proportion, E = hc/λ, where λ means the wavelength). The photons emitted precisely match the quantum energy difference between the excited state and the ground state.

A salt is a type of compound that include a metal ion and a non-metal ion. Sodium chloride (NaCl) is the most familiar example of a salt but others include calcium chloride(CaCl2) and copper(II) chloride (CuCl2). These compounds look very much the same to the unaided eye. For example both sodium chloride and calcium chloride are colorless crystals and as a fine powder they are white. In order to identify which salt you have, you have to do a test on the material. A traditional method for identifying elements in compounds is called a flame test.

In flame tests salts that are dissolved in water are evaporated using a hot flame. In the flame the atoms become excited and produce a characteristic color. This happens when the atoms’ excited electrons fall back down to the ground state. The color we see depends on the difference in energy between the excited and ground state. For some atoms this is a large difference and for others it is a smaller difference. The color can be used to tell if the energy difference is large or small. The purple end of the spectrum has the smallest wavelength and so the energy of that light is the largest. The red end of the spectrum has the largest wavelength and so that color represents the smallest energy changes. Remember the order of colors in a rainbow is ROY G BIV (red orange yellow green blue indigo violet). Energy changes can be judged against the spectrum because as you go from red to violet the energy change that made the color gets larger. Different atoms have different possible excited states, which vary in their distance from the atom’s ground state. Each element has a unique set of possible excited states. Because of this it is possible to identify an element by doing a flame test and recording the observed color. Every element has a unique flame test color.

It is a traditional art of the chemistry laboratory to use these colors to identify specimens of compounds that contain unknown metals. This is no longer a common laboratory technique though highly advanced technologies use the same production of light to identify elements and to measure how much is present. The production of light in a flame is still used outside the lab, though. It is put to use by practitioners of the art of fireworks manufacture. By including different metal salts, or mixtures of metal salts, in the exploding shell of a firework, these artists can produce beautiful displays in nearly all the colors of the rainbow.

Color Representative
Wavelength (nm)
Wavelength
Region (nm)
Violet 420 400 - 440
Blue 455 440 - 470
Blue-green 480 470 - 490
Green 525 490 - 560
Yellow-green 565 560 - 570
Yellow 580 570 - 585
Orange 620 585 - 630
Red 660 630 - 700

Light is a kind of wave, an electromagnetic wave. Our ability to perceive color depends on the different wavelengths of light. If you observe a rainbow or use a prism to split white light then you are looking at how our eyes and brain work together to give us the sensation of the colors of the rainbow. Because we see different ranges of the visible spectrum as different colors it’s possible to roughly assign a color we see to a particular wavelength of light. It is not a perfect match, but it’s a good start and is good enough for now. Take a look at the chart at right. It details the colors people in our culture usually assign to the rainbow and gives each color a range of wavelengths in units of nanometers. A nanometer (nm) is a billionth of a meter (1 m = 1 × 109 nm). Each color can be assigned a ‘representative wavelength’. You will use these to identify colors in this activity.




page break

Procedure

Carefully read and follow the instructions below. Record your data and observations in a Google Doc while you work to make things efficient for yourself.

  1. Create a Data Table (you may copy-paste this one into your document to get started:
    Ionic Compound Formula Cation (positive ion) Anion (negative ion) Color Observed Representative Wavelength (nm) Element Responsible for Color
               
  2. Watch the videos:
    Flame Tests (by Mr. Keller)
    Flame Tests of Metal Salts
  3. For every different chemical which is used in a flame test in either video, record the appropriate information in the data table you created. Do not fill in the final column: “Element Responsible for Color”. You will fill in this column as part of the post-lab questions. To help identify ions, you may use your Ions Reference Sheet. Choose whatever color is closest from the table of representative wavelengths in the introduction of this lab.

Grading

For this lab you must turn in the following items:

  1. Your data table recording flame test colors and wavelengths
  2. Answers to the following questions
  3. A one-page essay about how fireworks are made, concentrating on how they produce different colors but including details of their basic construction and operation

Questions

  1. According to the introduction on this lab handout, what is the chemical definition of a salt?
  2. According to the introduction on this lab handout, what is the ground state of an atom?
  3. According to the introduction on this lab handout, what is an excited state of an atom?
  4. Based on your reading of the introduction, what has to happen within an atom (that is, what happens to the electrons in an atom) before it can produce light?
  5. Based on your reading of the introduction, why do atoms need to be heated in a hot flame or subjected to high-voltage electricity before they will produce any light?
  6. Based on your reading of the introduction, which color represents a larger energy change within an atom: blue or orange? Explain.
  7. Determine the element that is responsible for the color of each flame test by considering the following questions. Answer each with appropriately formatted chemical formulas and complete sentences as necessary.
    1. What are the names and formulas of all of the compounds that contain the chloride ion (Cl)?
    2. Do any compounds containing the chloride ion (Cl) have the same color? Explain.
    3. What are the names and formulas of all of the compounds that contain the sodium ion (Na+)?
    4. Do any compounds containing the sodium ion (Na+) have the same color? Explain.
    5. What are the names and formulas of all of the compounds that contain the copper ion (Cu2+)?
    6. Do any compounds containing the copper ion (Cu2+) have the same color? Explain.
    7. Is it the cation or the anion that is responsible for the color of a flame test?
  8. After completing the questions above, fill in the final column on your table.

Web Links for Part III of the Lab
NOVA from WGBH: Fireworks! (http://www.pbs.org/wgbh/nova/fireworks/)
Chemical of the Week: Fireworks! from the University of Wisconsin (http://www.scifun.org/CHEMWEEK/fireworks/Fireworks2017.htm)
A Chemical and Engineering News article about Fireworks (http://pubs.acs.org/cen/whatstuff/stuff/7927sci3.html)
A “What’s That Stuff” article about Fireworks (https://cen.acs.org/articles/95/i27/s-fireworks-produces-those-colorful.html)
Instructions on how to light a bunsen burner and use it safely.
Pre-lab Questions for this lab
Pre-lab Questions for Atomic Emission Spectroscopy Lab
Atomic Emission Spectroscopy Lab
Last updated: May 14, 2020       Home