Using Flame Tests to Identify Unknowns

 

Valerie Stecher

New Rochelle High School, Westchester

Summer Research Program for Science Teachers

August 2011

 

 

Subject:  Chemistry

Grade Level: 9 - 12

Unit: Atomic Theory & Structure

Time Required:  One 48 minute period plus time at home for analysis questions to be completed

Purpose:  Students will investigate how the electromagnetic spectrum can be used to identify the unknown composition of a Colorflame candle by performing flame tests.

Objectives: SWBAT:

 

Aim:

To identify the metal in a Colorflame candle by comparison to known compounds

 

Materials:

Colorflame Candles                                         5 known samples of metal-containing compounds

Wooden splints                                                Bunsen burner

Matches                                                           Beaker of cold water

Colored Pencils                                                Video: How It's Made Fireworks ( link here)           

 

Materials note: When choosing the 5 known metal samples, consult MSDS for flammability or inhalation hazards!

 

Introduction:

Energy can be added to atoms many different ways. It can be in the form of light, an electric discharge or heat. This added or extra energy is emitted when the excited electrons in the atoms give off light and fall back to lower shells. The light emitted has wavelengths and colors that depend on the amount of energy originally absorbed by the atoms. Usually each individual excited atom will emit one type of light. Since we have billions and billions of atoms we get billions of excitations and emissions The energy levels in atoms and ions are the key to the production and detection of light. Energy levels or "shells" exist for electrons in atoms and molecules. The colors of dyes and other compounds results from electron jumps between these shells or levels. The colors of fireworks result from jumps of electrons from one shell to another. Observations of light emitted by the elements is also evidence for the existence of shells, sub shells and energy levels. Different elements emit different emission spectra when they are excited because each type of element has a unique energy shell or energy level system. Each element has a different set of emission colors because they have different energy level spacings. We will make qualitative observations of the emission spectra, or pattern of wavelengths (atomic spectra), emitted by five different elements in this lab. While we will not be looking the full spectra produced, the color that we can see with our naked eye can help us to identify the wavelength of light produced. We will then identify an unknown element by comparing the color of the unknown with the flame color of our known samples. If you miss anything, additional information and a virtual flame test can be found here: http://www.800mainstreet.com/spect/emission-flame-exp.html

 

What metals do colors indicate?

Color

Metal

Red

Carmine: Lithium compounds. Masked by barium or sodium.
Scarlet or Crimson: Strontium compounds. Masked by barium.
Yellow-Red: Calcium compounds. Masked by barium.

Yellow

Sodium compounds, even in trace amounts. A yellow flame is not indicative of sodium unless it persists and is not intensified by addition of 1% NaCl to the dry compound.

White

White-Green: Zinc

Green

Emerald: Copper compounds, other than halides. Thallium.
Blue-Green: Phosphates, when moistened with H2SO4 or B2O3.
Faint Green: Antimony and NH4 compounds.
Yellow-Green: Barium, molybdenum.

Blue

Azure: Lead, selenium, bismuth, CuCl2 and other copper compounds moistened with hydrochloric acid.
Light Blue: Arsenic and come of its compounds.
Greenish Blue: CuBr2, antimony

Violet

Potassium compounds other than borates, phosphates, and silicates. Masked by sodium or lithium.
Purple-Red: Potassium, rubudium, and/or cesium in the presence of sodium when viewed through a blue glass.

Procedure

 

                                  1.       Light your Colorflame candle. Record the color you observe.

                                  2.       Blow out your candle.

                                  3.       Light the bunsen burner

                                  4.       Take a small amount of known sample on a clean wooden splint.

                                  5.       Wave the splint through the flame. DO NOT HOLD IT IN THE FLAME. 

                                  6.       Record the color you see.

                                  7.       Dip the splint in cold water to extinguish.

                                  8.       Repeat with other known compounds, using a clean splint each time.

                                  9.       Turn off gas to burner and clean up lab area.

 

Data

 

Analysis: 

1.       Identify the metal your candle contained. Explain how you know.

 

2.       a) Describe, in terms of subatomic particles, how the light you observed in produced.

 

 

 

         b) What region of the electromagnetic spectrum is this light found? Identify the specific wavelength range that this   light could be found at.

 

 

          c) Using the visible light spectrum, estimate the wavelength and frequency of your unknown.

 

 

 

          d) Using your estimates in c, calculate the energy produced by the light.

 

 

 

3.       Arrange the following types of light in order of energy: x-rays, vis light, gamma rays, radio waves, UV

 

 

 

 

 

4.       From the August 2008 Regents exam. Base your answers to questions 66 through 68 on the information below.

          In a laboratory, a glass tube is filled with hydrogen gas at a very low pressure. When a scientist applies a high voltage between metal electrodes in the tube, light is emitted. The scientist analyzes the light with a spectroscope and observes four distinct spectral lines. The table below gives the color, frequency, and energy for each of the four spectral lines. The unit for frequency is hertz, Hz.

 

                                                 Visible Spectrum of Hydrogen

 

      Color                          Frequency (x 1014 Hz)                    Energy(x 1019 J)

        red                                              4.6                                         3.0                             

   blue green                                       6.2                                         4.1

       blue                                             6.9                                         4.6          

      violet                                            7.3                                         4.8

 

      66. On a separate sheet of graph paper or electronically, plot the data from the data table for frequency and energy. Circle and connect the points [1]

 

      67. A spectral line in the infrared region of the spectrum of hydrogen has a frequency of 2.3 x1014 hertz. Using your graph, estimate the energy associated with this spectral line. [1]

 

 

 

      68. Explain, in terms of subatomic particles and energy states, why light is emitted by the hydrogen gas. [1]

 

 

5.     From the June 2011 Regents exam. Base your answers to questions 52 through 54 on the information below:

 

                The bright-line spectra for three elements and a mixture of elements are shown below.


      52. Explain, in terms of  both electrons and energy, how the bright-line spectrum of an element is produced.   [1]

 

 

 

 

      53. Identify all the elements in the mixture.   [1]

 

 

      54. State the total number of valence electrons in a cadmium atom in the ground state.   [1]

 

 

6. Suppose you were a firefighter and you were called to a chemical plant fire. Upon arrival you see a bright violet/purple flame. What chemical would that tell you is burning?

 

 

7. After viewing the video on fireworks, use the information in your data table to design a firework and then predict what it would look like as it burns. Within the firework, specify which metal(s) your firework contains. Along the fuse, draw the fire as it burns. 

Assessment: Students will be assessed on participation in experimentation and final lab write up. An exit card will be given with the following question to be completed before leaving the lab: In one sentence - describe, in terms of energy and electrons, how a firework functions.


National Science Education Standards Grades 9 to 12

NS.9-12.1 Science As Inquiry

As a result of activities in grades 9-12, all students should develop abilities necessary to do scientific inquiry and understandings about scientific inquiry

NS.9-12.2 Physical  Science

As a result of their activities in grades 9-12, all students should develop an understanding of

New York State Standards:

Standard 1:  Students will use mathematical analysis, scientific inquiry, and engineering design, as appropriate, to pose questions, seek answers, and develop solutions.   

                Mathematical Analysis Key Idea 1: Abstraction and symbolic representation are used to communicate mathematically

                Scientific Inquiry Key Idea 1: The central purpose of scientific inquiry is to develop explanations of natural phenomena in a continuing, creative process.

                Key Idea 3: The observations made while testing proposed explanations, when analyzed using conventional and invented methods, provide new insights into phenomena.

Standard 4: Students will understand and apply scientific concepts, principles, and theories pertaining to the physical setting and living environment and recognize the historical development of ideas in science.

               Key Idea 3: Matter is made up of particles whose properties determine the observable characteristics of matter and its reactivity (performance indicator 3.1).

               Key Idea 4: Energy exists in many forms, and when these forms change, energy is conserved. (Performance Indicator 4.1)

Standard 7:  Students will apply the knowledge and thinking skills of mathematics, science, and technology to address real-life problems and make informed decisions.

 Key Idea 2: Solving interdisciplinary problems involves a variety of skills and strategies, including effective work habits; gathering and processing information; generating and analyzing ideas;  realizing ideas; making connections among the common themes of mathematics, science, and technology; and presenting results.