Materials Included In Kit

Additional Materials Required

Safety Precautions

Cupric chloride is highly toxic by ingestion; avoid contact with eyes, skin and mucous membranes. Lithium chloride is moderately toxic by ingestion and is a body tissue irritant. Fully extinguish the wooden splints by immersing them in a beaker of water before discarding them in the trash to avoid trashcan fires. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Remaining amounts of the metallic salts included in this kit may be saved and reused or disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• The flame colors are best observed if the wooden splints are soaked overnight in water. To soak, fill a large beaker about one-third full with water and put the entire package of wooden splints in the water. They only need to be submerged about halfway.
• The flame colors are due to the metal in each compound, not the chloride ion. If other metallic salts were used, such as nitrates [Ca(NO₃)₂, Cu(NO₃)₂, LiNO₃, NaNO₃, KNO₃ and Sr(NO₃)₂], the same results would be observed.
• Avoid mixing the various solids. If mixing occurs, the flames observed will either be mixtures of the two colors, or one of the colors will mask the other.
• Make sure students fully extinguish the wooden splints by immersing them in a beaker of water before discarding them in the trash to avoid trashcan fires.
• Have students be careful not to drop excess solid into the laboratory burner. If this happens, the flame will burn the color corresponding to the dropped solid instead of blue.
• Depending on the level of your students and your teaching style, you may want to copy the example calculation on the following page and hand it out to your students.

Further Extensions

• After your students have finished with the lab, test them with this exciting demonstration. Place a scoopful of each metallic salt in a separate half of a Pyrex^® Petri dish. Add enough methyl alcohol to cover the bottom of each dish. Light each dish individually with a butane safety lighter and have the students determine which metallic salt is in each dish.
• Have students look at the flames through a diffraction grating or piece of Flinn C-Spectra^® to observe the line emission spectra for each metal. Each element has a unique line emission spectrum. Students can sketch the line emission spectrum for each metal, then use the spectra to identify unknowns.

Sample Data

Example Calculation

Assume a substance emitted light with a wavelength of 450 nm when placed in a flame, but the difference in energy, ΔE, between the two energy levels is actually desired. The following two steps outline how to calculate ΔE from a wavelength in nanometers.

Step 1—Convert the wavelength from nanometers to meters.

Step 2—Substitute into equation 1 and solve for ΔE.

Data Tables

Note: Student interpretations of the flame colors may vary. Allow for slight variation in answers.

Answers to Questions

1. Use Table 1 in the Background section to record the approximate wavelength of light emitted for each metal in Data Table 2.

   See Data Analysis Table.

2. Convert each of the wavelengths in the data table from nanometers to meters. Record the wavelengths in meters in Data Table 2. Show at least one sample calculation in the space provided.

   Divide each wavelength value in nanometers by 1 x 10⁹ to convert to meters. (This is the same as multiplying each value in nanometers by 1 x 10^–9.) See the example calculation for setup. See Data Table 2 for answers.

3. Use Equation 1 from the Background section to calculate the change in energy, ΔE, for each metal. Show all work. Record the values in Joules in Data Table 2.

   Use the wavelength values in meters calculated in Question 2 and the equation

   {11869_Answers_Equation_1}
   {11869_Answers_Table_1}
4. Predict the color of the flame if the following materials were heated in the flame. Explain your prediction.
   1. Cupric nitrate, Cu(NO₃)₂—green
   2. Sodium sulfate, Na₂SO₄—yellow
   3. Potassium nitrate, KNO₃—violet

The colors are predicted by looking at the metal in each salt because it is the metal cation, not the anion, that determines the color of the flame.

References

Thanks to Sue Zoltewicz, Eastside High School, Gainesville, FL, and Otto Phanstiell, Episcopal High School, Jacksonville, FL, for providing us with the idea for this activity.

Introduction

Just as a fingerprint is unique to each person, the color of light emitted by metals heated in a flame is unique to each metal. In this laboratory activity, the characteristic color of light emitted for calcium, copper, lithium, potassium, sodium and strontium will be observed.

Concepts

• Flame tests
• Absorption/emission

Background

Absorption and Emission of Light in a Flame

When a substance is heated in a flame, the substance’s electrons absorb energy from the flame. This absorbed energy allows the electrons to be promoted to excited energy levels. From these excited energy levels, the electrons naturally want to make a transition, or relax, back down to the ground state. When an electron makes a transition from a higher energy level to a lower energy level, a particle of light called a photon is emitted. A photon is commonly represented by a squiggly line (see Figure 1).

An electron may relax all the way back down to the ground state in a single step, emitting a photon in the process. Or an electron may relax back down to the ground state in a series of smaller steps, emitting a photon with each step. In either case, the energy of each emitted photon is equal to the difference in energy between the excited state and the state to which the electron relaxes. The energy of the emitted photon determines the color of light observed in the flame. Because colors of light are commonly referred to in terms of their wavelength, equation 1 is used to convert the energy of the emitted photon to its wavelength.
In Equation 1,

ΔE is the difference in energy between the two energy levels in Joules
h is Plank’s constant (h = 6.626 x 10^–34 J sec)
c is the speed of light (c = 2.998 x 10⁸ m/sec)
λ is the wavelength of light in meters

Wavelengths are commonly listed in units of nanometers (1 m = 1 x 10⁹ nm), so a conversion between meters and nanometers is generally made.

The color of light observed when a substance is heated in a flame varies from substance to substance. Because each element has a different electronic configuration, the electronic transitions for a given substance are unique. Therefore, the difference in energy between energy levels, the exact energy of the emitted photon, and its corresponding wavelength and color are unique to each substance. As a result, the color observed when a substance is heated in a flame can be used as a means of identification.

The Visible Portion of the Electromagnetic Spectrum

Visible light is a form of electromagnetic radiation. Other familiar forms of electromagnetic radiation include γ-rays, such as those from radioactive materials and from space, X-rays which are used to detect bones and teeth, ultraviolet (UV) radiation from the sun, infrared (IR) radiation, which is given off in the form of heat, the microwaves used in radar signals and microwave ovens and radio waves used for radio and television communication. Together, all forms of electromagnetic radiation make up the electromagnetic spectrum (see Figure 2). The visible portion of the electromagnetic spectrum is the only portion that can be detected by the human eye—all other forms of electromagnetic radiation are invisible to the human eye. The visible portion of the electromagnetic spectrum is only a small part of the entire spectrum. It spans the wavelength region from about 400 to 700 nm. Light of 400 nm is seen as violet and light of 700 nm is seen as red. According to equation 1, wavelength is inversely proportional to energy. Therefore, violet light (400 nm) is higher energy light than red light (700 nm). The color of light observed by the human eye varies from red to violet according to the familiar mnemonic ROY G BIV: red, orange, yellow, green, blue, indigo and violet. As the color of light changes, so does the amount of energy it possesses.

Table 1 lists the wavelengths associated with each of the colors in the visible spectrum. The representative wavelengths are used as a benchmark for each color. For example, instead of referring to green as light in the wavelength range 500–560 nm, one may simply refer to green light as 520 nm light.

Materials

Safety Precautions

Cupric chloride is highly toxic by ingestion; avoid contact with eyes, skin and mucous membranes. Lithium chloride is moderately toxic by ingestion and is a body tissue irritant. Fully extinguish the wooden splints by immersing them in a beaker of water before discarding them in the trash to avoid trashcan fires. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Fill a 250-mL beaker about half-full with distilled or deionized water. Obtain six wooden splints that have been soaked in distilled or deionized water. Place them in this beaker of water to continue soaking at your lab station.
2. Fill a second 250-mL beaker about half-full with tap water. Label this beaker “rinse water.”
3. Label six watchglasses (or weighing dishes) CaCl₂, CuCl₂, LiCl, NaCl, KCl, SrCl₂. Place a small scoopful (about 1–1.5 g) of each metallic solid in the corresponding watchglass (or weighing dish).
4. Light the laboratory burner.
5. Dip the soaked end of one of the wooden splints in one of the metallic salts, then place it in the flame. Observe the color of the flame. Allow the splint to burn until the color fades. Try not to allow any of the solid to fall into the barrel of the laboratory burner. If necessary, repeat the test with the same splint and salt.
6. Immerse the wooden splint in the “rinse water” to fully extinguish it, then discard it in the trash.
7. Record your observations for the flame color produced by the metallic salt in the data table.
8. Repeat steps 5–7 for the other five metallic salts. Record your observations for the flame color produced by each metallic salt in the data table.

Student Worksheet PDF

11869_Student.pdf