Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. To make 1.0 M solutions of each salt, mass the following amounts and place them into individual, labeled beakers.

1. Calcium chloride dihydrate, 52 g
2. Copper(II) chloride dihydrate, 60 g
3. Lithium chloride, 15 g
4. Potassium chloride, 26 g
5. Sodium chloride, 20 g
6. Strontium chloride hexahydrate, 93 g

2. Place 350 mL of distilled or deionized water into each of the six labeled beakers. Stir until the salts dissolve.
3. Place wood splints in each solution, and allow them to soak at least 20 minutes.
4. Transfer solutions and soaking wood splints to labeled test tubes for each group.
5. Create six “mystery” test tubes. These can contain a single salt solution or a mixture of two. Label them Unknown X, Unknown Y, Unknown Z, etc. Salts that work well and are easy for students to identify include strontium, potassium and copper. Hint: If copper is used, you can dye the other solutions with blue food dye so all “mystery” solutions will be blue but will still emit the color matching its metallic salt.

Safety Precautions

Copper(II) chloride is moderately toxic; avoid contact with eyes, skin and mucous membranes. Lithium chloride is a body tissue irritant. Rinse the wooden splints before discarding them in the trash to avoid trashcan fires. Wear chemical splash goggles, chemical resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. 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. Each of the metallic solutions may be disposed of down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period.
• In addition to having students calculate energy and wavelength, you can also extend the activity by having students write the various electron configurations of the different elements and their corresponding ions.
• Instead of each group having their own chemicals, you can place the various solutions at different lab stations and have students rotate. Remind students to turn off the Bunsen burner, before moving to the next station.
• For the unknown solutions, you can dye all the unknowns blue with 1–2 drops of blue food coloring. Then students will not be able to identify the copper ion by the solution alone. Adding food dye to the solutions does not affect the flame test results.
• Small beakers may be used in place of test tubes.
• Instead of preparing the test tubes for students, you can have them come and collect the solutions needed for each group 

  themselves.

• 1.0 M solutions are not necessary. If desired, you can dilute the entire amount of each salt into 350 mL of distilled or 

  deionized water. For example, 100 g of calcium chloride dihydrate, 100 g of copper(II) chloride dihydrate, 50 g of lithium 

  chloride, 50 g of potassium chloride, 50 g of sodium chloride and 100 g of strontium chloride hexahydrate.

Teacher Tips

• The flame colors are due to the metal in each compound, not the chloride ion. If other ionic metal solutions were used, such as nitrates (e.g., LiNO₃, Cu(NO₃)₂), the same results would be observed.
• Avoid all contact between the wooden splints used for the various salts. If cross-contamination occurs, the flames observed will either be mixtures of the two colors or one of the colors will mask the other.
• As an extension, have students look at the flames through a diffraction grating or piece of C-Spectra^® to observe the line emission spectrum for each metal. Each element has a unique line emission spectrum. Students can sketch the line emission spectra for each solution, then use them to identify unknown solutions.

Sample Data

Data Table 1

Data Table 2

Answers to Questions

Energy/Wavelength Calculations

Use Table 1 in the Background section to determine the approximate wavelength of light emitted for each metal. Record your wavelength in nanometers and meters in Data Table 2.

1. Calculate the change in energy, ΔE, for each metal. Show all work. Record the values in joules in Data Table 2.

Sample calculation for calcium:
ΔE = hc/λ ΔE = (6.626 x10^–34 J•s)(2.998 x 10⁸ m/s)/(6.0 x 10^–7 m) = 3.3 x 10^–19 J

2. Predict the color of the flame if the following materials were heated in the flame. Explain your predictions.

1. Copper(II) nitrate

   Green

2. Sodium sulfate

   Yellow

3. Potassium nitrate

   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.

Extension Question

3. Write the electron configurations for the metals used and their corresponding ions.

Introduction

In this lab, you’ll be examining the color that various metallic salts emit when heated; similar to what happens during firework displays.

Watch the introductory video.

All light travels at the same speed, 2.998 x 10⁸ m/s. For a fun way to figure out the speed of light using your microwave and marshmallows, check out the video.

For a brief explanation on the colors of fireworks, check out the video at https://www.youtube.com/watch?v=dW5OBrB4MRM

Concepts

• Flame tests
• Absorption and emission
• Speed of light
• Wavelength and frequency

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 promotes the electrons to excited energy levels. From these excited energy levels, the electrons eventually 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 single 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 the corresponding 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•s)
c is the speed of light (c = 2.998 x 10⁸ m/s)
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.

Another item you can calculate from the wavelength is the frequency, using the following equation:

c is the speed of light (c = 2.998 x 10⁸ m/s)
λ is the wavelength of the light in meters
υ is the frequency of the wave in hertz (1 Hz = 1/s)

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 photon emitted 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 (e.g., those from radioactive materials and from space), X-rays (e.g., those used to detect bones and teeth), ultraviolet (UV) radiation from the sun, infrared (IR) radiation given off in the form of heat, microwaves (e.g. those 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, the photons in violet light (400 nm) are higher in energy than the photons in 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

Copper(II) chloride is moderately toxic; avoid contact with eyes, skin and mucous membranes. Lithium chloride is a body tissue irritant. Rinse the wooden splints 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. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part A

1. Obtain six test tubes labeled with each of the salt solutions listed in the Materials section. Wood splints should already be soaking in the solutions.
2. Make sure to have a 150-mL beaker half-full with distilled or deionized water. Label this beaker “rinse water.”
3. Light the laboratory burner.
4. Place the soaked end of one of the wooden splints in the flame. Observe the color of the flame. When done, immerse the wooden splint in the “rinse water” to fully extinguish it, then discard it in the trash.
5. Repeat for the remaining salt solutions.

Part B

6. Obtain an unknown sample from your instructor.
7. Identify which salt (or salts) are in the solution, and explain your choice(s).
8. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

14142_Student1.pdf