1.10: Experiment_610_Line Emission Spectra and Flame Tests_1_1_3

Student Name		Laboratory Date: Date Report Submitted:	___________________________
Student ID		Experiment Number and Title	Experiment 610: Line Emission Spectra and Flame Tests

Experiment 610: Flame Tests and Line Emission Spectra

Section 1: Purpose and Summary

• Observe the color of light emitted when a sample of a salt of a metal cation is heated
• Use the characteristic color of flames to identify ions in various laboratory samples
• Observe the emission spectra of various elements in gas discharge tubes

In the atomic structure of an element, electrons occupy shells and orbitals that correspond to a specific amount of energy. When energy is absorbed by the atom, these electrons are excited to higher energy shells or orbitals. Electrons in the excited state are unstable and will spontaneously go back to lower energy level, giving off light in the process. The wavelengths of light given off correspond to the energies involved when electrons move from one energy level to another. These electron movements or transitions show as lines in an emission spectrum for the element.

The flame test is a qualitative tool to help determine the identity of a metal ion in a compound. A solution containing metal ions is heated in a Bunsen burner flame where a characteristic color is given off that is visible to the naked eye. Heat excites the electrons in these metal ions to higher energy levels. When the electrons relax back to its original state, the same amount of energy is released and corresponds to the wavelength of light in the electromagnetic spectrum.

In this experiment, students will heat metal ion solutions and observe the color of the flame upon heating and use this information to identify ions present in a laboratory sample. Students will also observe the emission spectra of elements in gas discharge tubes.

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Use of eye protection is recommended for all experimental procedures.

Use caution when handling hydrochloric acid (HCl).

Waste Disposal:

Metal ion solutions should be disposed of in the Inorganic Hazardous Waste container.

Section 3: Procedure

Part 1: Flame tests

Different solutions of metal ions (such as K⁺, Ba²⁺, ^Ca2+, Cu²⁺, Sr²⁺, and Na⁺) may be used in this

experiment. Ask your instructor which ones you will be testing.

Note: Na⁺ ion exhibits a particularly strong and persistent color. This ion is present as an impurity in most solutions since glass bottles contain Na⁺ ions that can leach out. The strong color of Na⁺ in the flame can mask other colors such as the pale lavender color of K⁺. It is a good idea to test the Na⁺ solution last.

Obtain a piece of nickel/chrome wire 10 – 15 cm long. Bend one end of the wire into the smallest possible loop, about 1 mm in diameter. This loop will hold a drop of solution when you are doing the flame test.

Obtain a few milliliters of 6 M HCl solution in a medium-size test tube. This will be used to clean the nickel/chrome wire.

Fill two additional medium sized test tubes with laboratory water. Label them “Rinse #1” and “Rinse #2”.

Cross contamination is a major issue for flame tests. Clean the nickel/chrome wire by first dipping into the 6 M HCl solution and then holding it in the hottest part of the Bunsen burner flame.

Dip the wire in “Rinse #1” and then hold it in the Bunsen burner flame. Repeat this with “Rinse #2”. Continue this process until there is no significant color to the flame that comes off the wire. Return to the 6 M HCl solution if needed. This step will be done between testing of each metal ion solution.

Obtain a sample of known metal ion solutions from your instructor into clean small test tubes. Only a few drops of each solution are needed.

Using a dropper, let a drop of the metal ion solution fall onto the loop of the wire. Do not let the dropper itself touch the wire as this can cause contamination of the solution.

Hold the wire in the hottest part of the Bunsen burner flame and observe the color that you see. Record your observation in the data table below.

Clean the wire by repeating steps #4-5. Repeat for all known solutions.

Obtain an unknown solution from your instructor. Each student should test his or her own unknown. You and your lab partner must use different unknowns.

Record the identification code of your unknown. Repeat steps 3 to 5 to test the unknown solution. Report the color of the flame you observed and determine the identity of the metal ion.

Metal Ion	Observation/Flame Color
K^+
Ba^2^+
Ca^2^+
Cu^2^+
Sr^2^+
Na^+

Identification Code of Unknown	Observation/Flame Color	Identity of the Metal Ion

Part 2: Spectroscopic Examination

A flame test will give a characteristic color which is the sum of all specific emissions for that element. The color from the flame test can be separated into its individual components using a prism, diffraction grating, or reflection grating. The following procedure uses a diffraction grating, but may be adapted with spectroscopes or prisms as appropriate.

As seen from the flame tests, the color of the metal ion tends to be brief, which makes it difficult to observe. Gas discharge tubes can be used to show a continuous set of colors from the specific element of interest. This makes an emission spectrum easier to observe.

Look through a diffraction grating at daylight or a regular tungsten light bulb. All of these sources give "continuous" rather than "discrete line" spectra; that is, they give the whole rainbow of colors, each merging into the next.

Darken the area and use the diffraction grating to look at the displayed elements in gas discharge tubes. Record the spectrum produced by each. Use crayons or colored pencils to record the colors seen by making vertical lines on the space below to represent the spectra obtained.

Spectra #1 Element in gas discharge tube: ______________

Spectra #2 Element in gas discharge tube: ______________

Spectra #3 Element in gas discharge tube: ______________

Spectra #4 Element in gas discharge tube: ______________

Spectra #5 Element in gas discharge tube: ______________

Post Lab Questions:

To help you answer the following questions, refer to your textbook and your lecture notes.

1. Fill in the following table regarding the hydrogen spectrum. Use units of electron volts (eV) where 1 eV = 1.60218 x 10^-19 Joules. Refer to your textbook for other unit conversions.

Color	Known λ(nm)	E (in eV)
Red	656.4
Turquoise	486.3
Purple	434.2

2. Show the calculation for energy, E, of any two of the above colors using the following equation:

E= \(\frac{\text {hc}}{\text {λ}}\)

3. Show the calculation for the conversion of one of the above energies to the corresponding photon energy in units of kJ/mole. (Hint: E in the above equations is the energy per photon.).

4. What causes light to be emitted by an atom?

5. Why are only certain wavelengths emitted from an atom? (In other words, why do the spectra show lines of light instead of a continuous rainbow of light?)

6. Why does hydrogen emit different wavelengths of light than mercury?

Notes: