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Flame Tests

Atomic Emission and Electron Energy Levels

Introduction:

Just as a fingerprint is unique to each person, the color of light emitted by an

element heated in a flame is also unique to each element. In this experiment, the characteristics color of light emitted by calcium, copper, lithium, potassium, sodium, barium, and strontium ions will be observed.

Background information:

Visible light is a form of electromagnetic radiation. Other familiar forms of electromagnetic radiation

include X-rays, ultraviolet (UV) radiation, infrared (IR) radiation, microwave radiation, and radio waves. Together, all forms of electromagnetic radiation make up the electromagnetic spectrum. 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.

The visible spectrum 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. Wavelength is inversely proportional to energy. This means lower wavelength corresponds to higher energy and vice versa. See the table below for specific wavelengths.

Representative
Wavelength, nm / Wavelength Region,
nm / Color
410 / 400-425 / Violet
470 / 425-480 / Blue
490 / 480-500 / Blue-green
520 / 500-560 / Green
565 / 560-580 / Yellow-green
580 / 580-585 / Yellow
600 / 585-650 / Orange
650 / 650-700 / Red

Experiment Overview:

The purpose of this lab is to observe the characteristic flame test colors of different metal compounds and to use this information to identify an unknown metal ion.

Pre-Lab Questions:

1. Fill in the blanks: When an atom absorbs energy, the electrons move from their ______state to an ______state. When an atom emits energy, the electrons move from a(n) ______state to their ______state and give off ______.

Word Bank: ground, excited, light (words may be used more than once)

2. Is a flame test a qualitative or quantitative test for the identity of an unknown? Explain.

Materials:

Calcium chloride solution lithium chloride solution sodium chloride solution

unknown metal chloride solution Copper (II) chloride solution potassium chloride

strontium chloride solution wooden splints Bunsen burner

250 mL beaker striker latex gloves

**All chloride solutions are saturated. Wood splints will be soaked in the saturated solutions overnight.

Safety Precautions:

Copper (II) chloride is highly toxic; avoid contact with eyes, skin, and mucous membranes. Lithium chloride is moderately toxic and a body tissue irritants. Fully extinguish the wooden splints by immersing them in water before discarding them in the trash can. Wear goggles and apron. Wash hands thoroughly with soap and water before leaving the lab. Wear gloves!

Procedure:

1.  Divide a piece of paper into 7 columns.

2.  Label the columns: Ca, Li, Na, Cu, K, Sr, unknown

3.  Obtain one saturated wood splint of each metal from the labeled beaker.

4.  Place the correct splint in the correct column. (THIS IS VERY IMPORTANT TO GET RIGHT!)

5.  Fill a 250 mL beaker with tap water. This will be your rinse beaker.

6.  Light the Bunsen burner.

7.  Place the end of the saturated wood splint into the flame.

8.  Observe the color of the flame. Allow only half of the wood splint to burn.

9.  Extinguish the wooden splint by placing it in your rinse beaker.

10.  Record your observations for the flame color by the metal chloride in the Data Table.

11.  Repeat steps 7-10 for the other metal ions. Make sure to record the color of each flame.

12.  Perform steps 7-10 for the unknown. Record the color that you see. Predict what metal you believe the unknown to be.

Data:

Metal Ion / Color of Flame / Approximate
Wavelength (nm) / Approximate
Wavelength (m)
Calcium
Copper
Lithium
Sodium
Potassium
Strontium
Unknown

Post-Lab Questions:

1.  Use the table to record the approximate wavelength of light emitted by each metal ion.

2.  Convert each wavelength from nanometers to meters. Show one sample calculation in the space below.

3.  What is the identity of the unknown? Why do you believe this to be true?

4.  A glass rod was heated in a burner and gave off a bright yellow flame. What metal ion dominates the glass rod? Why do you think this to be the case?

5.  The alkali metals cesium and rubidium were discovered based on their characteristic flame colors. Cesium was named after the sky and rubidium after the gem color. What colors of light do you think these metals give off when heated in a flame?

**This lab was adapted from Flinn ChemTopic™ Labs volume 3.