Chapter 13
Experiment: Flame Tests and 
Spectral Lines for Elements
   

Purpose: To observe and identify metallic ions using flame tests and to observe several elements spectral line patterns using a diffraction grating.

Background: Have you ever wondered why a candle flame is yellow?  The characteristic yellow of a candle flame comes from the glow of burning carbon fragments.  The carbon fragments are produced by the incomplete combustion of the wick and the candle wax.  When elements, such as carbon, are heated to high temperatures, some of their electrons are excited to higher energy levels.  When these excited electrons fall back to the lower energy levels, they release excess energy in packages of light energy called PHOTONS or QUANTA.  The color of the emitted light depends on its energy.  Blue light has more energy than red light.  When heated, each element emits a characteristic pattern of light energies which is useful in identifying the element.  In this experiment, you will perform flame tests used to identify several metallic elements.

In the second part of the experiment, you will look at several samples of elemental gases which are being excited, not by heat (as in the flame tests), but by running an electrical current through the pure gases (in a vacuum).   In the same way that heat causes some electrons to get excited and jump up to  higher energy levels, so too does an electrical current when passed through the pure gas sample.  When these excited electrons fall back to the lower energy levels, they release their excess energy in the form of light.  When you look through a diffraction grating (which behaves similarly to a prism), the light is diffracted (differentiated) into discrete, distinct bands of light.  Each and every element has a unique emission spectrum (pattern of light bands).  You will be drawing these patterned bands of light for several elements using colored pencils.

Materials:

• Nichrome wire loop

• Beaker

• Paper towels

• Bunsen burner

• Chemicals:

  • 6 M hydrochloric acid

  • Solutions of: 

    • Potassium nitrate

    • Calcium nitrate

    • Strontium nitrate

    • Lithium nitrate

    • Copper (II) nitrate

    • Sodium nitrate

    • Barium nitrate

• CRTs with the following elements: 

  • Helium

  • Neon

  • Mercury

  • Hydrogen

  • Argon

• Diffraction grating

• Colored pencils

Procedure:

PART A - FLAME TESTS

1. Pour about 15 mL of 6M HCl into a small beaker.

2. Between each sample of metallic compound, you will dip your nichrome loop into a small amount of 6M HCl and heat it in the hot flame of the bunsen burner - keep dipping and heating until no color comes from the wire when it is put into the flame.

Hydrochloric acid is corrosive and cause severe burns!.

3. Dip the clean wire loop into the metallic compound solution and heat the sample in the bunsen burner flame.  Record on DATA TABLE (see below).

4. Repeat step 2.

5. Continue with each metal - being absolutely certain that the nichrome loop is completely clean between samples.

Data Table:

Copy/paste this data table into Microsoft Word.  Bring data table to class on the day of the experiment.

PART B - DIFFRACTION GRATING

Print off the data table here and bring to class on the day of the lab.

1. Pick up a diffraction grating from the front lab table.

2. Holding the diffraction grating up at your eye, look up at one of the fluorescent lights in the ceiling (white light).  Draw what you see on the data table you printed off in the section labeled WHITE LIGHT.  Since red light has the lowest frequency, always draw the red to the LEFT - as compared to violet light which has the highest frequency, always draw violet towards the RIGHT.  Use the colored pencils.  As you draw your bands of color, remember that white light emits a CONTINUOUS SPECTRUM so your colors should "blend" together as you draw from red to violet light.

3. In the lab, there will be different cathode ray tubes (CRTs) with 5 different elements.

4. Using the diffraction grating, draw the patterns of the bands of colored light you see onto your data sheet.  Again, draw lowest frequencies of light to the left, higher frequencies to the right.  On the 5 different samples, you will NOT see a continuous spectrum - you see an EMISSION SPECTRUM and so you should be drawing DISTINCT color stripes - NOT blended together!  These will look TOTALLY different from the white light!  You will be graded both on the accuracy in the placement of the lines relative to each other and their colors.  NEATNESS counts!

Analysis:

Copy/paste these questions into Microsoft Word and then answer the questions.

1. In the flame tests, what elements produced the most easily identified colors? 

2. In the flame tests, what element produced the most intense color?

3. The energy of colored light increases in this order - the colors of a rainbow....

ROY G BIV - red, orange, yellow, green, blue, indigo, violet.  List the elements you tested in the flame tests in increasing order of the energy of light emitted.

4. The production of fireworks uses the same exact principles that you observed in this experiment.  Read this short discussion about Fireworks: http://chemistry.about.com/library/weekly/aa062701a.htm Compare the colors you observed in the flame tests to the colors listed on the website in the table.  Did you get the same results?  Explain why or why not.

5. Your white light drawing should look very different from the emission spectra for the 5 pure elements.  How is your white light drawing different from your drawings of the other elements?  Why is the white light so different?  

6. Compare your bands of light (when you used the diffraction gratings) to the bands of light on this website: http://www.colorado.edu/physics/2000/quantumzone/  Read carefully the first part of this website.  Scroll down to the part where you can select different elements.  Use this information to compare what YOU saw to what you SHOULD have seen.  Be specific.

Lab Report:

For your lab report, staple together in this same order:

• Part A data sheet
• Part B data sheet
• Analysis questions and answers (word processed)