LESSON 17: Technicolor Atoms: Flame Tests

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LESSON 17
Technicolor Atoms
Flame Tests

THINK ABOUT IT

HISTORY CONNECTION

HISTORY

CONNECTION

The original firecracker, called baozhu, was created in China around 200 B.C.E. It was a segment of green bamboo that was thrown onto a fire. Upon heating, the trapped gases inside the bamboo expanded, causing the bamboo to explode.

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peng wu/Getty Images

On the Fourth of July, you can watch colorful fireworks without considering the chemistry behind them. But each color that bursts forth in the sky is associated with a particular chemical compound. For example, any green sparkles you might see are probably due to a compound such as barium sulfate. But what’s responsible for the color? Is it the entire barium sulfate compound, or is it one of the atoms in the compound?

What evidence is there that certain atoms are present in a compound?

To answer this question, you will explore

Flame Tests

EXPLORING THE TOPIC

Flame Tests

Fireworks originated in China about 2000 years ago. The legend surrounding the discovery suggests that fireworks were discovered by a Chinese alchemist who mixed charcoal, sulfur, and potassium nitrate and accidentally produced a colorful gunpowder. Since then, the noise and the bright colors associated with fireworks have been used in celebrations all over the world.

In a chemistry lab, it is relatively easy to obtain the colorful flames associated with fireworks. You can do so by heating certain compounds in a flame, such as the flames of a Bunsen burner. The flame colors produced by heating four different compounds are shown here.

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© Lester V. Bergman/CORBIS
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© Lester V. Bergman/CORBIS
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© Lester V. Bergman/CORBIS
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© Lester V. Bergman/CORBIS

Different compounds produce different colors in a flame test. The metal in the compound may determine color.

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The colors for several more compounds are provided in the table. Take a moment to examine the data. What patterns do you notice?

Notice that each metal atom, Li, Ca, Na, and so on, is associated with a specific flame color. Lithium compounds all make a red flame, while the barium compounds all make a green flame. The nonmetal atoms in these compounds do not seem to affect the color of the compound. So CaSO4 and CaCl2 and Ca(NO3)2 all produce the same color flame, but CaCl2 and NaCl and CuCl2 do not. The metal atom must somehow be responsible for the color of the flame.

Evidence for Atoms in Compounds

Evidence for Atoms in Compounds

ASTRONOMY CONNECTION

ASTRONOMY

CONNECTION

Each element has a light “signature.” Astronomers can deduce the composition of a star from the colors of light that it emits. This photo shows the light emitted by the star Arcturus, one of the brightest stars in the night sky. When the light is separated by a prism in this way, you can see that a particular shade of blue is missing because it has been absorbed by iron in the star’s corona.

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Dr Juerg Alean/Science Source

Chemists have found these flame color patterns to be quite helpful. A flame test can be used to quickly confirm the presence of certain metal atoms in an unknown sample. So, a potassium compound can quickly be distinguished from a calcium compound by heating samples of the compounds.

Also, the color patterns associated with the flame tests clearly indicate whether certain metal atoms are present within the compounds. For example, copper metal turns the flame blue-green and so does copper chloride. This is evidence that there are copper atoms in the copper chloride.

Excited Electrons

Excited Electrons

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Heating metal compounds in a Bunsen burner does not destroy the atoms in them or create new elements. This is because the temperature of a flame is not high enough to change the nuclei of metal atoms. However, the heat does have an impact on the electrons of these metal atoms.

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Electrons are located at different average distances from the nucleus of an atom. When heated, some electrons of metal atoms get “excited” and move to distances farther from the nucleus. This move is only temporary. When the electrons move back to their original distance from the nucleus, they release energy in the form of colored light. This is what you see during a flame test.

The fact that it is relatively easy to affect the electrons in atoms opens up new possibilities for exploration. Perhaps the solution to creating substances with the properties of gold rests in altering the electrons in some way. The nuclei of atoms cannot be changed easily, so it is time to explore ways in which the electrons in atoms can be changed. The rest of this unit explores the role of the electrons in the chemistry of atoms and compounds.

Example

Flame Colors

Which of these compounds will give similar flame colors when heated?

NaCl CaCl2 SrCl2 Sr(NO3)2 Cu(NO3)2

Solution

The color of the flame depends on the metal. The only two compounds with the same metal are strontium chloride, SrCl2, and strontium nitrate, Sr(NO3)2.

LESSON SUMMARY

LESSON SUMMARY

What evidence is there that certain atoms are present in a compound?

KEY TERM

flame test

Many metal atoms produce a characteristic colored flame when compounds containing those atoms are heated. The colors are a result of light energy produced when excited electrons return to their original distance from the nucleus in the metal atoms. Flame tests provide evidence that certain atoms are present in compounds and that they are not destroyed during chemical changes. The flame test also demonstrates that it is easier to add or remove electrons in atoms, which can be done with a small amount of energy, than to alter the protons and neutrons in the nucleus, which requires a great deal of energy.

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Exercises

Reading Questions

  1. How did the flame test provide evidence that specific atoms are present in compounds?

  2. Explain what is responsible for the colors during a flame test.

Reason and Apply

  1. Find out why fireworks are so colorful. What substances are used to produce the colors?

  2. Predict the color of the flame for the compound sodium hydroxide, NaOH. Explain your reasoning.

  3. Imagine that you were in charge of creating a red and purple fireworks display. Name two combinations of compounds you could use.

  4. What evidence do you have from flame tests that copper is responsible for producing a flame with a blue-green color?

  5. Does nitrate produce a colored flame? Explain your thinking.

  6. Would it matter whether you did a flame test with sodium chloride, NaCl, in solid form or sodium chloride as an aqueous solution? Explain.

  7. What flame colors would be produced by these compounds? Explain your choices.

    1. Na2CO3

    2. Ba(OH)2

    3. KOH

    4. K2CO3

    5. BaO

  8. If two chemical samples both produce an orange flame upon testing, which statement is true?

    1. The two samples contain identical compounds.

    2. The samples both contain chlorine atoms.

    3. The samples both contain calcium atoms.

    4. The samples both contain potassium atoms.

    5. The samples contain different compounds.

  9. What evidence supports the claim that chloride, Cl–, does not cause the flame to have a color?

    1. Lithium chloride, LiCl, and sodium chloride, NaCl, have different colors.

    2. Ammonium chloride, NH4Cl, does not cause the flame to have a color.

    3. Sodium chloride, NaCl, and sodium nitrate, NaNO3, both produce flames with a yellow-orange color.

    4. All of the above.