The Question:
How is radiation measured?
Background Information (can be used as Post-lab Discussion).
The beautiful colored flames that you observe in this experiment are put to work commercially in the form of fireworks. Most fireworks incorporate the metal ions used in this lab to produce the common fireworks colors: blue(copper), green(barium), red(strontium), and orange(calcium). Magnesium is also used to produce white light.Although the color of a flame is said to be particular to the kind of metal atom, the color of the fireworks flame is considerably different from that seen in the laboratory. This effect is primarily to the presence of oxidizers and fuels that produce unwanted colors or otherwise mask the light given off by the coloring agents. Sodium is often viewed as an impurity because a small amount of sodium can cause a significant change in flame color. The metal ions are usually bound to an anion (a negative ion), forming a solid at room temperature. The anions influence the temperature of the flame. For example, a flame containing barium nitrate will burn at a higher temperature than chloride. This difference in temperature sometimes causes different hues to be emitted from the same metal ions.
Introduce students to an African-American scientist, Dr. George Edward Alcorn. He works with various spectrometers and designs space instrumentation.Please see this web site: http://www.lib.lsu.edu/lib/chem/display/alcorn.html
Caution: High voltages are required when using gas spectrum tubes. Do not touch the spectrum tubes or high voltage apparatus. The teacher should change the spectrum tubes as necessary.
Caution: When using a flame, make sure that long hair and loose clothing are secured before proceeding with the lab.
Caution: Some of the salts used to saturate the toothpicks are poisonous or irritating to the skin. Do not pick up the toothpicks with your fingers. Keep your hands away from your mouth until after you have washed them and have left the laboratory.
Caution: Do not let the spectroscope get too close to the flame. The heat could melt the diffraction grating inside the spectroscope.
Procedure.
1. Obtain a spectroscope or diffraction grating from your teacher. View an incandescent light source through the spectroscope. Record your observations. Note the positions and order of the colors.2. Your teacher will supply power to a spectrum tube of a particular gas. View the glowing gas through the spectroscope. Record your observations. Note the color, position, and relative number of the bright emission lines.
3. Repeat step 2 (the teacher will change the spectrum tube) for the remaining gas tubes.
4. Set up and light a Bunsen burner. Using forceps (not your fingers!) pick up a saturated toothpick, holding it at one end. Hold the toothpick in the hottest part of the flame and observe what happens. Note the color(s) of the flame and the duration of each color. When the toothpick ignites or begins to char, remove it from the flame.
5. Place the charred toothpick on a watchglass and record your observations of the flame in a data table.
6. For this next step, assign these roles to members of your lab group:
a. one person to hold the toothpick in the flame,b. one person to view the flame through the spectroscope,
c. a third person (if available) to record data as it is called out by the viewer; otherwise, record the data after the flame has been extinguished.
With the second toothpick for a given metallic ion, observe the flame through the spectroscope. Record your observations. Note the color, position, and relative number of bright emission lines.
7. Repeat steps 4 through 6 for each of the five remaining known metals.
8. Repeat steps 4 through 6 for each of the unknown metals. If extra toothpicks are available, you may wish to view the flames of the known metals again to compare with the unknowns.
9. When finished, empty the watchglass onto a wet paper towel. Roll up the paper towel and discard in the trash. Return the spectroscope to your teacher.
Assessment 1.
Students answer the questions below.Analysis 1. Which of the observed light sources displayed a continuous spectrum? List the colors of the spectrum in order.
2. Is the number of visible emission lines equal to the number of electrons for an element? Give evidence in support of your answer.
3. How are the spectrum tubes similar to the Crookes tube?
4. What was (were) the identity of the metal ion(s) in the unknown solutions? Give evidence in support of your conclusions.
5. Predict the flame color if metallic ions of copper and strontium were mixed.
6. Based on the flame colors in this experience, which single element would most likely be used to produce yellow fireworks? Why?