Recommendations and Observations A watch glass can be used to extinguish the flame as one desires. Combustine onlv 5-10 mL of methanol a t a time reduces the hazarz of large fire and produces only a small auantitv of heat. allowinn the Petri dishes to be handled secommendedusinga polyethvlene bottle to deliver the methanol for both convenience and safety. L q e r amounts of methanol increase the length of the burn of the flame tests, a s well a s the intensity of the flame. I n most cases the comnounds can be stored in the Petri dish between denlonst;ations by covering each with a watch glass or matching Petri dish. Adding water to the methanol to increase the solubility of the salts did not improve the results. The success of this flame test is due to several factors.
a
the high temperature achieved by the methanol the clean-burning properties of methanol the greater solubilities of the salts in methanol over other alcohols After a short b u m the optimum temperature is achieved. Although some of the~ompoundshad some solubility in ethanol, it did not prove a s satisfactory as methanol because ethanol burns with a yello\r-orango flame d,.Generally the halides are the most effrctive in producing the desired results. Excellent results have heen ohtnined with the compounds listed in the table. Literature Cited 1.Gouge, E . M. J. Ckom. Ed=. 1988.65.545. 2. Arthur. PLPefvnDDmomlrotiom in Gomml Chem&fry:M&aw-Hill: NewYorkand Landon, 1939;p 311. 3. Infernoflsaol C~itLmll h b h ofNumrim1 Data, Pkwics, Chemlstq and l&hnoiogy, Vol. IY:Washburn. E. W.Ed.: Mffiraw-HiU New Yorkand London, 1928: pp 2 0 6
2w.
Line Spectra Using a CD Disc Henry Brouwer Redeemer College Ancaster, ON, Canada L9G 3N6
If you have ever viewed reflected light from a CD disc, you will have noticed that the full spectrum of the light source is resolved by the d i s c t h e disc behaves a s a diffraction grating. Such a CD disc also provides a n excellent method of viewing the emission spectrum from a gas discharge tube or from a n element heated in a flame.
Compounds Used and Colors Produced
Group IA lithium
Color / Group IIA I red
chloride sodium yellow iodide potassium faint calcium iodide purple chloride rubidium
chloride cesium chloride
faint violet faint blue
strontium chloride barium chloride
Color
orangered
Others boric acid
Color light green
copper(l1) green bromide with blue fringes
crimson yellowist
To view the spectrum, hold the CD disc behind the gas discharge tube and look a t the reflected light. In order to see the lines clearly, it is best to view the spectrum in a darkened mom. I t also may be necessary to move the disc slightly in order for others to see the lines. For example, the light from hydrogen shows three very distinct lines-violet, blue and red; whereas, helium shows four lines -blue, green, yellow, and red. The lines are clearly visible when viewed within 2-3 meters, but a t greater distances the violet, blue and green lines are more dimcult to see. To view the spectrum of an element heated i n a flame, place a slit (about 2-3 mm wide) made of paper or cardboard between the flame and the disc. Then view the light passing through this slit using the CD disc to observe the emission spedrum for a given element. [To excite the element in a flame, dip a nichrome wire having a loop a t one end into a compound or a solution of the c~mpoundcontaining the desired element and heat in the flame. I t is convenient to make a set of wires by inserting the nonlooped end of the nichrome wire into the wide end of a cork stopper, which is then used to stopper a vial containing the compound.] The line spectra are not a s distinct nor a s bright as with the gas discharge tubes and thus only students close to the apparatus will be able to view the spectra. Although replica plastic diffraction gratings can also be used for viewing the spectra, the lines are not a s bright a s with the CD disc. Unfortunately, due to the circular nature of the CD disc, the lines are curved, not parallel. Bending the disc straightens the line spectra somewhat, but i t decreases the distance between the lines.
Volume 69 Number 10 October 1992
829
