Question
What is the chemical composition of a cd rom?
Wanda
Answer
The materials that make up a compact disc are a transparent polycarbonate plastic and a reflective metal layer. Most plastics, polycarbonates included, are materials whose chemical structure has many repeating groups of atoms. Such materials are called polymers (where poly- means many ). The repeating unit of polycarbonate polymers has a carbon and three oxygen atoms (the "carbonate" unit ) along with another carbon-based portion. The metal layer is usually aluminum or silver. There is often another protective layer of transparent plastic called acrylic deposited on the metal layer. <BR>Music CDs and CD-ROMs have the structure described above. CD-R discs have an additional component, a light-sensitive dye, between the polycarbonate and metal layers. The ability to make changes in the light-sensitive dye is what allows a CD-R "burner" to "write" information onto a CD-R. A re-writable compact disc (CD-RW ) is yet another variation: the re-writable part between the polycarbonate and the reflective metal is made up of several very thin layers. That re-writable region has a layer containing an alloy (mixture ) of elements (silver, indium, antimony, and tellurium ) sandwiched between transparent layers of zinc sulfide or silicon dioxide. The ability to change the reflectance of the alloy between higher and lower reflective states is what allows a CD-RW "burner" to "write" information onto a CD-RW--and to write different information on it at a later time. <BR>
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