|glass (glās) Pronunciation Key
A usually transparent or translucent material that has no crystalline structure yet behaves like a solid. Common glass is generally composed of a silicate (such as silicon oxide, or quartz) combined with an alkali and sometimes other substances. The glass used in windows and windshields, called soda glass, is made by melting a silicate with sodium carbonate (soda) and calcium oxide (lime). Other types of glass are made by adding other chemical compounds. Adding boron oxide causes some silicon atoms to be replaced by boron atoms, resulting in a tougher glass that remains solid at high temperatures, used for cooking utensils and scientific apparatuses. Glass used for decorative purposes often has iron in it to alter its optical properties.
Our Living Language : Common sand and glass are both made primarily of silicon and oxygen, yet sand is opaque and glass is transparent. Glass owes its transparency partly to the fact that it is not a typical solid. On the molecular level, solids usually have a highly regular, three-dimensional crystalline structure; the regularities distributed throughout the solid act as mirrors that scatter incoming light. Glass, however, consists of molecules which, though relatively motionless like a typical solid, are not arranged in regular patterns and thus exhibit little scattering; light passes directly through. At a specific temperature, called the melting point, the intermolecular forces holding together the components of a typical solid can no longer maintain the regular structure, which then breaks down, and the material undergoes a phase transition from solid to liquid. The phase transition in glass, however, depends on how quickly the glass is heated (or how quickly it cools), due to its irregular solid structure.
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