tailieunhanh - English for students of Physics_Unit 13

The solid state and the structure of Solids We all live on terra firma, the 29 percent of our planet’s solid crust that lies above sea level. And almost everything we do is tied to solids, living in houses, creating and marketing solid goods, eating solid foods, and so on. But if you are asked to define a solid, it might be difficult. A solid is one of those familiar things that are hard to put into words. A good definition of a solid is that it tends to keep its shape when it is left alone. But that doesn’t. | 125 UniT THIRTEEN PHASE OF MATTER READING PASSAGE The solid state and the structure of Solids We all live on terra firma the 29 percent of our planet s solid crust that lies above sea level. And almost everything we do is tied to solids living in houses creating and marketing solid goods eating solid foods and so on. But if you are asked to define a solid it might be difficult. A solid is one of those familiar things that are hard to put into words. A good definition of a solid is that it tends to keep its shape when it is left alone. But that doesn t mean a solid is necessarily rigid. Rubber bands books and the clothes you wear- these flexible materials maintain their shape to some degree. They aren t rigid but they are solid. We ve seen that at the atomic level the atoms or molecules bonded together in a solid stay in place with respect to their neighbors. The strength and rigidity of the solid then depends to some degree on how strong the bonds are between those atoms or molecules. But more is involved than just bonds. Diamond the hardest natural substance and graphite which is so soft and slippery that it its used to lubricate door locks are both pure forms of carbon atoms held together with covalent bonds. The difference that makes one so hard and the other soft is the structural arrangements of their atoms. In any solid the atoms or molecules are in fixed positions. When there is an order that is a pattern in the placement of the molecules or atoms that repeats throughout the solid it is called crystalline. Examples of crystalline solids are table salt diamonds quartz and ice. If the molecules or atoms in a solid have no particular arrangement fitting together in a seemingly random way the solid is called amorphous. Plastics glass and the cement in concrete are examples of amorphous solids. However many solids have mixed structures. Rocks such as sandstone and granite are amorphous composites of small crystals of different chemical compositions. Whether a .

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