Physical Science 8th grade (1).pdf

The regularity of the silicon atoms in the crystal is

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The regularity of the silicon atoms in the crystal is what allows millions of tiny circuits on a computer “chip” to function identically. Figure 5.14: The shape of a salt crystal is due to the arrangement of sodium and chlorine atoms at the submicroscopic level. crystalline - solids that have an orderly, repeating pattern of molecules or atoms.
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106 U NIT 2 P ROPERTIES OF M ATTER Figure 5.15: The difference between crystalline solids and amorphous solids. Figure 5.16: Making a molded plastic part. amorphous - solids that do not have a repeating pattern of molecules or atoms. polymer - material in which individual molecules are made of long chains of repeating units Polymers Plastics You can probably look around you and see a dozen objects made of plastic. Because plastic can be created with an extremely wide range of physical properties, this material is used for many things. Some plastics are soft, like the polyurethane wheels on in-line skates. Other plastics are hard, like the polycarbonate used to make safety glasses. Still other plastics are slippery, like the nonstick surfaces on cooking pans. Amorphous solids Most plastics are examples of amorphous solids. The word amorphous comes from the Greek for “without shape.” Unlike crystalline solids, amorphous solids do not have a repeating pattern of molecules or atoms (Figure 5.15). Other examples of amorphous solids include rubber, wax, and glass. Polymers Plastics belong to a family of materials called polymers . The prefix “poly” means many. Polymers are materials in which individual molecules are made of long chains of repeating units. For example, ethylene is a molecule with two carbon and four hydrogen atoms. Polyethylene is a polymer made by joining ethylene molecules together in a long chain. Pure ethylene is a gas at room temperature. Polyethylene is a solid plastic that is used in containers, sandwich bags, and innumerable other applications. Why polymers are so useful Polymers are useful because they have melting points that are well above room temperature but much lower than most metals. In their liquid state, polymers (plastics) can be easily formed using molds (Figure 5.16). When the liquid cools and solidifies, the plastic object has good strength and elasticity. By altering the recipe and molecular structure it is possible to design polymers that have an incredible variety of physical properties.
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107 5.2 S OLID M ATTER C HAPTER 5: S TATES OF M ATTER Heat conduction in solids What is conduction? Heat conduction is the transfer of heat by the direct contact of particles of matter. When you hold a warm mug of tea or cocoa, you experience conduction. Heat is transferred from the mug to your hand. Conduction occurs between two materials at different temperatures when they are touching each other. Heat can also be transferred by conduction through materials. If you stir hot cocoa with a metal spoon, heat is transferred from the cocoa through the spoon and to your hand.
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