Physical Science 8th grade (1).pdf

Elasticity describes a solids ability to be stretched

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Elasticity describes a solid’s ability to be stretched and then return to its original size. This property also gives objects the ability to bounce and to withstand impact without breaking. Brittleness Brittleness is defined as the tendency of a solid to crack or break before stretching very much. Glass is a good example of a brittle material. You cannot stretch glass even one-tenth of a percent (0.001) before it breaks. To stretch or shape glass you need to heat the glass until it is almost melted. Heating causes molecules to move faster, temporarily breaking the forces that hold them together. Ductility One of the most useful properties of metals is that they are ductile. A ductile material can be bent a relatively large amount without breaking. For example, a steel fork can be bent in half and the steel does not break. A plastic fork cracks when it is bent only a small amount. Steel’s high ductility means steel can be formed into useful shapes by pounding, rolling, and bending. These processes would destroy a brittle material like glass.
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105 5.2 S OLID M ATTER C HAPTER 5: S TATES OF M ATTER Crystalline solids Crystalline solids Almost everyone would recognize this solid as a crystal. In science, however, crystal has a broader meaning. The atoms (or molecules) in a solid can be arranged in two fundamentally different ways. If the atoms are in an orderly, repeating pattern, the solid is called crystalline . Examples of crystalline solids include salts, minerals, and metals. The geode in the picture is a mineral crystal. Many solids are crystalline Most naturally occurring solids on Earth are crystalline . This is most evident when materials exist as single crystals, like salt, for instance. If you look at a crystal of table salt under a microscope, you see it is cubic in shape. If you could examine the arrangement of atoms, you would see the shape of the crystal comes from the cubic arrangement of sodium and chlorine atoms (Figure 5.14). The external shape of a crystal reflects the internal arrangement of atoms and molecules. Multicrystal solids Metals like steel are also crystalline. They don’t look like “crystals” because solid metal is made from very tiny crystals fused together in a jumble of different orientations (Figure 5.14). But on the microscopic level, atoms in a metal are arranged in regular crystalline patterns. The diagram shows two common patterns, cubic and hexagonal. Crystal silicon One of the most important crystalline elements is silicon. Silicon crystals are the foundation of microelectronics. Almost all the electronic circuits in cell phones, computers, and innumerable other devices are made from pure silicon crystals that have been sliced into wafers. Microscopic electric circuits are printed on the silicon wafers.
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