Physical Science 8th grade (1).pdf

B all oxidation numbers from the same molecule must

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b. All oxidation numbers from the same molecule must be positive. c. All oxidation numbers from the same molecule must be negative Figure 8.14: Question 2 Figure 8.15: Question 4 Figure 8.16: Question 7
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172 Salt: Common, But Uncommonly Important Timbuktu sounds like a faraway place, isolated, inaccessible - almost imaginary. In fact, it is in a remote location: at the edge of the Sahara in the West African nation of Mali. And today, Timbuktu is a ruined city, a harsh place under constant threat from desert sand and winds. It is hard to picture it as it once was - a thriving center of trade and culture. What could this old city have in common with salt? Beginning more than 900 years ago, Timbuktu became an important crossroads of African trade. Caravans of thousands of camels crossed hundreds of miles of desert to and from the city. Two commodities, which often traded at equal value and were used as money, drove the city’s fortunes: gold and salt. Today salt is among the most common substances we know. Why was it so valuable back then, literally worth its weight in gold? There were two big reasons. First, salt was much harder to find than it is today. Second, it was very important for preserving foods. Before refrigeration, salt was used to keep meat edible for long periods of time. The use of salt as a preservative contributed to the survival and expansion of human civilization. Salt in the modern world Today, salt is still important, but not as a food preservative. Of course, it is commonly used to season food. It is also used as a supplement in raising livestock and poultry. Salt has many industrial uses, too, in making paper, soap, detergent, and a variety of chemicals. In many parts of the United States, huge amounts of salt are used to melt ice on highways in winter. One of the chief ways to get salt is to mine it. Rock salt, also called halite, is mined from salt deposits which are usually deep in the ground. A mammoth salt deposit lies beneath portions of Ohio, Michigan, Pennsylvania, New York, West Virginia, and Ontario, Canada. This Great Eastern Salt Basin is one of the largest salt beds in the world. The salt was evaporated from ancient seas and deposited more than 300 million years ago. Today, in Cleveland, Ohio, this salt is mined from beneath Lake Erie. The salt mine is immense; its two main shafts are nearly 2,000 feet deep. The other common way to get salt is through evaporation. In the evaporation process, salt is gathered from seawater, salt marshes, or salt lakes. San Francisco Bay in California has one of the largest salt operations in the country; it is scooped from the bottom of coastal ponds. The Great Salt Lake in Utah is another huge evaporation site. Chapter 8 Connection
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173 U NIT 3 A TOMS AND THE P ERIODIC T ABLE The chemistry of salt Salt is a chemical compound composed of two elements, sodium and chlorine. It is called sodium chloride. The chemical symbol for salt is NaCl. A salt crystal contains one atom of sodium for each atom of chlorine.
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