Physical Science 8th grade (1).pdf

In this reaction iron replaces copper in the solution

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copper forming on the iron nail. In this reaction, iron replaces copper in the solution and the copper falls out of the solution as a metal: Precipitation occurs when one product is insoluble In many reactions dissolved substances react to form substances that are no longer soluble. The insoluble product drops out of solution, forming a precipitate . A precipitate is a solid product that comes out of solution in a chemical reaction. Precipitates usually form many small particles which cause a cloudy appearance in a solution (Figure 10.8). The limewater test for carbon dioxide is a precipitation reaction. In this test, a gas suspected of containing carbon dioxide is bubbled through a solution of CaO 2 H 2 (limewater). Any carbon dioxide in the gas reacts to form a precipitate, turning the solution milky-white.
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213 10.2 E NERGY AND CHEMICAL REACTIONS C HAPTER 10: C HEMICAL R EACTIONS Combustion reactions Petroleum is a mixture of hydrocarbons Almost 40 percent of all the energy we use comes from petroleum (oil) and 2/3 of that is gasoline and diesel fuel. Petroleum is not a single substance but a complex mixture of many substances created over millions of years by the decay of plants and animals. The major elements in petroleum are hydrogen and carbon, with smaller amounts of oxygen, nitrogen, and sulfur. Refining The refining process separates petroleum into molecules with different numbers of carbon atoms. The smaller molecules are used in gasoline. Heavier molecules become kerosene and heating oil. The heaviest molecules become tar and asphalt used for roads (Figure 10.9). The reactions of burning gasoline In a perfect reaction, all the hydrocarbon molecules are completely burned to into carbon dioxide and water. Unfortunately, in an engine not all the fuel burns completely and pollutants such as carbon monoxide are also formed. Impurities in fuel, such as sulfur and nitrogen in the air, also have reactions that form pollutants such as oxides of nitrogen and sulfuric acid. Range of molecule sizes End use C 1 - C 12 Gasoline and light fuels, such as aviation fuel C 12 - C 18 Kerosene and heating oil C 19 - C 30 Grease, motor oil, wax C 31 - C 36+ Tar and asphalt Figure 10.9: Some of the many molecules found in gasoline.These are examples of hydrocarbons, molecules made with only hydrogen and carbon.
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214 U NIT 4 M ATTER AND C HANGE Figure 10.10: Comparing nuclear and chemical reactions. nuclear reaction - a process that changes the nucleus of an atom and may turn one element into a completely different element. Nuclear reactions What is a nuclear reaction? Nuclear reactions change the nucleus of an atom. Until just 100 years ago people looked for a way to turn lead into gold. With today’s understanding of nuclear reactions, it is now possible. However, we don’t do it very often because the process is much more expensive than gold itself!
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