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Unit 2 The atmosphere Teaching Notes 2.2 Classification of matter: pure substance and mixture Homogeneous and heterogeneous mixtures Mixtures can be homogeneous or heterogeneous. Homogeneous mixtures have uniform composition and properties throughout. For example, when sugar dissolves in water, it forms a solution in which the sugar and water are uniformly mixed. The mixture is therefore homogeneous. A container of air is another homogeneous mixture. Heterogeneous mixtures are those which are not uniform in composition and properties throughout. For example, granite is a heterogeneous mixture consisting of the minerals quartz, feldspar and mica. It is usually possible to identify these components by the eyes. Concrete is a heterogeneous mixture consisting of cement, sand and rock. 2.3 Elements and compounds Main differences between a mixture and a compound Summary of difference between a mixture and a compound Mixture Compound 1 Composition by mass The amounts of the different substances in the mixture can vary The amounts of the elements in the compound are in a fixed ratio 2 Separating the constituents Constituents can be separated by simple physical methods, such as evaporation or distillation Can only be separated into its constituents by chemical methods 3 General properties Properties are similar to the substances making up the mixture Properties are very different from the elements in it 4 Melting point and boiling point Does not have a sharp melting point or boiling point With sharp melting point and boiling point 5 Energy change during formation No or little energy change when a mixture is made Energy is usually given out or absorbed when a compound is formed 2.4 The atmosphere The four layers of the Earth’s atmosphere The four layers of the Earth’s atmosphere are the troposphere, the stratosphere, the mesosphere and the ionosphere. We live in the troposphere with trips into the stratosphere. Only astronauts pass out of the atmosphere. The atmosphere in balance All living things depend on oxygen, nitrogen, carbon dioxide and water vapour. These substances are recycled. The process of photosynthesis and respiration recycle oxygen. oxygen in air oxygen taken in during respiration oxygen given off during photosynthesis plants build up starches from glucose animals break down plant starches to glucose during respiration Nitrogen circulates between the air, the soil and living things in the nitrogen cycle. nitrogen in air proteins in living things bacteria, lightning, fertilizers decay nitrates in soil denitrifying bacteria Carbon dioxide circulates between the air, the soil and living things ?and in and out of the ocean ?in the carbon cycle. carbon dioxide in air bruning fossil fuels respiration Photosynthesis deca y carbon compounds in plants carbon compounds in animals carbon compounds in fossil fuels Water circulates between the air, the ocean and living things in the water cycle. water vapour in air condensation evaporation rain ocean In the last 200 years (ever since the Industrial Revolution) the balance of carbon dioxide has been upset by the burning of too much fossil fuel. Most scientists agree that this is the cause of global warming. 2.6 Separating oxygen and nitrogen from the air Uses of oxygen Manufacturing steel - Oxygen is blown through the molten iron to remove carbon and sulphur impurities. The carbon and sulphur are converted to carbon dioxide and sulphur dioxide and escape as gases. Welding and cutting - Oxygen is used in oxy-acetylene welding and cutting. When acetylene burns in oxygen, the temperature reaches 3 200 ℃. This is hot enough to melt most metals which can then be cut or welded together. Breathing apparatus - Oxygen is used in life-support machines in hospitals. It is also mixed with anaesthetizing gases during surgical operations. Mountaineers and deep sea divers also use oxygen. Space rockets and shuttles - Oxygen is carried in space rockets so that the hydrogen and kerosene fuels can burn. Space shuttles use oxygen gas in fuel cells which convert chemical energy into electrical energy. Uses of nitrogen Manufacturing ammonia and nitric acid - Nitrogen is used in large quantities in the production of ammonia gas, which is used to produce nitric acid. Nitric acid is used in the manufacture of dyes, explosives and fertilizers. As a refrigerant - Liquid nitrogen is used as a refrigerant. Its low temperature makes it useful for freezing food quickly. Providing an inert atmosphere - Nitrogen is used as an inert atmosphere for some processes because it is unreactive. For example, empty oil tankers are filled with nitrogen to prevent fires. 2.9 Air quality in Hong Kong Chemistry and Society The disappearing ozone layer Why is the ozone layer so important? Ozone is a gas found in the Earth’s atmosphere some 10 to 30 km above the Earth. This layer of ozone gas absorbs some of the sun’s radiation that is harmful to living things. Too much of the harmful radiation can cause skin cancer, as well as other health problems. The ozone layer is breaking down In 1986, scientists discovered two holes in the ozone layer - a small hole over the North Pole, and a much larger hole over Antarctica. Scientists reported that the holes have grown bigger and the ozone layer has become thinner all around the Earth. October 1979 October 1990 October 1996 October 1982 October 1991 October 1997 October 1984 October 1992 October 1998 October 1986 October 1993 October 1999 October 1988 October 1994 October 2000 Dobson units 100 500 Chlorofluorocarbons A possible cause of the holes is a group of chemicals called chlorofluorocarbons (CFCs). CFCs are used in automobile air conditioners, in some aerosol sprays, in refrigerators and foam packaging for fast-food containers. CFCs enter the atmosphere when these products are made and used. In the upper atmosphere, CFCs are broken down by ultraviolet radiation, releasing chlorine atoms. These chlorine atoms destroy ozone. Government action and future trends International concerns regarding ozone depletion has led governments around the world to phase out the use of CFCs. Recently, CFCs have been replaced as aerosol propellants by hydrocarbons and the use of manual pressure packs. Alternative chemicals have been developed for use in refrigeration and air-conditioning systems. However, some scientists say that even if we stop using CFCs today, the ozone layer would continue to thin for the next 10 years. How can you help? You may feel that you cannot change laws or influence the policies of international companies. However, politicians and business people can be affected by the way you act. If people stopped buying products containing CFCs, companies that make them would need to find an alternative ?and that process is already starting. Imagine you have to buy a box of eggs. If you buy eggs packed in a recycled paper container, instead of in a foam container that contains CFCs, you have made a decision that will help the environment. Questions 1 Suggest how you can help to protect the ozone layer. 2 Visit the website of the Environmental Protection Department of Hong Kong Government http://www.epd.gov.hk/epd/ Find out the status of phasing out of ozone depleting substances in Hong Kong. Notes for questions 1 Students may suggest the following points: Buy air-conditioning equipment that does not use CFC as refrigerant. Conduct regular inspection and maintenance of air-conditioning and refrigeration appliances to prevent and minimize refrigerant leakage. Replace air-conditioning and refrigeration appliances operating on CFC refrigerant with those that use non-CFC refrigerant. 2 Phase out schedule of ozone depleting substances in Hong Kong Ozone depleting substance Schedule Halons Import for local consumption banned by 1.1.1994 CFCs Carbon tetrachloride 1,1,1-trichloroethane Import for local consumption banned by 1.1.1996 HCFCs Freeze consumption at base level starting 1.1.1996 Import for local consumption banned by 1.1.2030