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B) radiation d) flame f) chain reaction h) oxygen
1. The place where burning is happening and the gases are very hot and give out light. 2. A substance that burns to give heat energy which can be changed into other forms of energy. 3. An odoureless, invisible gas which supports combustion and is a reactive gas. 4. Brown deposit on iron due to corrosion by water and air. 5. The action of sending out heat, light, x – rays or radio waves. 6. A form of energy, flowing from a hot body. 7. A reaction which gives reactive products that go on to make more reactive products and more reaction and so on. 8. The temperature to which a substance must be heated before combustion can take place in air. XIX. Write five questions which will test whether others in your group have really understood the text
TEXT 2 Fire For a fire to start and spread three things are needed: fuel (to vaporize or burn), oxygen (to combine with fuel vapour), and heat (to raise the temperature of the fuel vapour to its ignition temperature). This is the classic "fire triangle." Fire is defined as rapid oxidation, accompanied by light and heat. Oxidation is simply the reaction of anything with oxygen. Remove any of the three factors from a fire and the fire will go out. The fire triangle also illustrates two important facts inpreventing and extinguishing fires: if any side of the fire triangle is missing, a fire cannot start, and if any side of the fire triangle is removed, the fire will go out. A fuel may be a solid, a liquid or a gas, but combustion will only occur when the fuel is in the form of a vapour. Solid and liquid fuels must have heat applied to them to vaporize before they can ignite. Gases are of course already in a vapour form. The oxygen side of the fire triangle refers to the oxygen of the surrounding air. Generally, a minimum concentration of 16 % oxygen in the air is needed to support flaming combustion. However, smouldering combustion can take place in about 3 % of oxygen. Air normally contains about 21 % of oxygen, 78 % of nitrogen and 1 % of other gases, mainly argon. The third side of the fire triangle is heat. Most materials require the application of heat in one form or another to bring them to the ignition temperature. Thus the ignition temperature of a substance (solid, liquid or gas) is the lowest temperature at which sustained combustion will occur without the application of a spark or flame. Ignition temperatures vary among substances. The ignition temperatures of common combustible materials lie between 149°C (300°F) and 538°C (1000°F). Heat from fire is transferred by one or more of three methods: conduction, convection and radiation. In the case1 of conduction heat travels from regions of high temperature to regions of low temperature, for example, the heat passes from one molecule to the next one and so on. The ability to conduct heat (thermal conductivity) varies between materials. Convection occurres only in liquids and gases. When the fluid is heated it expands and becomes denser. The lighter (hotter) fluid rises and cooler fluid takes its place2. The cool fluid is heated in turn3 and then also rises, and so on. In the case of radiation the energy from a heat source can be transmitted to objects at some distance by radiation similar to light waves, but with longer wavelengths. This occur in many residential fires starting with things like clothes drying in front of a fire. The fire triangle is a simple means of illustrating the three requirements for the fire existence. However, it does not explain the nature of fire. In particular4, it does notinclude the chain reaction that results from chemical reaction of the fuel, oxygen and heat. The fire tetrahedron is a better representation of the combustion process. A tetrahedron is a solid figure with four triangular faces. It is useful for illustrating and remembering the combustion process because it has room for the chain reaction and because each face touches the other three faces. The basic difference between the fire triangle and the fire tetrahedron is the following: the tetrahedron illustrates how flaming combustion is supported and sustained through the chain reaction. In a sense5, the chain reaction face keeps the other three faces from falling apart. This is an important point, because the extinguishing agents used in many modern portable fire extinguishers, automatic extinguishing systems and explosion suppression systems directly attack and break down the chain reaction sequence. Notes 1. in the case of – в случае, когда речь идет о 2. to take the place – заместить, замещать 3. in turn – в свою очередь 4. in particular – в частности, в особенности 5. in a sense – в некотором смысле, в известном отношении Дата добавления: 2015-09-18 | Просмотры: 831 | Нарушение авторских прав |