题目内容
The rocket engine, with its steady roar like that of a waterfall or a thunderstorm, is an impressive symbol of the new space age. Rocket engines have (76) powerful enough to shoot astronauts (77) the earth’s gravitational pull and (78) them on the moon. We have now become (79) in space. Impressive and complex (80) it may appear, the rocket, which was (81) in China over 800 years (82) , is a relatively simple device. Fuel that is (83) in the rocket engine changes (84) gas. The hot and rapidly (85) gas must escape, but it can do so only through an opening that faces backward. As the gas is radiated with great force, it pushes the rocket in the opposite direction.
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If we view a science as a body of systematized knowledge, then chemistry is usually called a natural science because it (26) knowledge of the natural world. At times we may wonder (27) there is no complete system (28) all of chemistry fits perfectly. Gaps in the present system, however, show that chemistry is still a(n) (29) subject and that we (30) all of its facts, laws, and theories. (31) , chemistry as a science is very much (32) us today, and its future holds the bright promise (33) much more to come. Man’s knowledge about himself and nature has grown into (34) sciences. The growth of the separate sciences has been more developmental than intentional. The separation of the natural sciences into physical and biological sciences, and physical sciences into physics and chemistry, happily (35) a larger body of knowledge into more manageable parts. At the same time we should remember that the concepts, techniques, and applications of the various sciences are interdependent and not exclusively a part of one science or (36) . In this (37) , chemistry is a key science among the natural sciences because everyone, (38) the area of natural science he wishes to pursue, needs at least an introduction to the principles and simpler applications of chemistry as a foundation for his specialty. Chemistry deals with the (39) of matter, changes in matter, the laws and principles (40) these changes, and the concepts and theories that interpret them.
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