美国高中化学教材一

1 C:ChemistryXIUnit-1Unit-1(3)-Lay-4(reprint).pmd 27.7.6 (reprint) 1SOME BASIC CONCEPTS OF CHEMISTRY UNIT 1 After studying this unit, you will be able to understand and appreciate the role of chemistry in different spheres of life; explain the characteristics of three states of matter; classify different substances into elements, compounds and mixtures; define SI base units and list some commonly used prefixes; use scientific notations and perform simple mathematical operations on numbers; differentiate between precision and accuracy; determine significant figures; convert physical quantities from one system of units to another; explain various laws of chemical combination; appreciate significance of atomic mass, average atomic mass, molecular mass and formula mass; describe the terms mole and molar mass; calculate the mass per cent of different elements constituting a compound; determine empirical formula and molecular formula for a compound from the given experimental data; perform the stoichiometric calculations. SOME BASIC CONCEPTS OF CHEMISTRY Chemistry is the science of molecules and their transformations. It is the science not so much of the one hundred elements but of the infinite variety of molecules that may be built from them . Roald Hoffmann Chemistry deals with the composition, structure and properties of matter. These aspects can be best described and understood in terms of basic constituents of matter: atoms and molecules. That is why chemistry is called the science of atoms and molecules. Can we see, weigh and perceive these entities? Is it possible to count the number of atoms and molecules in a given mass of matter and have a quantitative relationship between the mass and number of these particles (atoms and molecules)? We will like to answer some of these questions in this Unit. We would further describe how physical properties of matter can be quantitatively described using numerical values with suitable units. 1.1IMPORTANCE OF CHEMISTRY Science can be viewed as a continuing human effort to systematize knowledge for describing and understanding nature. For the sake of convenience science is sub-divided into various disciplines: chemistry, physics, biology, geology etc. Chemistry is the branch of science that studies the composition, properties and interaction of matter. Chemists are interested in knowing how chemical transformations occur. Chemistry plays a central role in science and is often intertwined with other branches of science like physics, biology, geology etc. Chemistry also plays an important role in daily life. Chemical principles are important in diverse areas, such as: weather patterns, functioning of brain and operation 2 C:ChemistryXIUnit-1Unit-1(3)-Lay-4.pmd 2CHEMISTRY of a computer. Chemical industries manufacturing fertilizers, alkalis, acids, salts, dyes, polymers, drugs, soaps, detergents, metals, alloys and other inorganic and organic chemicals, including new materials, contribute in a big way to the national economy. Chemistry plays an important role in meeting human needs for food, health care products and other materials aimed at improving the quality of life. This is exemplified by the large scale production of a variety of fertilizers, improved varieties of pesticides and insecticides. Similarly many life saving drugs such as cisplatin and taxol, are effective in cancer therapy and AZT (Azidothymidine) used for helping AIDS victims, have been isolated from plant and animal sources or prepared by synthetic methods. With a better understanding of chemical principles it has now become possible to design and synthesize new materials having specific magnetic, electric and optical properties. This has lead to the production of superconducting ceramics, conducting polymers, optical fibres and large scale miniaturization of solid state devices. In recent years chemistry has tackled with a fair degree of success some of the pressing aspects of environmental degradation. Safer alternatives to environmentally hazardous refrigerants like CFCs (chlorofluorocarbons), responsible for ozone depletion in the stratosphere, have been successfully synthesised. However, many big environmental problems continue to be matters of grave concern to the chemists. One such problem is the management of the Green House gases like methane, carbon dioxide etc. Understanding of bio-chemical processes, use of enzymes for large-scale production of chemicals and synthesis of new exotic materials are some of the intellectual challenges for the future generation of chemists. A developing country like India needs talented and creative chemists for accepting such challenges. 1.2NATURE OF MATTER You are already familiar with the term matter from your earlier classes. Anything which has mass and occupies space is called matter. Fig. 1.1Arrangement of particles in solid, liquid and gaseous state Everything around us, for example, book, pen, pencil, water, air, all living bei