Chemistry Grade 9 Textbook PDF

CHEMISTRY CHEMISTRY STUDENT TEXTBOOK GRADE 9 CHEMISTRY STUDENT T...

CHEMISTRY CHEMISTRY STUDENT TEXTBOOK GRADE 9 CHEMISTRY STUDENT TEXTBOOK GRADE 9 STUDENT TEXTBOOK GRADE 9 Agriculture Building Bonding Construction Chemistry Medicine Food Production Measurments CHEMISTRY Textbook FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF EDUCATION MINISTRY OF EDUCATION  CHEMISTRY STUDENT TEXTBOOK GRADE 9 Writers: Sisay Tadesse (Ph.D.) Tegene Tesfaye (Ph.D.) Editors: Tesfaye Semela (Ph.D.) (Curriculum Editor) Kenenisa Beresa (M.A.) (Language Editor) Ahmed Awel (M.Sc.) (Content Editor) Illustrator: Abinet Tilahun (M.Sc.) Designer: Konno B. Hirbaye (M.Sc.) Evaluators: Legesse Adane (Ph.D.) (Reviewer) Nega Gichile (B.Sc., M.A.) (Evaluator) Sefiw Melesse (M.Sc.) (Evaluator) Tolessa Mergo (B.Sc., M.Sc.) (Evaluator) Federal Democratic Republic of Ethiopia Hawassa University Ministry of Education First Published August 2023 by the Federal Democratic Republic of Ethiopia , Ministry of Education, under the General Education Quality Improvement Program for Equity (GEQIP-E) supported by the World Bank, UK’s Department for International Development/DFID-now merged with the Foreign, Common wealth and Development Office/FCDO, Finland Ministry for Foreign Affairs, the Royal Norwegian Embassy, United Nations Children’s Fund/UNICEF), the Global Partnership for Education (GPE), and Danish Ministry of Foreign Affairs, through a Multi Donor Trust Fund. © 2023 by the Federal Democratic Republic of Ethiopia, Ministry of Education. All rights reserved. The moral rights of the author have been asserted. No part of this textbook reproduced, copied in a retrieval system or transmitted in any form or by any means including electronic, mechanical, magnetic, photocopying, recording or otherwise, without the prior written permission of the Ministry of Education or licensing in accordance with the Federal Democratic Republic of Ethiopia as expressed in the Federal Negarit Gazeta, Proclamation No. 410/2004 - Copyright and Neighboring Rights Protection. The Ministry of Education wishes to thank the many individuals, groups and other bodies involved – directly or indirectly – in publishing this Textbook. Special thanks are due to Hawassa University for their huge contribution in the development of this textbook in collaboration with Addis Ababa University, Bahir Dar University and Jimma University. Copyrighted materials used by permission of their owners. If you are the owner of copyrighted material not cited or improperly cited, please contact the Ministry of Education, Head Office, Arat Kilo, (P.O.Box 1367), Addis Ababa Ethiopia. Printed by: GRAVITY GROUP IND LLC P.O.Box 13TH Industrial Area, Sharjah UNITED ARAB EMIRATES Under Ministry of Education Contract no. MOE/GEQIP-E/LICB/G-01/23 ISBN: 978-9999 0 - 0 -016-1 CONTENTS 1 Unit : CHEMISTRY AND ITS IMPORTANCE 1 1.1 Definition and Scope of Chemistry 2 1.2 Relationship between Chemistry and Other Natural Sciences 6 1.3 The Role Chemistry Plays in Production and in the Society 8 1.4 Some Common Chemical Industries in Ethiopia 12 Unit Summary 15 Review Exercise 16 Unit 2 : MEASUREMENTS AND SCIENTIFIC METHODS 18 2.1 Measurements and Units in Chemistry 19 2.2 Chemistry as Experimental Science 35 Unit Summary 47 Review Exercise 48 Unit 3 : STRUCTURE OF THE ATOM 50 3.1 Historical Development of the Atomic Theories of Matter 51 3.2 Fundamental Laws of Chemical Reactions 56 3.3 Atomic Theory 64 3.4 Discoveries of Fundamental Subatomic Particles and the Atomic Nucleus 69 3.5 Composition of an Atom and the Isotopes 85 Unit Summary 101 Review Exercise 105 I  Unit 4 : PERIODIC CLASSIFICATION OF ELEMENTS 109 4.1 Historical Development of Periodic Classification of the Elements 110 4.2 Mendleev’s Classification of the Elements 113 4.3 The Modern Periodic Table 116 4.4 The Major Trends in the Periodic Table 125 Unit Summary 137 Review Exercise 138 Unit 5 : CHEMICAL BONDING 140 5.1 Chemical Bonding 141 5.2 Ionic Bonding 143 5.3 Covalent Bonding 154 5.4 Metallic Bonding 167 Unit Summary 171 Review Exercise 173 II U NI T  CHEMISTRY AND ITS IMPORTANCE Unit Outcomes After completing the unit, you will be able to  define chemistry;  describe its’ scope;  discuss the relationships between chemistry with physics, biology, medicine geology and other subjects;  describe the application of chemistry in the field of agriculture, medicine, food production and building construction;  name some common chemical industries in Ethiopia and their Product. 1 Chemistry Grade 9 Make groups and discuss on the following questions and present your discussion points to the class. 1. What do you think that the following materials are made of? Start-up Activity  the air you breath,  the water you drink,  the cloth you wear, and  the food you eat? 2. Why is everything in this world changing from time to time? 3. Is it important to know the materials from which everything is made up of? Why? 4. Mention some of the materials you are using commonly. Did you know who manufactured them? Since chemistry is so fundamental to our world, it plays a role in everyone’s lives and touches almost every aspect of our existence in some way. Chemistry is essential for meeting our basic needs such as food, clothing, shelter, health, energy, clean air, water, and soil. The question is how? In this unit, the definition and the scope of chemistry, the relationship between chemistry and other natural sciences, the role it plays in production and in the society, and some common chemical industries in Ethiopia and their products are presented thoroughly. 1.1 Definition and Scope of Chemistry In this section two aspects of chemistry are going to be dealt with. The first is definition of chemistry, which will be followed by the scope of chemistry. We shall begin by defining chemistry first. 1.1.1 Definition of Chemistry At the end of this section, you will be able to define chemistry. Students, form a group of three or four and discuss the question given below. Present your discussion points to the class when asked by your teacher. 1. Did you know what the composition of the salt you are adding to your meal is? 2  Chemistry and its Importance 2. What do you think will happen to the sugar crystals when you add a teaspoon of it in a cup of tea and stir it? 3. How can you distinguish table salt from sugar? 4. What happens to the wood when you burn it? Activity 1.1 Chemistry is the science that deals with the properties, composition, and structure of substances (elements and compounds), the transformations they undergo, and the energy that is released or absorbed during these processes. A substance is a particular kind of matter with uniform properties. Example, gold, silver, water, soap, table salt, etc (Figure 1.1). Matter is a physical substance, that which occupies space and possesses rest mass. Example, book, pencil, television, stool, etc. The property of a substance is its attribute, quality, or characteristic. Every substance, in the universe in which we live, has its own properties by which we can distinguish it from other substances. This is because every substance has its own unique composition and structure. Example, water is a substance that has no color, taste, and shape. Composition is the nature of something’s ingredients or constituents; how a whole or mixture is made up. Example, table salt is chemically composed of the elements sodium and chlorine. The stainless-steel spoons are solid solution (alloy) of chromium, carbon and other elements. The arrangement and relationships between the parts or elements of something complex is known as its structure. Example, the school buildings are made up of roof, ceiling, doors, windows, walls, and floor arranged in a certain order. The arrangement of each of these parts are known as the structure of the school building. Salt Sulphur Gold Silver Figure 1.1 Substances around us. Every substance in our environment is continuously changing from time to time due to both external and internal forces. Due to this change, it transforms from one form into the other. The transformation of a substance is a marked change in form, nature, or appearance. These transformations are accompanied by energy changes. 3 Chemistry Grade 9 Exercise 1.1 1. Define the term chemistry. 2. Explain the meaning of the following phrases.  property of a substance  composition of a substance  structure of a substance  transformation of a substance 1.1.2 Scope of Chemistry At the end of this section, you will be able to explain the scope of chemistry. Students, form groups of three or four. Discuss the following questions and present your discussion points to the class. 1. How do you clean: your cloth when it gets dirt, the dishes after eating meal, and your hand? 2. How does the butcher in your town measure the weight of beef before selling it? 3. Do you know how the clothes and shoes you wear, the tyres of automobiles, the different medicines you take when you are ill, and the glasses in the windows of your house are made? 4. What happens to the food in your body, after you ate it? Activity 1.2 The study of modern chemistry has many branches, but can generally be broken down into five main disciplines, or areas of study: i. Physical chemistry: It is the study of macroscopic properties, atomic properties, and phenomena in chemical systems. A physical chemist may study such things as the rates of chemical reactions, the energy transfers that occur in reactions, or the physical structure of materials at the molecular level. ii. Organic chemistry: It is the study of substances containing carbon. Carbon is one of the most abundant elements on Earth and is capable of forming a tremendously vast number of chemicals (over twenty million so far). Most of the chemicals found in all living organisms are based on carbon. 4  Chemistry and its Importance iii. Inorganic chemistry: It is the study of substances that are not primarily based on carbon. Inorganic chemicals are commonly found in rocks and minerals. One current important area of inorganic chemistry deals with the design and properties of materials involved in energy and information technology. iv. Analytical chemistry: It is the study of the composition of matter. It focuses on separating, identifying, and quantifying chemicals in samples of matter. An analytical chemist may use complex instruments to analyze an unknown material in order to determine its various components. v. Biochemistry: It is the study of chemical processes that occur in living things. It may cover anything from basic cellular processes up to understanding disease states so that better treatments can be developed. All of the aforementioned disciplines of Chemistry are highly engaged in taking measurements, making observations, and using them to come to conclusions. Chemistry is about looking for patterns in the way substances behave. Because living and non- living things are made of matter Chemistry affects all aspects of life and most natural events. The scope of Chemistry can be extended to explaining the natural world, preparing people for career opportunities, and producing informed patriot citizens. The scope of chemistry includes agriculture, medicine food production, and building construction (Figure 1.2). Figure 1.2 Some chemical products. Chemistry, however, is not only involved in providing useful substances in the areas of development and technology, but it can also result in very dangerous substances that can negatively affect human being’s life and the environment (eg. fluorochlorohydrocarbons, oxides of nitrogen, carbon, and sulphur). 5 Chemistry Grade 9 Exercise 1.2 Provide correct answer for the following questions. 1. List down examples of chemicals or chemical products that are used in the following areas:  Agriculture  Medicine  Food production  Building construction Hint: Refer Figure 1.2 above. 2. Search on the internet, and write down some of the problems caused by dangerous chemicals affecting the environment. 3. Which of the problems you find in question #2 above are observed in your locality? 4. What do you think is the solution to the problem(s)? Remember that you are an Ethiopian citizen, and have the responsibility of protecting the nation from the problems caused due to chemical substances. 1.2 The Relationship Between Chemistry and Other Natural Sciences At the end of this section, you will be able to discuss the relationship of chemistry with physics, biology, medicine, geology and other subjects. Students, form groups of three or four, and discuss the following questions. Then, present your discussion points to the class, when asked by your teacher. 1. List down the subjects that are categorized under natural science. 2. In biology class you may studied about photosynthesis. Is it possible to explain photosynthesis without having the knowledge of a chemical reaction? Reason out Activity 1.3 why? Chemistry is one branch of science. Science is the process by which we learn about the natural universe by observing, testing, and then generating models that explain our observations. Because the physical universe is so vast, there are many different branches of science ( Figure 1.3). Thus, biology is the study of living things, and geology is the study of rocks and the earth. Physics is the branch of science concerned with the nature and properties of matter and energy. 6  Chemistry and its Importance Figure 1.3 The relationships between natural sciences and chemistry. Although we divide science into different fields, there is much overlap among them. For example, some biologists and chemists work in both fields so much that their work is called Biochemistry. Biochemistry is the study of the chemical processes occurring in living matter. To give you a specific example, there are chemists who are working on the isolation, characterization and biological activities of compounds from medicinal plants. Similarly, Geology and Chemistry overlap in the field called Geochemistry. Geochemistry is defined as the study of the processes that control the abundance, composition, and distribution of chemical compounds and isotopes in geologic environments. Chemistry and Physics overlap in the areas of atomic and small molecule properties. Both of them deal with matter and energy. Physical chemistry is the branch of chemistry concerned with the application of the techniques and theories of physics to the study of chemical systems. Chemical physics is a sub discipline of chemistry and physics that investigates physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics. It is the branch of physics that studies chemical processes from the point of view of physics. Chemistry and Medicine are related in the area of Medicinal chemistry. Figure 1.3 shows how many of the individual fields of science are related. At some level, all of these fields depend on the matter as they all involve ‘stuff’. Because of this, chemistry has been called the ‘central science’ linking them all together. Exercise 1.3 Provide correct answer the following questions. 1. What aspects of nature are studied in  Physics?  Geochemistry?  Biology?  Biochemistry?  Geology?  Physical chemistry? 2. What are the regions of an overlap between  Chemistry and biology?  Chemistry and geology?  Chemistry and physics? 7 Chemistry Grade 9 1.3 The Role Chemistry Plays in Production and in the Society At the end of this section, you will be able to describe the application of chemistry in the field of agriculture, medicine, food production and building construction. Students, form groups of two or three and discuss the questions below. Present your discussion points to the class when asked by your teacher. 1. What are the common types of fertilizers the Ethiopian farmers employ to increase their crop productivity? 2. Give some examples of household materials that are used for cleaning, baking ‘diffo dabbo’, disinfecting salad, preserving raw meat, and hair treatment. 3. What types of medications (traditional and modern) are used to treat the various diseases you know? 4. What are the common types of fuels that are sold in the gas stations? Activity 1.4 There are many instances in your everyday life that involves the knowledge of chemistry, its applications, and its rules. Let us look at some of them one by one. A. Agriculture The study of chemistry has brought the world with chemical fertilizers such as calcium super phosphate, urea, ammonium sulphate, and sodium nitrate. These chemicals have assisted greatly in increasing the yield of fruits, vegetables, and other crops (Figure 1.4). Chemistry has been effective in the manufacture of pesticides, which have lessened the crop damage. Depending on the targeted pest, pesticides include fungicides, herbicides, and insecticides. Thus, we can supply to the ever-growing demand for food. Chemistry has also an important role in the manufacturing of better- quality plastic pipes for irrigation, and is commonly used in farming. This has massively increased irrigation resulting in a better climate in which the crops grow. B. Food Production Other than its great contribution in the production of different agricultural products, chemistry has led to the discovery of different kinds of food preservatives. These chemicals have greatly assisted to preserve food products for a longer period. It has given methods to test the presence of adulterants which ensure the supply of pure foodstuff. Consumers have benefited from new technologies that have increased their food’s availability, appearance, nutritional contents and flavor. A local example of food processing and keeping it for a longer period of time is the preservation of raw meat. 8  Chemistry and its Importance Figure 1.4 Agricultural products. Students, form groups of two or three and discuss the questions below. Present your discussion points to the class when asked by your teacher. 1. What are the common pests that damage crops in your locality? 2. What are the common herbs that reduce the production of food in your locality? 3. What are the commercial and traditional pesticides and herbicides used by the local farmers? Activity 1.5 C. Medicine Chemistry has provided mankind with a large number of life-saving medicines. We could find a cure for dysentery and pneumonia as a result of the discovery of Sulphur drugs and penicillin. Besides this, life-saving drugs like cisplatin and Taxol are effective for cancer therapy, and AZT is used for AIDS victims. Although AZT does not cure HIV- AIDS, it fights the multiplication of the virus thereby prolonging the life of the victim. HIV-AIDS as we know is a pandemic that has no curative medication. We need to prevent ourselves from this killer disease by being Abstain, Be faithful or reduce the number of your sex partners, and/or use a Condom. 9 Chemistry Grade 9 Common Drugs Chemistry Provided Disinfectants Analgesics Anesthetics Antibiotics Antiseptics Tranquilizers  Disinfectants: Are used to kill the microbe present in toilets, floors, and drains. The sanitizers we use for Covid-19 belong to this group.  Analgesics: An analgesic or painkiller is any member of the group of drugs used to achieve analgesia, relief from pain.  Anesthetics: Has made medical operations more and more effective via relieving pain.  Antibiotics: Are used to control infection and cure diseases.  Antiseptics: Are used to contamination of the wounds by bacteria.  Tranquillizers: To reduce tension and bring about calm and peace to patients suffering from mental diseases. Students, form groups of two or three and discuss the questions below. Present your discussion points to the class when asked by your teacher. 1. Search on the Internet or from other sources and find examples of analgesics, antibiotics, tranquilizers, antiseptics, disinfectants, anesthetics, and insecticides. 2. Describe the composition and preparation of hand disinfectant or hand sanitizer. Activity 1.6 D. Building Construction Materials By providing building resources such as glass, steel and cement, chemistry helps in the construction of safer houses and multi-story structures. It also helps in the construction of long-lasting and durable dams and bridges. The best example here could be the Grand Ethiopian Renaissance Dam (GERD) which is under construction in the Benishangul-Gumuz Region (Figure 1.5). The GERD is a 6,450 MW hydro power project nearing completion on the Blue Nile in Ethiopia, located about 30 km upstream of the border with Sudan. It will be the largest hydro power project in Africa. 10  Chemistry and its Importance Figure 1.5 The Grand Ethiopian Renaissance Dam, Benishangul Gumuz Region, Ethiopia. Figure 1.6 The role of chemistry in different sectors. Exercise 1.4 Provide correct answer for the following questions. 1. List down the role of chemistry in your locality. Draw a spider diagram of your own. 2. What problems, do you think, will be observed in the livelihood of the community in which you are living in the absence of enough knowledge of chemistry? 3. In your opinion, what further roles can chemistry play? 11 Chemistry Grade 9 1.4 Some Common Chemical Industries in Ethiopia At the end of this section, you will be able to name some common chemical industries found in Ethiopia and their products. Students, make groups of how two or three and do the following activities. Present your answers to the class when asked by your teacher. 1. List down some of the house hold chemicals? 2. Do you know which industry is producing them? 3. List down the common chemical industries (enterprises) found in your locality or in the vicinity of your town. Which chemicals or chemical products are they producing? Activity 1.7 An industry is defined as an economic activity concerned with the processing of raw materials and the manufacture of goods in factories. It can also be interpreted as a group of companies that are linked based on their primary business activities. Individual companies are generally categorized into an industry based on their largest sources of revenue. The Ethiopian government is highly engaged in expanding industries in the past two decades. As part of this expansion several industrial parks have been under construction (Figure 1.7). Figure 1.7 One of the industrial parks found in Ethiopia. The chemical industries comprise the companies that manufacture inorganic- and organic-industrial chemicals, explosives, fragrances, agrochemicals, polymers and 12  Chemistry and its Importance rubber, ceramic products, petrochemicals, oleochemicals (oils, fats, and waxes), and flavors. Central to the world economy, it converts natural resources (oil, natural gas, air, water, metals, and minerals) into diverse products. They are further categorized into industrial inorganic chemicals; plastics, materials, and synthetics; drugs; soap, cleaners, and toilet goods; paints and allied products; industrial organic chemicals; agricultural chemicals; and miscellaneous chemical products. The chemical products mean products manufactured, processed, sold, or distributed by the company that are chemical substances, or that contained chemical substances. Three general classes of products are (1) basic chemicals such as alkalis, acids, organic chemicals, and salts (2) chemical products to be used in further manufactures such as plastic materials, synthetic fibers, pigments, and dry colors, and (3) finished chemical products to be used for ultimate consumption such as cosmetics, drugs, and soaps; or to be used as materials or supplies in other industries such as fertilizers, paints, and explosives. Currently, there are several medium and large-scale chemical and chemical products industries (enterprises) in Ethiopia (Table 1.1). These enterprises produce chemicals like aluminum sulphate, caustic soda, soda ash, carbon dioxide, bleaching chemicals, magnesium oxide, pesticides, and chemical products like soap and detergent, cement, paints, building materials, cosmetics, plastic, natural gum, candle, glass, sugar, tyre, pulp and paper, pharmaceuticals and tobacco. Table 1.1 Some of the large and medium scale chemical enterprises in Ethiopia. No. Name of the Enterprise City Product 1 Chorra Gas & Chemical A.A Plastic, chemicals, petroleum products products 2 Chorra Gas & Chemical A.A Aluminum sulphate and sulphuric acid products 3 Ziway Caustic Soda Ziway Sodium hydroxide factory 4 Abijata Soda Ash Bulbula Trona (Na3H(CO3)2.2H2O) Factory 5 Repi Soap & Detergent A.A Soap and detergent P.L.C 6 Adola Magnesium Adolla Magnesium oxide Oxide Factory 7 Adami Tulu Pesticide Adami-Tulu Formulates malathion, endosulfan, Processing Plant diazinon, fenitrothion and dimethoate 8 Nefas Silk Paints A.A Paints, varnishes, antirusts and glues factory 13 Chemistry Grade 9 No. Name of the Enterprise City Product 9 Modern Building A.A Cement and cement products, Industries ceramics, paints, sanitary ware, adhesives, glues, plastic rubber, terrazzo tiles, cultured marble 10 Kadisco Chemical A.A Paints, coatings and adhesives Industry 11 Tadesse Filatea PLC Woliso Soap, detergent, corrugated iron, nail, infant milk formula 12 Etab Laundry Soap Hawassa Soap and detergent Factory 13 Get-Eshet Detergent Bishoftu Detergent products and Leather Manufacturing and chemical inputs Packing P.L.C 14 Ethio-Asia Industries S.C A.A Soap and detergent 15 Y.B Cosmetics Sheger city Cosmetics and perfume 16 Mekab PLC (Cosmetics) A.A Hair oil, shampoo, conditioner, body oil, vaseline, body lotion, detergents and plastic mouldings 17 BEKAS Chemicals PLC A.A Detergents, cosmetic products, plastic packing materials, industrial surfactants and putty 18 Arbaminch Textile Arbaminch Textile and fabric products Share compary 19 Teamco Soap Factory Burayu Soap and detergent Note: A.A stands for Addis Ababa. Other chemical product industries in Ethiopia.  Cement (Mugher, Diredawa, Mesobo, Derba, Midroc, Dangote)  Sugar (Metehara, Wonji, Finchaa, Omokuraz)  Paper and pulp (Wonji)  Pharmaceuticals (Addis, Ethiopia, Adigrat)  Tyre (Horizon Addis Tyre) 14  Chemistry and its Importance Exercise 1.5 Provide the correct answer for the following questions. 1. List the common names together with the chemical names, formula, and use of 10 household chemicals. 2. Browse the Internet or use other sources and find out 10 other household chemicals that are commonly used in different homes with their corresponding molecular formula, chemical names, and use. 3. List at least 15 chemical industries that are found in Ethiopia. Project work 1.1 An industrial trip to the local industries In this project, you will be able to visit a local chemical industry and present your observations to the class in group. Students, your teacher will arrange an industrial tour so that you will practically observe the raw materials, the chemical processes involved, and the finished products in the industries that are located in your vicinity. You are required to write a report and present it to the class in group. Key Terms of the Unit  Analgesics  Composition  Metallurgy  Antibiotics  Disinfectants  Property  Anesthetics  Energy  Structure  Antiseptics  Industry  Substance  Chemistry  Insecticides  Tranquilizers  Chemical products  Matter  Chemical industry Unit Summary Chemistry is the science that deals with the properties, compositions, and structures of substances, the transformations they undergo, and the energy that is released or absorbed during these processes. The study of modern chemistry has many branches, but can generally be broken down into five main disciplines, or areas of study: Organic chemistry, Inorganic chemistry, Physical chemistry, Analytic chemistry and Biochemistry. The scope of chemistry includes agriculture, medicine, food production, and building construction. Because living and non-living things are made up of matter, chemistry affects all aspects of life, and most natural events. The scope of chemistry can also be extended to explaining the natural world, preparing people for career opportunities, and producing informed patriot citizens. 15 Chemistry Grade 9 Chemistry is the central science in the natural sciences. Being one of the basic sciences Chemistry is related to other natural sciences like Biology, Physics, Geology, Medicine, and Mathematics. Chemistry and Biology are related by the metabolic processes occurring inside living matter known as Biochemistry. Chemistry and Geology are related by the processes that control the abundance, composition, and distribution of chemical compounds and isotopes inside the crust of the earth known as Geochemistry. A sub-discipline of chemistry and physics that investigates physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics is known as Chemical physics. Chemistry and Medicine are related by a discipline that encloses the design, development, and synthesis of pharmaceutical drugs called Medicinal chemistry. Since Chemistry is so fundamental to our world, it plays a role in everyone’s lives and touches almost every aspect of our existence in some way. Chemistry is essential for meeting our basic needs such as food, clothing, shelter, health, energy, and clean air, water, and soil. Chemistry, however, can also affect our environment negatively through production of toxic substances. The release of theses toxic substances into the environment results in climate change, the prime issue the world is facing currently. It is, therefore, high time to seek for solution to this problem by using our chemical knowledge. Chemistry plays a significant role in the advancement and growth of several industries. There are several chemical and chemical products industries that produce a number of chemicals and chemical products. One way of categorizing them is into industrial inorganic chemicals; plastics, materials, and synthetics; drugs; soap, cleaners, and toilet goods; paints and allied products; industrial organic chemicals; agricultural chemicals; and miscellaneous chemical products. The Ethiopian government is working hard in establishing Industrial Zones throughout the nation. Currently, there are several medium and large-scale chemical and chemical products industries (enterprises) in Ethiopia. These enterprises produce chemicals like aluminum sulphate, caustic soda, soda ash, carbon dioxide, bleaching chemicals, magnesium oxide, pesticides; and chemical products like soap and detergent, cement, paints, building materials, cosmetics, plastic, natural gum, candle, glass, sugar, tyre, pulp and paper, pharmaceuticals and tobacco. Review Exercise Part I: Basic Level Questions. Identify each of the following statements as ‘True’ or ‘False’. Give your reason(s) for false statements. 1. Chemistry is a science that deals with the study of the way living things behave. 2. Every substance in the universe, in which we live, has its own properties by which we can distinguish it from other substances. 16  Chemistry and its Importance 3. The transformation of a substance is a marked change in form, nature, or appearance. 4. The study of chemistry involves only microscopic information. 5. Organic chemistry is the study of chemicals that are not based on carbon. Part II: Intermediary Level Questions. Fill in the blank spaces. 6. The property of a substance is its ___________ , ___________ or ____________. 7. ___________ is the nature of something’s ingredients or constituents; the way in which a whole or mixture is made up. 8. The arrangement of and relations between the parts or elements of something complex is known as its ___________. 9. ___________ is a power derived from the utilization of physical or chemical resources, especially to provide light and heat. 10. ___________ is the study of macroscopic properties, atomic properties, and phenomena in chemical systems. 11. ___________ is the study of the composition of matter. Part III: Challenge Level Questions. Provide appropriate answers to the following questions. 12. Define the terms industry, chemical industry, and chemical products. 13. What are the roles chemistry played in production and society? 14. How does chemistry play a role in increasing comfort, pleasure, and luxuries? 15. Mention at least 10 chemical industries found in Ethiopia and their chemical products. 16. Which branch of chemistry has the highest scope? Why? 17. What are the five fields of chemistry? 18. What will be the future efforts of chemistry? 19. What jobs can you do with chemistry? 17 U NI T  MEASUREMENTS AND SCIENTIFIC METHODS Unit Outcomes After completing the unit, you will be able to  use proper SI units;  identify the causes of uncertainty in measurement;  express the result of any calculation involving experimental data to the appropriate number of decimal places or significant figures;  apply scientific methods in solving problems;  demonstrate an understanding of experimental skills in chemistry;  demonstrate a knowledge of basic laboratory apparatuses and safety rules;  describe scientific inquiry skills along this unit: observing, inferring, predicting, comparing & contrasting, communicating, analyzing, classifying, applying, theorizing, measuring, asking question, developing hypothesis, designing experiment, interpreting data, drawing conclusion, making generalizations and problem solving.  Measurements and Scientific Methods 2.1 Measurements and Units in Chemistry Learning competencies At the end of this section, you should be able to  list the seven SI units and their prefixes;  describe the seven SI units and their prefixes;  write the names and symbols of derived SI units;  use the factor label method for solving problems and making conversion of SI units;  describe uncertainty of measurement;  identify the digits that are certain and the ones that are uncertain given a number representing measurement;  identify causes of uncertainty in measurement;  define precision and accuracy;  estimate the precision possible for any instrument they use in the laboratory;  explain systematic and random errors;  analyze given data in terms of precision and accuracy;  define significant figures;  determine the number of significant figures in a calculated result;  use the scientific notation in writing very large or very small numbers. Conduct the following activity and present your finding to the class. 1. In group, discuss and list down different traditional ways of measuring mass of solid and liquid substances Start-up Activity sold in the market places in your area. 2. Mention indigenous methods of measurements (length, mass, time, volume) Measurement is the comparison of a physical quantity to be measured with a unit of measurement that is, with a fixed standard of measurement. On a centimeter scale, the centimeter unit is the standard of comparison. In traditional markets people buy and sell goods by estimating their size in traditional way or use traditional measurement method. Figure 2.1 shows traditional market and people exchanging goods by estimating their size using indigenous methods of measurements. 19 Chemistry Grade 9 Figure 2.1 Traditional market. The study of chemistry depends heavily on measurement. For instance, chemists use measurements to compare the properties of different substances and to assess changes resulting from an experiment. A number of common devices enable us to make simple measurements of a substance’s properties: The meter stick measures length; the burette, the pipette, the graduated cylinder, and the volumetric flask measure volume (see Figure 2.2); the balance measures mass; the thermometer measures temperature. The instruments illustrated on Figure 2.2 provide measurements of macroscopic properties, which can be determined directly. Microscopic properties, on the atomic or molecular scale, must be determined by an indirect method. A measured quantity is usually written as a number with an appropriate unit. To say the distance between Addis Ababa and Hawassa by car along a certain route is 275 is meaningless. We must specify that the distance is 275 kilometers. In science, units are essential to state measurements correctly. 20  Measurements and Scientific Methods Figure 2.2 Some common measuring devices found in a chemistry laboratory. Conduct the following activity and present your finding to the class. a. Using thermometer with oC and oF scale measure your body temperature. Compare the results in oC with the result in oF. b. What is the distance between Addis Ababa and your town (village) in kilometers? c. Which basic SI units are appropriate to express the: i. average room temperature, and ii. time duration for the earth to have one rotation A ctivity 2.1 around its axis? 2.1.1 SI Units (The International System of Units) For many years, scientists recorded measurements in metric units, which are related decimally, that is, by powers of 10. In 1960, however, the General Conference of Weights and Measures, the international authority on units, proposed a revised metric 21 Chemistry Grade 9 system called the International System of Units. Table 2.1 shows the seven SI base units. Measurements that we will utilize frequently in our study of chemistry include time, mass, volume, density, and temperature. Table 2.1 SI Base Units. Base Quantity Name of Unit Symbol Length Meter m Mass Kilogram kg Time Second s Electrical current Ampere A Temperature Kelvin K Amount of substance Mole mol Luminous intensity Candela cd Heat and Temperature Temperature measures the intensity of heat, the “hotness” or “coldness” of a body. Heat is a form of energy that always flows spontaneously from a hotter body to a colder body — never in the reverse direction. Relationships among the three temperature scales are illustrated in Figure 2.4. Between the freezing point of water and the boiling point of water, there are 100 steps (°C or Kelvins, respectively) on the Celsius and Kelvin scales. Thus, the “degree” is the same size on the Celsius and Kelvin scales. But every Kelvin temperature is 273.15 units above the corresponding Celsius temperature. The relationship between these two scales is as follows: K = 0C + 273.15 0C or 0C = K - 273.150 In the SI system, “degrees Kelvin” are abbreviated simply as K rather than °K and are called kelvins. Any temperature change has the same numerical value whether expressed on the Celsius scale or on the Kelvin scale. For example, a change from 25°C to 59°C represents a change of 34 Celsius degrees. Converting these to the Kelvin scale, the same change is expressed as (273 + 25) = 298 K to (59 + 273) = 332 K, or a change of 34 kelvins. Comparing the Fahrenheit and Celsius scales, we find that the intervals between the same reference points are 180 Fahrenheit degrees and 100 Celsius degrees, respectively. Thus, a Fahrenheit degree must be smaller than a Celsius degree. It takes 180 Fahrenheit degrees to cover the same temperature interval as 100 Celsius degrees. From this information, we can construct the unit factors for temperature changes: 22  Measurements and Scientific Methods 180 0F or 1.8 0C and 100 0C 1.0 0C or 100 0C 1.0 0F 180 0F 1.8 0F But the starting points of the two scales are different, so we cannot convert a temperature on one scale to a temperature on the other just by multiplying by the unit factor. In converting from °F to °C, we must subtract 32 Fahrenheit degrees to reach the zero point on the Celsius scale (Figure 2.3).  1.8o F   9o F  1.0o C o 5o C o o F =  x oC × o  + 32o F =  x oC × o  + 32o F and C o = 1.8 ( o F x F − 32 ) o F= 9 o ( F x F − 32o F )  1.0 C   5 C  Figure 2.3 The relationships among the Kelvin, Celsius (centigrade), and Fahrenheit temperature scales. Example 2.1: Temperature conversion When the temperature reaches “100.°F in the shade,” it’s hot. What is this temperature on the Celsius scale? Solution 1.0o C o We use the relationship = o C o 1.8 F ( x F − 32o F ) 1.0o C 1.0o C to carry out the desired conversion. = o C 1.8o F (100 o F − = 32 o F ) 1.8o F = (68o F ) 38o C 23 Chemistry Grade 9 Exercise 2.1 Temperature Conversion When the absolute temperature is 400 K, what is the Fahrenheit temperature? 2.1.2 Derived Units All other SI units of measurement can be derived from base units (called derived units). Table 2.2 shows some of the common derived units. Once base units have been defined for a system of measurement, you can derive other units from them. You do this by using the base units in equations that define other physical quantities. For example, area is defined as length times width. Therefore, SI unit of area = (SI unit of length) × (SI unit of width) From this, SI unit of area is meter × meter, or m2. Similarly, speed is defined as the rate of change of distance with time; that is, speed = distance/time. Consequently, SI unit of distance SI unit of speed = SI unit of me The SI unit of speed is meters per second (that is, meters divided by seconds). The unit is symbolized m/s or m s-1. The unit of speed is an example of an SI derived unit, which is a unit derived by combining SI base units. Table 2.2 displays a number of derived units. Volume and density are discussed in this section. Volume Volume is defined as length cubed and has the SI unit of cubic meter (m3). This is too large a unit for normal laboratory work, so we use either cubic decimeters (dm3) or cubic centimeters (cm3, also written cc). Traditionally, chemists have used the liter (L), which is a unit of volume equal to a cubic decimeter. In fact, most laboratory glassware (Figure 2.2) is calibrated in liters or milliliters (1000 mL = 1 L). Because 1 dm equals 10 cm, a cubic decimeter, or one liter, equals (10 cm)3 = 1000 cm3. Therefore, a milliliter equals a cubic centimeter. In summary, 1 L = 1 dm3 and 1 mL = 1 cm3. 24  Measurements and Scientific Methods Table 2.2 Derived units. Quantity Definition of Quantity SI Unit Area Length squared m2 Volume Length cubed m3 Density Mass per unit volume kg/m3 Speed Distance traveled per unit time m/s Acceleration Speed changed per unit time m/s2 Force Mass times acceleration of object kg.m/s2 (= newton, N) Pressure Force per unit area kg/(m.s2)(= pascal,Pa) Energy Force times distance traveled kg.m2/s2 (= joule,J) Density The density of an object is its mass per unit volume. You can express this as m d= v where d is the density, m is the mass, and V is the volume. Suppose an object has a mass of 15.0 g and a volume of 10.0 cm3. Therefore, the density will be 15.0 g d= = 1.50 g/cm3 10.0 cm3 The density of the object is 1.50 g/cm3. Density is an important characteristic property of a material. Water, for example, has a density of 1.000 g/cm3 at 4 oC and a density of 0.998 g/cm3 at 20 oC. Lead has a density of 11.3 g/cm3 at 20 oC. Density can also be useful in determining whether a substance is pure. Consider a gold bar whose purity is questioned. The metals likely to be mixed with gold, such as silver or copper, have lower densities than gold. Therefore, an adulterated (impure) gold bar can be expected to be far less dense than pure gold. Example 2.1: Calculating the density of a substance A colorless liquid, used as a solvent (a liquid that dissolve other substances), is believed to be one of the following (Table 2.3): Table 2.3 Density of different liquids. Substance Density (in g/mL) n-butyl alcohol 0.810 ethylene glycol 1.114 isopropyl alcohol 0.785 toluene 0.866 25 Chemistry Grade 9 To identify the substance, Chaltu determined its density. By pouring a sample of the liquid into a graduated cylinder, she found that the volume was 35.1 mL. She also found that the sample weighed 30.5 g. What was the density of the liquid? What was the substance? Solution The solution to this problem lies in finding the density of the unknown substance. Once the density of the unknown substance is known, you can compare it to the list of known substances presented in the problem and look for a match. Density is the relationship of the mass of a substance per volume of that substance. Expressed as an equation, density is the mass divided by the volume: d = m/V. You substitute 30.5 g for the mass and35.1 mL for the volume into the equation. m 30.5 g d= = = 0.869 g/mL v 35.1mL The density of the liquid equals that of toluene (within experimental error). Answer Check Always be sure to report the density in the units used when performing the calculation. Density is not always reported in units of g/ml org/cm3, for example; gases are often reported with the units of g/L. Exercise 2.2 A piece of metal wire has a volume of 20.2 cm3 and a mass of 159 g. What is the density of the metal? We know that the metal is manganese, iron, or nickel, and these have densities of 7.21 g/cm3, 7.87 g/cm3, and 8.90 g/cm3, respectively. From which metal is the wire made? 2.1.3 Common Prefixes Used in SI Units The factor expressed as a factor to the power of ten, SI/ metric prefix, the symbol used and the actual decimal number are tabulated in Table 2.4. They are widely used and are easy to add to the basic units. Like metric units, SI units are modified in decimal fashion by a series of prefixes, as shown in Table 2.4. Measurements that we will utilize frequently in our study of chemistry include time, mass, volume, density, and temperature. 26  Measurements and Scientific Methods Table 2.4 SI/ Metric Units, Symbols and Numbers. Factor Prefix Symbol Decimal Example 1012 Tera T 1,000,000,000,000 1 Terameter (Tm)=1×1012 m 109 Giga G 1,000,000,000 1 Gigameter (Gm)= 1×109 m 106 Mega M 1,000,000 1 Megameter (Mm)= 1×106 m 103 Kilo k 1,000 1 kilometer (km)= 1×103 m 102 Hecto h 100 1 hectometer (hm)= 1×102 m 101 Deca da 10 1 decameter (dam)= 1×101 m 10-1 Deci d 0.1 1 decimeter (dm)= 1×10-1 m 10-2 Centi c 0.01 1 centimeter (cm)= 1×10-2 m 10-3 milli- m 0.001 1 millimeter (mm)= 1×10-3 m 10-6 Micro µ 0.000 001 1 micrometer (µm)= 1×10-6 m 10-9 Nano n 0.000 000 001 1 nanometer (nm)= 1×10-9 m 10-12 Pico p 0.000 000 000 001 1 picometer (pm)= 1×10-12 m Examples of SI prefixes The SI prefixes/ metric prefixes are easily used as demonstrated by the few simple examples given below:  1 Megawatt = 1,000,000 watts  1 kilogram = 1,000 grams  1 µF = 1 microFarad = 1/1,000,000 Farad Along with these the abbreviations or symbols can also be used. For example, kV for kilovolts, kW for kilowatts, and km for kilometer. The other symbols or abbreviations can be used in exactly the same manner. 2.1.4 Uncertainty in Measurements Whenever you measure something, there is always some uncertainty. There are two categories of uncertainty: systematic and random. 1. Systematic uncertainties are those which consistently cause the value to be too large or too small. Systematic uncertainties include such things as reaction time, inaccurate meter sticks, optical parallax and miscalibrated balances. In principle, systematic uncertainties can be eliminated if you know they exist. 2. Random uncertainties are variations in the measurements that occur without a pre- dictable pattern. If you make precise measurements, these uncertainties arise from the estimated part of the measurement. Random uncertainty can be reduced, but never eliminated. We need a technique to report the contribution. 27 Chemistry Grade 9 The uncertainty shall rather be understood as an interval within which the result can be found with a given probability. Thus, the result will be within the interval but all values within the interval have the same probability to represent the result. Except when all the numbers involved are integers (for example, in counting the number of students in a class), obtaining the exact value of the quantity under investigation is often impossible. For this reason, it is important to indicate the margin of error in a measurement by clearly indicating the number of significant figures, which are the meaningful digits in a measured or calculated quantity. When significant figures are used, the last digit is understood to be uncertain. For example, we might measure the volume of a given amount of liquid using a graduated cylinder (see Figure 2.4) with a scale that gives an uncertainty of 1 mL in the measurement. If the volume is found to be 6 mL, then the actual volume is in the range of 5 mL to 7 mL. We represent the volume of the liquid as (6±1) mL. In this case, there is only one significant figure (the digit 6) that is uncertain by either plus or minus 1 mL. For greater accuracy, we might use a graduated cylinder that has finer divisions, so that the volume we measure is now uncertain by only 0.1 mL. If the volume of the liquid is now found to be 6.0 mL, we may express the quantity as (6.0± 0.1) mL, and the actual value is somewhere between 5.9 mL and 6.1 mL. We can further improve the measuring device and obtain more significant figures, but in every case, the last digit is always uncertain; the amount of this uncertainty depends on the particular measuring device we use and the user’s ability. Figure 2.4 Uncertainty in volume measurement using a measuring cylinder. 28  Measurements and Scientific Methods Conduct the following activity and present your finding to the class. 1. Make a chain of paper clips or other objects of uniform length. Then use a meter stick to measure a series of lengths on the chain. For example, measure sections containing one, two, three, etc., clips. Record your results and share them with your classmates. 2. Using laboratory scale, take several mass reading for one, two, three objects of uniform size. You can use any convenient objects you find in the laboratory. Record your results and discuss them in your group. Focus especially on the similarities and differences in your measurement. Did you all find the same reading for the same object? What do you think are the cause of the uncertainties, if any? Discuss the results with the Activity 2.2 rest of the class. Calculating uncertainties There are several techniques that will produce an estimate of the uncertainty in the value of the mean. Since we are expecting students to produce an estimate of the uncertainty any suitable value that indicates half the range is acceptable. Example 2.2: A student measures the diameter of a metal canister using a ruler graduated in mm and records these results: Diameter/mm Reading 1 Reading 2 Reading 3 Mean 66 65 61 64 The uncertainty in the mean value (64 mm) can be calculated as follows: a. Using the half range The range of readings is 61 mm – 66 mm so half the range is used to determine the uncertainty. Uncertainty in the mean diameter = (66 mm – 61 mm)/2 = 2.5 mm Therefore, the diameter of the metal canister is 64 mm ± 2.5 mm. Since a ruler graduated in mm could easily be read to ± 0.5 mm, it is acceptable to quote the uncertainty as ± 2.5 mm for this experiment. b. Using the reading furthest from the mean In this case, the measurement of 61 mm is further from the average value than 66 mm therefore we can use this value to calculate the uncertainty in the mean. Uncertainty in the mean diameter = 64 mm – 61 mm = 3 mm. 29 Chemistry Grade 9 Therefore, the diameter of the metal canister is 64 mm ± 3 mm. c. Using the resolution of the instrument This is used if a single reading is taken or if repeated readings have the same value. This is because there is an uncertainty in the measurement because the instrument used to take the measurement has its own limitations. If the three readings obtained above were all 64 mm then the value of the diameter being measured lies somewhere between 63.5 mm and 64.5 mm since a meter rule could easily be read to half a millimeter. In this case, the uncertainty in the diameter is 0.5 mm. Therefore, the diameter of the metal canister is 64 mm ± 0.5 mm. This also applies to digital instruments. An ammeter records currents to 0.1 A. A current of 0.36 A would be displayed as 0.4 A, and a current of 0.44 A would also be displayed as 0.4 A. The resolution of the instrument is 0.1 A but the uncertainty in a reading is 0.05 A. The typical uncertainty of a top loading balance is 0.05 g. How would you report on weighing of 23.25 g made on this top loading balance? The result should be reported as 23.25 g ± 0.05 g. Such an item of data means that the correct reading lies between 23.20 g and 23.30 g. The uncertainty in a measurement can be expressed in two useful ways: a. as the absolute uncertainty in the last digit written b. as the percent uncertainty calculated as follows absolute uncertainty % uncertainty = × 100 measurement % uncertainty = (0.05 g)/( 23.25 g) × 100 = 0.2% Therefore the answer may be reported as: Absolute uncertainty: 23.25 g ± 0.05 g Percent uncertainty: 23.25 g ± 0.2% Exercise 2.3 Absolute uncertainty and percent uncertainty in a single reading: Use the given uncertainty to calculate the % uncertainty in each of the following readings and report the result of measurement in terms of absolute uncertainty and percent uncertainty: a. A barometer reading of 723.5 torr. The absolute uncertainty is 0.1 torr b. 2.75 g weighed on a top loading balance. The absolute uncertainty is 0.05 g c. 2.7413 g weighed on an analytical balance. The absolute uncertainty is 0.0002 g 30  Measurements and Scientific Methods d. A temperature reading of 75.6 °C on a thermometer graduated to the nearest degree. The absolute uncertainty is 0.2 °C e. 18.6 ml measured in 100 ml graduated cylinder. The absolute uncertainty is 0.4 mL f. 43.7 ml measured in 100 ml graduated cylinder. The absolute uncertainty is 0.4 mL 2.1.5 Precision and Accuracy Measurements may be accurate, meaning that the measured value is the same as the true value; they may be precise, meaning that multiple measurements give nearly identical values (i.e., reproducible results); they may be both accurate and precise; or they may be neither accurate nor precise. The goal of scientists is to obtain measured values that are both accurate and precise. If you repeat a particular measurement, you usually do not obtain precisely the same result, because each measurement is subject to experimental error. The measured values vary slightly from one another. Suppose you perform a series of identical measurements of a quantity. The term precision refers to the closeness of the set of values obtained from identical measurements of a quantity. Accuracy is a related term; it refers to the closeness of a single measurement to its true value. Example 2.4: Precision and Accuracy The archery targets in Figure 2.5 show marks that represent the results of four sets of measurements. a. a precise but inaccurate set of measurements. b. an accurate but imprecise set of measurements. c. a set of measurements that is both precise and accurate. d. a set of measurements that is neither precise nor accurate. Figure 2.5 The distribution of darts on a dart board showing the difference between precise and accurate. 31 Chemistry Grade 9 Form groups, discuss and present your conclusion to the class 1. Mohammed measured the mass of a sample of gold using one balance and found 1.896 g. On a different balance, the same sample was found to have a mass of 1.125 g. Which measurement was correct the first or the second measurement? Careful and repeated measurements made by Mohammed, including measurements on a calibrated third balance, showed the sample to have a mass of 1.895 g. The masses obtained from the three balances are in the Table 2.5: Table 2.5 Mass measurement Balance 1 Balance 2 Balance 3 1.896 g 1.125 g 1.893 g 1.895 g 1.158 g 1.895 g 1.894 g 1.067 g 1.895 g A ctivity 2.3 Exercise 2.4 a. A 2-carat diamond has a mass of 400.0 mg. When a jeweler repeatedly weighed a 2-carat diamond, he obtained measurements of 450.0 mg, 459.0 mg, and 463.0 mg. Were the jeweler’s measurements accurate? Were they precise? b. A single copper penny was tested three times to determine its composition. The first analysis gave a composition of 93.2% zinc and 2.8% copper, the second gave 92.9% zinc and 3.1% copper, and the third gave 93.5% zinc and 2.5% copper. The actual composition of the penny was 97.6% zinc and 2.4% copper. Were the results accurate? Were they precise? 2.1.6 Significant Figures We must always be careful in scientific work to write the proper number of significant figures. In general, it is fairly easy to determine how many significant figures a number has by using the following rules: 1. Any digit that is not zero is significant. Thus, 845 cm has three significant figures, 1.234 kg has four significant figures, and so on. 2. Zeros between nonzero digits are significant. Thus, 606 m contains three significant 32  Measurements and Scientific Methods figures, 40,501 kg contains five significant figures, and so on. 3. Zeros to the left of the first nonzero digit are not significant. Their purpose is to indicate the placement of the decimal point. For example, 0.08 L contains one significant figure, 0.0000349 g contains three significant figures, and so on. 4. If a number is greater than 1, then all the zeros written to the right of the decimal point count as significant figures. Thus, 2.0 mg has two significant figures, 40.062 mL has five significant figures, and 3.040 dm has four significant figures. If a number is less than 1, then only the zeros that are at the end of the number and the zeros that are between nonzero digits are significant. This means that 0.090 kg has two significant figures, 0.3005 L has four significant figures, 0.00420 min has three significant figures, and so on. 5. For numbers that do not contain decimal points, the trailing zeros (that is, zeros after the last nonzero digit) may or may not be significant. Thus, 400 cm may have one significant figure (the digit 4), two significant figures (40), or three significant figures (400). We cannot know which is correct without more information. By using scientific notation, however, we avoid this ambiguity. In this particular case, we can express the number 400 as 4×102 for one significant figure, 4.0×102 for two significant figures, or 4.00×102 for three significant figures. A second set of rules specifies how to handle significant figures in calculations. 1. In addition and subtraction, the answer cannot have more digits to the right of the decimal point than either of the original numbers. Consider these examples: 89.332 +1.1 ← one digit after the decimal point 90.432 ← round off to 90.4 2.097 -0.12 ← two digits after the decimal point 1.977 ← round off to 1.98 The rounding-off procedure is as follows. To round off a number at a certain point we simply drop the digits that follow if the first of them is less than 5. Thus, 8.724 rounds off to 8.72 if we want only two digits after the decimal point. If the first digit following the point of rounding off is equal to or greater than 5, we add 1 to the preceding digit. Thus, 8.727 rounds off to 8.73, and 0.425 rounds off to 0.43. 2. In multiplication and division, the number of significant figures in the final product or quotient is determined by the original number that has the smallest number of significant figures. The following examples illustrate this rule: 2.8×4.5039=12.61092 → round off to 13 6.85 = 0.0611388789 → round off to 0.0611 112.04 33 Chemistry Grade 9 3. Keep in mind that exact numbers obtained from definitions (such as 1 ft, 12 in, where 12 is an exact number) or by counting numbers of objects can be considered to have an infinite number of significant figures. Example 2.5 Significant figures Determine the number of significant figures in the following measurements: (a) 394 cm, (b) 5.03 g (c) 0.714 m, (d) 0.052 kg, (e) 2.720 × 1022 atoms, (f ) 3000 mL. Solution: (a)Three, because each digit is a nonzero digit. (b) Three, because zeros between nonzero digits are significant. (c) Three, because zeros to the left of the first non zero digit do not count as significant figures. (d) Two. Same reason as in (c). (e) Four, because the number is greater than one, all the zeros written to the right of the decimal point count as significant figures. (f) This is an ambiguous case. The number of significant figures may be four (3.000 × 103), three (3.00 × 103), two (3.0 × 103), or one (3 × 103). This example illustrates why scientific notation must be used to show the proper number of significant figures. Exercise 2.5 Significant figures: Determine the number of significant figures in each of the following measurements: a. 35 mL d. 7.2 × 104 molecules b. 2008 g e. 830 kg. c. 0.0580 m 3 Exercise 2.6 1. Arithmetic operations: Carry out the following arithmetic operations to the correct number of significant figures: a. 11,254.1 g + 0.1983 g d. 0.0154 kg ÷ 88.3 mL b. 66.59 L – 3.113 L e. 2.64 × 103 cm + 3.27 × 102 c. 8.16 m × 5.1355 cm. 2. Significant Figures (Addition and Subtraction) a. Add 37.24 mL and 10.3 mL. b. Subtract 21.2342 g from 27.87 g. 3. Significant Figures (Multiplication) What is the area of a rectangle 1.23 cm wide and 12.34 cm long? 34  Measurements and Scientific Methods 2.1.7 Scientific Notation and Decimal Places We use scientific notation when we deal with very large and very small numbers. For example, 197 grams of gold contains approximately 602,000,000,000,000,000,000,000 gold atoms. The mass of one gold atom is approximately 0.000 000 000 000 000 000 000 327 gram. In using such large and small numbers, it is inconvenient to write down all the zeros. In scientific (exponential) notation, we place one nonzero digit to the left of the decimal. 602,000,000,000,000,000,000,000 = 6.02×1023 23 places to the left, therefore exponent of 10 is 23 0.000 000 000 000 000 000 000 327 =3.27×10-22 22 places to the right, therefore exponent of 10 is -22. The reverse process converts numbers from exponential to decimal form. Example 2.6 Unit Conversions The Ångstrom (Å) is a unit of length, 1×10-10 m,that provides a convenient scale on which to express the radii of atoms. Radii of atoms are often expressed in nanometers. The radius of a phosphorus atom is 1.10 Å. What is the distance expressed in centimeters and nanometers? Plan We use the equalities 1 Å= 1×10-10m, 1 cm = 1×10-2 m, and 1nm = 1× 10-9 m to construct the unit factors that convert 1.10 Å to the desired units. Solution: Let x be the length in cm unit and y the length in nm. 1 x 10-10 m x 1cm x cm = 1.10 0A x 10A 1 x 10-2 m = 1.10 x 10 cm -8 1 x 10-10 m x 1nm y nm= 1.10 0A x 10A 1 x 10-9 m = 1.10 x 10 nm -1 Exercise 2.7 Volume Calculation Assuming a phosphorus atom is spherical; calculate its volume in Å3, cm3, and 4 nm3. The formula for the volume of a sphere is V = 3 πr 3 Refer to Example above. 35 Chemistry Grade 9 2.2 Chemistry as Experimental Science Learning competencies At the end of this section, you should be able to  define scientific method;  describe the major steps of the scientific method;  use scientific methods in solving problems;  demonstrate some experimental skills in chemistry;  describe the procedures of writing laboratory report. Chemistry is largely an experimental science, and a great deal of knowledge comes from laboratory research. In addition, however, today’s chemist may use a computer to study the microscopic structure and chemical properties of substances or employ sophisticated electronic equipment to analyze pollutants from auto emissions or toxic substances in a soil. Many frontiers in biology and medicine are currently being explored at the level of atoms and molecules the structural units on which the study of chemistry is based. Chemists participate in the development of new drugs and in agricultural research. What’s more, they are seeking solutions to the problem of environmental pollution along with replacements for energy sources. And most industries, whatever their products, have a basis in chemistry. For example, chemists developed polymers (very large molecules) that manufacturers use to make a wide variety of goods, including clothing, cooking utensils, artificial organs, and toys. Chemistry is evidence based. All chemical statements are based on experiment. Chemistry is part of the body of modern science. It shares the experimental method of all sciences. It improves in time also using new discoveries and concepts from other sciences. In turn, it provides both theoretical and experimental tools to different sciences. Biology and Geology cannot be studied without a thorough understanding of chemical phenomena. Indeed, because of its diverse applications, chemistry is often called the “central science.” 2.2.1 The Scientific Method Conduct the following activity and present your finding to the class. a. Collect a plastic bag filled with different items provided by your teacher. b. Decide on the question you would like to answer about your bag. Write it down. (Do not open the bag) 36  Measurements and Scientific Methods c. Guess what the answer to your question might be. Write down. (Do not open the bag) d. Open your bag and answer the questions. e. Be sure to count the total number of items. Now, discuss which part of the activity (a, b, c, d, or e) introduces the scientific terminology: hypothesis, data A ctivity 2.4 collection, experimentation, etc. The Scientific method What is Scientific Method? The Scientific method is a process with the help of which scientists try to investigate, verify, or construct an accurate and reliable version of any natural phenomena. They are done by creating an objective framework for the purpose of scientific inquiry and analyzing the results scientifically to come to a conclusion which either supports or contradicts the observation made at the beginning. Scientific Method Steps The aim of all scientific methods is the same, that is, to analyze the observation made at the beginning but there are various steps adopted as per the requirement of any given observation. However, there is a generally accepted sequence of steps of scientific methods as it is shown in Figure 2.6. Figure 2.6 The four main steps of the research process in studying chemistry and their relationships. 37 Chemistry Grade 9 i. Observation and formulation of a Question: This is the first step of a scientific method. In order to start one, an observation has to be made into any observable aspect or phenomena of the universe and a question needs to be asked pertaining to that aspect. For example, you can ask, “Why is the sky black at night? or “Why is air invisible?” ii. Data Collection and Hypothesis: The next step involved in the scientific method is to collect all related data and formulate a hypothesis based on the observation. The hypothesis could be the cause of the phenomena, its effect, or its relation to any other phenomena. iii. Testing the Hypothesis: After the hypothesis is made, it needs to be tested scientifically. Scientists do this by conducting experiments. The aim of these experiments is to determine whether the hypothesis agrees with or contradicts the observations made in the real world. The confidence in the hypothesis increases or decreases based on the result of the experiments. iv. Analysis and Conclusion: This step involves the use of proper mathematical and other scientific procedures to determine the results of the experiment. Based on the analysis, the future course of action can be determined. If the data found in the analysis is consistent with the hypothesis, it is accepted. If not, then it is rejected or modified and analyzed again. It must be remembered that a hypothesis cannot be proved or disproved by doing one experiment. It needs to be done repeatedly until there are no discrepancies in the data and the result. When there are no discrepancies and the hypothesis is proved beyond any doubt, it is accepted as a ‘theory’. 2.2.2 Some Experimental Skills in Chemistry Laboratory Safety Rules In the home, the kitchen and bathroom are the sites of most accidents. The chemical laboratory poses similar hazards and yet it can be no more dangerous than any other classroom if the following safety rules are always observed. Most of them are based on simple common sense. 1. Responsible behaviour is essential. The dangers of spilled acids and chemicals and broken glassware created by thoughtless actions are too great to be tolerated. 2. Wear approved eye protection at all times in the laboratory and in any area where chemicals are stored or handled. The only exception is when explicit instructions to the contrary are given by your teacher. 3. Perform no unauthorized experiments. This includes using only the quantities instructed, no more. Consult your teacher if you have any doubts about the instructions in the laboratory manual. 4. Do not smoke in the laboratory at any time. Not only is smoking a fire hazard, but smoking draws chemicals in laboratory air (both as vapours and as dust) into the lungs. 5. In case of fire or accident, call the teacher at once. Note the location of fire 38  Measurements and Scientific Methods extinguishers and safety showers now so that you can use them if needed. 6. Report all injuries to your instructor at once. Except for very superficial injuries, you will be required to get medical treatment for cuts, burns, or fume inhalation. 7. Do not eat or drink anything in the laboratory. 8. Avoid breathing fumes of any kind. 9. Never use mouth suction in filling pipets with chemical reagents. Always use a suction device. 10. Never work alone in the laboratory. There must be at least one other person present in the same room. In addition, your teacher should be quickly available. 11. Wear shoes in the laboratory. Bare feet are prohibited because of the danger from broken glass. Sandals are prohibited because of the hazard from chemical spills. 12. Confine long hair and loose clothing (such as ties) in the laboratory. They may either catch fire or be chemically contaminated. a. A laboratory apron or lab coat provides protection at all times. A lab apron or lab coat is required when you are wearing easily combustible clothing (synthetic and light fabrics). b. It is advisable to wear old clothing to laboratory, because it is both generally not as loose and flammable as new clothing, and not as expensive to replace. 13. Keep your work area neat at all times. Clean up spills and broken glass immediately. Clutter not only will slow your work, but it leads to accidents. Clean up your work space, including wiping the surface and putting away all chemicals and equipment, at the end of the laboratory period. 14. Be careful when heating liquids; add boiling chips to avoid “bumping”. Flammable liquids such as ethers, hydrocarbons, alcohols, acetone, and carbon disulfide must never be heated over an open flame. 15. Always pour acids into water when mixing. Otherwise the acid can spatter, often quite violently. Pour acid into water. 16. Do not force a rubber stopper onto glass tubing or thermometers. Lubricate the tubing and the stopper with glycerol or water. Use paper or cloth towelling to protect your hands. Grasp the glass close to the stopper. 17. Dispose of excess liquid reagents by flushing small quantities down the sink. Consult the teacher about large quantities. Dispose of solids in crocks. Never return reagents to the dispensing bottle. 18. Carefully read the experiment and answer the questions in the prelab before coming to the laboratory. An unprepared student is a hazard to everyone in the room. 19. Spatters are common in chemistry laboratories. Test tubes being heated or containing reacting mixtures should never be pointed at anyone. If you observe this practice in a neighbour, speak to him or her or the teacher, if needed. 20. If you have a cut on your hand, be sure to cover with a bandage or wear appropriate laboratory gloves. 21. Finally, and most important, think about what you are doing. Plan ahead. Do 39 Chemistry Grade 9 not cookbook. If you give no thought to what you are doing, you predispose yourself to an accident. The first and foremost rule of any laboratory is to be safe! This may seem obvious, but people often disregard safety protocols for one reason or another, putting themselves and those around them in danger. The best thing you can do is to make sure you follow all safety protocols at all times. Safety goggles are required wear in all chemistry labs. Not wearing them puts you in danger of eye irritation and possibly blindness in the case of an accident. A small droplet of acid could splash out of the container at any time. Better safe than permanently blinded! Latex gloves should be used when there is a possibility of corrosive chemicals spilling onto your hands. A lab apron or coat can also prevent injury in case of spills or splashes. A beaker is a common container in most labs. It is used for mixing, stirring, and heating chemicals. Most beakers have spouts on their rims to aid in pouring. They also commonly have lips around their rims and markings to measure the volume they contain, although they are not a precise way to measure liquids. Beakers come in a wide range of sizes. Because of the lip that runs around the rim, a lid for a beaker does not exist. However, a watch glass can be used to cover the opening to prevent contamination or splashing. Figure 2.7 shows some of the commonly used laboratory equipments. Crucible tongs Ring support Bunsen burner Ring stand Scoopula Test tube brush Utility clamp Clay triangle Wire gauze Test tube Spatula holder Test tube rack Beaker Elenmeyer Florence Graduated Funnel Evaporating dish flask flask cylinder Crucible & cover Medicine Watch Wash bottle Test tube Buret Graduated Volumetric dropper glass pipet pipet Figure 2.7 Commonly used Laboratory Equipment. 40  Measurements and Scientific Methods Conduct the following activity and present your finding to the class. Laboratory equipment List down laboratory equipment you know and that are not shown in Figure 2.7. Describe their use. Which of A ctivity 2.5 them are used for measurement? 2.2.3 Writing a Laboratory Report A. The Pre-laboratory Report Each experiment in this manual includes a pre-laboratory (prelab) report. The prelab report is to be completed before the experiment is begun in the laboratory. Its purpose is to ensure familiarity with the procedure and provide for a more efficient utilization of limited laboratory time. The prelab questions can be answered after a careful reading of the introduction and procedure of the experiment. Sample calculations are sometimes included to provide awareness of data that needs to be collected and how it is treated. Your teacher may prefer to administer prelab quizzes instead of collecting prelab reports. B. The Laboratory Report A good laboratory report is the essential final step in performing an experiment. It is in this way that you communicate what you have done and what you have discovered. Since it is the only means, in many instances, of reporting results, it is important that it be prepared properly. A laboratory report is a final draft. As such it is always written in ink or typed. A typed laboratory report is necessary if your handwriting is hard to read. There must be no erasures or crossed out areas. The initial draft of a laboratory report belongs in your laboratory notebook for two reasons. 1. It is unlikely that you will get everything correct on the first attempt and, thus, a first draft written on the report form itself could be very messy. 2. If the report itself is lost or destroyed, you can easily and quickly rewrite the report from the notebook. It is essential that a laboratory report be neat. Studies have shown that when the same work is submitted in both neat and sloppy form, the neat version makes the better impression. Neat work indicates that the writer knows and cares about the subject matter. All data should be presented with the correct significant figures and units. The omission of units makes it difficult for the reader to know the size of the numbers being reported. And writing down the wrong number of significant figures amounts to lying about the 41 Chemistry Grade 9 precision of the data. Too many significant figures imply that you know a number more precisely than you actually do. All questions should be answered with complete and grammatically correct sentences. Abbreviations should not be included in written answers. Read the sentence out loud to make sure that it makes sense. Your sample computations should be labeled with their purpose, for example; “mass of the liquid”. Within the computation, all numbers must have the correct units and the correct number of significant figures. Laboratory reports that extend to more than one page should either be stapled together or have your name and the page number at the top right of each page. For example: Terhas Asgedom, page 2 of 4 pages. This makes it more difficult for the instructor to inadvertently misplace pages. Using a paper clip or tearing corners to hold pages together is not acceptable. Reports should also be dated. Graphs Graphs are used to present the data in picture form so that they can be more readily grasped by the reader. Occasionally, a graph is used to follow a trend. Notice that the best smooth curve is drawn through the data points. This is not the same as connecting the dots; all of the data points will not fall on the line. Often, however, a graph is used to show how well data fit a straight line. The line drawn may either be visually estimated (“eyeballed”) or computed mathematically. There are many essential features of a good graph. 1. The axes must be both numbered and labeled. The abscissa is the right-to-left or the horizontal axis or x-axis. 2. The graph must have a title. When we speak of graphing, we always mention the quantity plotted on the ordinate first. 3. The data points are never graphed as little dots. One may use small circles, small circles with a dot inside, crosses, asterisks, or X’s. If dots are used, data are too easily lost on the graph or “created” by stray blobs of ink. 4. Any lines that appear on the graph in addition to data points should be explained. Thus, the line drawn is explained in the title as “(visually estimated best straight line).” 5. The scales of the axes should be adjusted so that the graph fills the page as much as possible. Measurements and Density Chemistry is very much an experimental science in which careful and accurate measurements are the very essence of meaningful experimentation. It is, therefore, essential for the beginning student to learn how scientific measurements are carried out properly through the use of common measuring instruments. It is equally important for the student to acquire an appreciation of the significance of measurements and to 42  Measurements and Scientific Methods apply learned technique to a common specific experiment. In the following experiment you will become familiar with how mass and volume measurements are carried out and how an evaluation of the measurements is reflected in the number of significant figures recorded. These mass and volume measurements will then be used to determine the density of (1) a metal bar and (2) a salt solution by two different methods. Finally, the results of the density measurements will be evaluated with respect to their precision and accuracy. The density of an object is one of its most fundamental and useful characteristics. As an intensive property it is independent of the quantity of material measured since it is the ratio of the mass of an object to its volume. The density of an object can be determined by a variety of methods. In this experiment you will practice using a balance to measure mass. In