Grade 8 General Science Textbook PDF

Fetena.net : Ethiopian No#1 Educational Resource General Science Student Textbook Grade 8 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook...

Fetena.net : Ethiopian No#1 Educational Resource General Science Student Textbook Grade 8 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook General Science Grade 8 Student Textbook Authors: Yonas Nibret (BSc., MA) Sefiw Melesse (Msc.) Abebe Habte (Msc) Editors and Evaluators: Getahun Getachew (BEd.) Muluneh T/Birhan (BEd.) Ali Kemal (MEd.) Coordinator Getachew Talema (MA.) layout design & Art: Entoto poly technic college (T.M.S) II Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Take Good Care of This Textbook This textbook is the property of your school take good care not to damage or lose it. Here are 10 ideas to help take care of the book 1. Cover the book with protective materials, such as plastic , old newspapers or magazine. 2. Always keep the book in a clean dry place. 3. Be sure your hands are clear when you use the book 4. Do not write on the cover or inside pages. 5. Use a piece of paper or cardboard as a bookmark. 6. Never tear or cut out any pictures or pages. 7. Repair any torn page with paste or tape. 8. Pack the book carefully when you place it in your school bag. 9. Handle the book with care when passing it to another person. 10. When using a new book for the first time, lay it on its back, open a few page at a time. Press lightly along the bound edge as you turn the pages. This will keep the cover in good condition. III Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook c 2021 by Addis Ababa Education Bureau While every attempt has been made to trace and acknowledge copyright, the authors and publishers apologies for any accidental infringment where copyright has proved untraceable. Acknowledgement Above all, Ato Zelalem Mulatu, AAEB Head, should receive the most ac- knowledgements for his outstanding leadership from the outset to the end of the Textbook and Teacher’s guide preparation. Just to mention his chief roles but a few: he generated valuable ideas, shared his vast experience during most panels, initiated and convinced his conviction to all stakeholders that the Addis Ababa City Government School teachers have to take the lion’s share in the Textbook and Teacher Guide development. His unabated inspiration and urge to the team for diligence, deep sense of patriotism, synergy and true professional ethics has been energy to all actors partaking in the task. The next bottom-heart gratitude has to be extended to the following management members of the bureau: Ato Admasu Dechasa, Deputy Head of the Curriculum Division, Ato Dagnew Gebru, Deputy Head of the Education Technology, Ato Samson Melese, deputy Head of Teacher Development Division, W/ro Abebech Negash, Bureau Head Advisor, Ato Desta Mersha, Bureau Technical Advisor and Ato Sisay Endale, Head of Education Bureau Office. Members of the AAEB management, have to be commended for their painstaking efforts in addressing instantly each issue of challenge, reviewing all drafts and providing immediate feedbacks. Without their unreserved devotion, the timely and successful realiza- tion of this huge work would not have been possible. The Last deepest acknowledgement needs to go to the school principals for allowing the textbook writers to be free of their regular job of teaching and to focus on the material preparation. Moreover, their earnest and consistent moral support deserves special words of praise. IV Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Table of Contents UNIT ONE Basics of Scientific Investigation.....................................................1 1.1 Scientific Measurements.............................................................2 1.2 Doing Scientific Investigation...................................................16 Review Exercise...............................................................................23 UNIT TWO Composition of Matter.....................................................................25 2.1 Early Thinking about the Composition of Matter......................26 2.2 Inside of an Atom.......................................................................27 2.3 Molecules...................................................................................31 Review Exercise...............................................................................35 UNIT THREE Classification Of Compounds..........................................................38 3.1 Introduction...............................................................................39 3. 2 Organic Compounds.................................................................40 3.3 Inorganic Compounds................................................................45 3.4 Neutralization Reaction and Salts..............................................64 Review Exercise...............................................................................71 UNIT FOUR Human Body Systems and Health..................................................74 4.1 Integumentary Systems.............................................................75 4.2 Muscular System.......................................................................85 4.3. Skeletal System........................................................................89 4.4. Digestive System......................................................................98 4.5 Respiratory System...................................................................106 4.6 Circulatory System...................................................................109 4.7 Reproductive System..............................................................114 Review Exercise.............................................................................125 V Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook UNIT FIVE Ecosystem and Conservation of Natural Resources......................127 5.1. Ecosystem and Interactions....................................................129 5.2. Conservation of Natural Resources........................................142 Review Exercise............................................................................162 UNIT SIX The Solar System...........................................................................164 6.1 Family of the Solar System......................................................165 6.2 Formation of the Solar System.................................................177 6.3 Earth in Comparison with Solar System..................................180 6.4 Our Planet’s Suitability for Life (uniqueness)..........................183 Review Exercises...........................................................................185 UNIT SEVEN Physical Phenomena in the Surrounding........................................187 7.1 Phenomena of Light (source & properties)..............................188 7.2 Vision and Imaging..................................................................194 7.3 Sound.......................................................................................199 7.4 Heat..........................................................................................207 7.5. Simple Circuit.........................................................................212 7.6 Magnetism...............................................................................216 Review Exercises...........................................................................223 VI Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook UNIT ONE BASICS OF SCIENTIFIC INVESTIGATION Learning Outcomes: At the end of this unit, You will be able to: identify the basic and derived units of measurements; explain the concept of measuring physical quantities; describe the components of a scientific investigation; demonstrate ability to work effectively and respectfully with others in performing fair testing. Main contents 1.1 Scientific Measurments 1.2 Doing Scientific Investigation 1 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Introduction This unit contains two sub units: scientific measurement and do- ing scientific investigation. Under scientific measurement the in- digenous and modern methods of measurement, the classification of physical quantities into fundamental and derived quantity and the difference between accuracy and precision will be discussed. Under doing scientific investigation, the importance, procedures and ethical issues of a scientific investigation will be discussed. Finally using locally available materials, a simple investigation will be conducted. 1.1 Scientific Measurements At the end of this section, you will be able to: explain the concept of measuring physical quantities; describe the various indigenous methods of measurement; distinguish between the basic and derived physical quantities; categorize the basic and derived units of measurements (length, mass, time, temperature, volume, area, density, force); identify prefixes and perform conversions among units of measurements; distinguish between accuracy and precision in measurements. Introduction Making observation is common experience in science. Similarly, it is usual asking the basic questions like how big an object is? How tall are you? To answer these questions, measurements have to be made. Measurement is the process of obtaining the magnitude of a quantity relative to an agreed standard. 2 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook In this section both the indigenous and modern methods of measurement will be discussed. The indigenous method of measurement refers to a measurement practiced locally while the modern method refers to a measurement applied by the scientific community. Indigenous Methods of Measurements An indigenous method of measurement refers to measurement methods that are practiced locally for a long period of time and are passed from generations to generation. In this section, we will pay attention to the measurement of length, mass, and time. A. Length Length is a measure of the distance between two points. In Ethiopia we use different indigenous units of length measurement. The commonly used ones are: 1. Hand-span: The hand-span is the measure from the tip of your little finger to the tip of your thumb when your hand is stretched out, Fig 1.1 (a). 2.Digit: A digit is the width of an adult human male fingertip, Fig 1.1 (b). 3.Cubit: A measure of distance from the tip of one’s elbow to the tip of the middle finger when your arm is extended, Fig 1.1 (c). 4.Foot: A measure of distance from the back of the heel to the tip of the big toe, Fig 1.1 (d). 5.Pace: A linear distance measure of a person’s extended walk. A pace is a unit of length consisting either of one normal walking step. The pace is the distance measured from the heel of one foot to the heel of the same foot when it next touched the ground, Fig 1.1 (e). 6. Arm span: Arm span also known as fathom is the distance from the middle fingertip of the left hand to that of the right hand when you stretch your arms out as far as they can reach, Fig 1.1 (f). 3 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Figure 1.1 Indigenous Length measurements Figure 1.1 Indigenous Measurement Length Activity 1.1: Make a group containing 5 students. Using your hand Activity 1.1: Make a group containing 5 students. Using your hand span, span,cubit cubit and digit digit measure measurethe thewidth widthof of a table a table or aor a desk desk in your in your classroom.Record classroom. Recordyour yourmeasurement measurement in in the the table table below. below. No Name of the student Measurement result making measurement 1 2 3 4 Question: Did each of you obtain the same measure for that bench? Question: Did each of you obtain the same measure for that table or Justify desk? the difference Justify of students‘ the difference measurement. of students’ measurement. 11 hand-span, digit, cubit, foot, pace Exercise 1.1: Compare the size of your and arm-span and write them in order of increasing value. B. Mass The amount of matter present in a substance is called mass. Like length, there is also an indigenous method of measuring mass. The following are some examples of the indigenous unit of mass measurement used in Ethiopia. 4 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 1. Weqet- Weqet is a mass measuring unit usually used to measure the mass of powder of gold in local markets. 2. Quntal – Quntal (may be taken from the English word quintal) is a bag used to measure the mass of grains. It is equal to a hundred kilogram. 3. Feresula:- is used to measure the mass of pepper and coffee. It is equal to 17 kilogram. Figure 1.2 Indigenous mass measurements Exercise 1.2: Discuss about the reliability of the above three indigenous mass measuring methods. C. Time Time is the measure of the duration for an interval.There is also an indigenous method of measuring time. Our elders were used the shadow of a tree to measure time. As the position of the Sun changes from morning to evening the length of the shadow of a tree varies. In the morning and late in the afternoon, the length of the shadow is high. At noon when the Sun is overhead no shadow will be seen. Using this fact they could tell the approximate time of the day by just looking at the position of the shadow of a tree found at or near their home. Activity 1.2: Using a long tree found in your school, mark the time at different height of the shadow of the tree. Use this shadow clock for some time. Discuss your observation. 5 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Project 1.1: In ancient time three commonly known time measuring devices were used: They are known as sundial, sand clock and water clock. Using internet and other sources explore how these devices were used to measure time and report your finding to the class. D. Volume Volume is the measure of the space occupied by an object. In the local markets of Addis Ababa the following tools are used for different size volume measurements. 1. Jog: A plastic cup used for measuring the volume of liquids. 2. Tassa: A can used to measure cereals, pulses ,liquids and solids. 3. Sini: A small ceramic cup often used for measuring coffee, pulses and spices. 4. Birchiko: A glass often for measuring pulses and liquids. 5. Kubaya: A mug, often used for measuring cereals, pulses and liquids. Figure 1.3 Some examples of Indigenous volume measurements Exercise 1.3: 1. Discuss about the problems there could be in using the above indigenous volume measuring devices. 2. Discuss in group about the pros and cons of indigenous measurements used in your locality 6 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Project 1.2: With the help of your teacher go to the local market found near to your school. Gather information about the indigenous measuring devices used for different measurements in the market. You can also ask your elder family members and present a report to your classmates. Physical Quantities and Scientific Methods of Measurement In our day to day life, we measure many things such as the mass of vegetables, the volume of liquids, the speed of a car, the temperature of the day etc. Such quantities which could be measured are called physical quantities. A physical quantity is a property of an object that can be measured or calculated from other physical quantity. Examples of physical quantities are: length, mass, time, temperature, area, volume, density, force etc. Generally, physical quantities are classified into two types, namely: fundamental quantities and derived quantities 1.Fundamental Physical quantities and their units Fundamental quantities, also known as base quantities, are quantities which cannot be expressed in terms of any other quantity. They are the bases for other quantities. There are seven fundamental (basic) physical quantities: length, mass, time, temperature, electric current, luminous intensity and amount of a substance. In this section we will discuss only about the first four commonly measured fundamental quantities: length, mass, time and temperature. The names and symbols of the units of the fundamental quantities in the International System of units (SI) are shown in table 1.1.The International System of Units (SI, abbreviated from the French Système international (d’unités)) is a system of measurement based on base units. An International System of units (SI) is currently used all over the world. Measurement is the comparison of an unknown quantity with some known quantity. This known fixed quantity is called a unit. Thus, the result of a measurement is expressed in two parts. One part is a number and the other part is the unit of the measurement. For example, if a student has a mass of 32 kg: 7 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook is mass, the value of the measurement is 32 and the unit of measure is kilograms (kg). the quantity being measured is mass, the value of the measurement is 32 This tells us that any measurement consists of two parts. The first is the and the unit of measure is kilograms (kg). number This which tells us that indicates the magnitude any measurement of theofquantity consists andThe two parts. the second first is theindicates number the which indicates of unit (standard) thethat magnitude quantity. of the quantity and the second indicates the unit (standard) of that quantity. Units can be classified into two groups: fundamental units and derived Units can be classified into two groups: fundamental units and derived units. units. TheThe units units usedused to tomeasure measurefundamental fundamental quantities quantities are are called called fundamental fundamental units. units.ItItdoes doesnotnotdepend depend onon any anyother otherunit. unit. Table 1. 1 Fundamental quantities and their SI units Quantity Name of Unit Symbol of the unit Length Meter m Mass kilogram kg Time Second S Temperature Kelvin K Derived Physical 2.Derived Physical Quantities Quantitiesand andtheir Units their Units Physical quantities Physical which quantities depend which on oneon depend or more one fundamental quantities or more fundamental for quantities their measurements derived are called are for their measurements called quantities. Speed,Speed, derived quantities. area, volume, density and force are examples of derived quantities. The area, volume, density and force are examples of derived quantities. The units used to measure derived quantities are called derived units. It units used depends to measure units on fundamental derived forquantities are called SI their measurement. derived units. derived It units aredepends described on by mathematically fundamental combining units for (dividing,SI their measurement. multiplying or derived units powering) the base units. Some of the derived quantities and their are described by mathematically combining (dividing, multiplying or units are given in table 1.2. powering) Tablethe1.base units. Some 2 Derived of the derived quantities quantities and their and their units SI units are given in table 1.2. No. Derived quantity Symbol Unit 1 Table Area1. 2 Derived quantities A and their mSI units xm = m2 2 Volume V m x m x m = m3 3 Speed V 16 m/s 4 Density ῤ Kg/m3 8 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Example 1.1: Show how the unit of (a) area and (b) speed is derived from the fundamental units. Solution: (a) The equation for the area of rectangular surface is Area = length x width. Both length and width are length measurements. Hence they are measured in meter. Unit of area = unit of length x unit of width Unit of area = m x m = m2 (b) The equation for speed is Speed = distance/time Thus the unit of speed is the unit of distance (m) over the unit of time (s) = m/s Activity 1.3: Discuss in group about the importance of scientific measurement to the study of science. Let the representative of your group present what you have agreed to your classmate. Exercise 1.4: Show how the units of the following derived quantities are derived from the unit of base quantities. (a) volume, (b) density and (c) force. Prefixes and Conversion of Base Units Prefix In science we deal with quantities which are both very large and very small. A short hand form of writing very large and very small numbers is known as a prefix. A few of the prefixes used in the SI system of units are shown in Table 1.3. 9  In science we deal with quantities which are both very large and very Fetena.net : Ethiopian No#1 Educational Resource small. A short hand form of writing very large and very small numbers General Science GRADE 8 Student TextBook is known as a prefix. A few of the prefixes used in the SI system of units are shown in Table 1.3. Table 1.3. SI prefixes Prefix Symbol Name Decimal representation Mega M million 1 000 000 Kilo k thousand 1 000 Centi c hundredth 0.01 milli m thousandth 0.001 Conversion micro of base µ units millionth 0.000001 It is often necessary to convert between units of measurement. For Conversion of base units example, It is oftena necessary mass measured in grams to convert may beunits between required to convert intoFor of measurement. example, kilogram. a mass measured in grams may be required to convert into kilogram. To convert from one unit to another within the SI, usually means To convert from one unit to another within the SI, usually means moving a decimal point. If you can remember what the prefixes mean, moving a decimal point. If you can remember what the prefixes mean, you can you can convert convert within withinthe theSISIsystem systemrelatively relativelyeasily by by easily simply simply multiplying multiplying orordividing dividing thethe number number by value by the the value of theofprefix. the prefix. Example 1.2: Convert 6.5 kilogram (kg) to gram (g). Example 1.2: Convert 6.5 kilogram (kg) to gram (g). Solution: Since killo (k) is a prefix representing 1000, so: Solution: 6.5 Since kg = 6.5 k is a prefix × (1000) representing g = 6500 g 1000, so: Example 6.5 kg = 6.51.3: Convert × (1000) g = 200 6500meters g to kilometers. We know1.3: Example thatConvert 1 km 200= 1000m. Then meters to we will ask if 1000m is 1km kilometers. then what will be 200m in km? 18 Solution: 1 km = 1000 m 200 m = 1 km × 200 m 200 km = = 0.2 km ? = 200m 1000 m 1000 Exercise 1.5 1. Convert the following: a) 0.6 km to cm b) 500 g to kg c) 30 min to hour d) 50 m to mm e) 0.25 kg to g f) 0.5 hour to second 2. Write the following quantities in units with the appropriate prefixes: a) 3500 m b) 0.0012 sec c) 0.01 g 10 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Measuring Physical Quantities The measurement of a physical quantity is done by using measuring instruments. In this section we will discuss how to measure mass, length, time, and temperature using their appropriate devices. Measuring the mass of objects Instruments which are used to measure mass are known as balances. Theused to measure balance mass. Itthe compares workmass basedofonan the object principlewith that the amount mass. a known of extension (or Different compression) types of a springare of balances is proportional there, to seethe mass Fig of1.4. the object attached to it. Figure 1.4: Instruments Used to Measure Mass Note that, before taking measurement check that the balance is on a level surface, and reads zero when no load is placed on it. Note that, before taking measurement check that the balance is on a Thesurface, level SI or base andunit of Mass reads zero is kilogram when (kg).isFor no load smallon placed mass it. we use gram (g). To measure the mass of objects less than 1 gram, we can use Themilligram. SI unit of To mass measure kilogram is the (kg). mass of big For we objects small mass we use quintal anduse tone.gram (g). To measure the mass of objects less than 1 gram, we can use 1 kg = 1000 g. milligram. To measure the mass of big objects we use quintal 1 gtone. and = 1000 mg 1 quintal = 100 kg 1 tone = 1000 kg 20 11 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook The relationship between different units of Length. 1 kg = 1000 g. 1 g = 1000 mg 1 quintal = 100 kg 1 tone = 1000 kg Example 1.4: How much is 1200 gram in kilogram? 1 Solution: 1200 g = 1200 × kg = 1.2 kg 1000 Exercise 1.6: Convert the following measurement: (a) 2.5 kg to gram, (b) 200 gram to milligram. Measuring Length Length is a measure of how long an object is. Depending on the size of the length of the object, we are going to use different types of length measuring instrument, see Fig 1.5. Figure 1.5 Instruments used to Measure Length The SI unit of length is meter (m). When we want to measure larger lengths, we can use kilometers. If we want to measure small lengths, we can use centimeters or millimeters. 12 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook The relationship between different units of Length. 1km = 1000 m 1 m = 100 cm 1cm = 10mm Note that when we are measuring length using these device do not forget to place the zero mark exactly at one end of the thing you are measuring and read the scale at the other end. Example 1.5: How many millimeters are there in a meter? Solution: 1m = 100 cm = 100 x 10 mm = 1000 mm Exercise 1.7: Convert the following into the required measures: (a) 8 meters to millimeter. (b) 5500 meters to kilometer. Measuring time Time is used to quantify the duration of events. Time is measured with a stop watch or clock. Figure 1.6 Time measuring Instruments The SI unit of time is second (s). For longer intervals of time we use: day, month , year, decades, century and millennium. 13 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook The relationship between different units of time 1 hour = 60 minutes 1 minute = 60 seconds 1 day = 24 hours 1 week = 7 days 1 year = 365 or 366 days Example 1.6: Convert one hour into seconds. Solution: 1 hour = 60 minutes = 60 × 60 second = 3600 seconds. Exercise 1.8: How many (a) minutes, and (b) seconds are there in one day? Measuring Temperature Thermometer is the device used to measure the temperature of an object or place. The SI unit of temperature is Kelvin. Degree Celsius (°C) and degree Fahrenheit (0F) are other units of temperature Thermometers could be analogue or digital, see Figure 1.7 Figure 1.7 Temperature Measuring Devices Activity 1.4:Measuring body temperature. measure the body temperature of two students by using thermometer. Compare the two temperatures with the standard temperature of a body which is 37°C Discuss about your observations. 14 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook In using thermometer, hold the thermometer at the top, do not hold the bulb of a thermometer and do not let the bulb touch the glass. Activity 1.5: Measuring the temperature of water. Using a laboratory thermometer, measure the temperature of a warm water. Record your observations Safety!! Never put a laboratory thermometer into your mouth. Accuracy and Precision in Measurement Accuracy refers to how close a measurement is to its accepted or known value. Example 1.7: If in a laboratory you obtain a mass measurement of 8.2 kg for a given substance, but the actual or known mass is 10 kg, is your measurement accurate? Answer: This measurement is not accurate, because your measurement (8.2 kg) is not close to the known value (10kg). Precision refers to how close two or more measurements are to each other, regardless of whether those measurements are accurate or not. Example 1.8: In the above example 1.4, if you measure the mass of the given substance five times, and get 3.2 kg, 3.1 kg, 3.25 kg, 3.3 kg and 3.2 kg. Is your measurement precise? Answer: This measurement is precise, because the values are close to each other but not accurate because it is far from the known value (10 kg). This shows that precision is independent of accuracy. You can be very precise but inaccurate. You can also be accurate but not precise. Exercise 1.9: The figure below shows 3 results of a student playing a dart game. In the space provided below each figure, write whether the result is (a) accurate but not precise (c) precise but not accurate (b) accurate and precise (d) neither precise noraccurate 15 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Figure 1.8 Dart game Exercise 1.10: 1. Define the following terms: physical quantity, fundamental quantity, derived quantity. 2. State the various indigenous methods of measurement used in Addis Ababa. 3. What are prefixes? 4. What is the difference between accuracy and precision in measurements? 1.2 Doing Scientific Investigation At the end of this section, you will be able to: describe the components of a scientific investigation; demonstrate ability to work effectively and respectfully with others in performing fair testing; practice scientific investigation procedures using appropriate contents to their age levels. Introduction to Scientific Investigation Science is a process of learning about the world through observation, inquiry, formulating and testing hypotheses, gathering and analyzing data, and reporting and evaluating findings.This process is referred as the scientific investigation or scientific method. 16 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 1.2 Scientific Method Activity 1.6 What are the applications of scientific method? All sciences, including the social sciences, employ variations of what is called the scientific method. Scientific method is the process by which scientists approach their work. The Steps of the Scientific Method Based on the type of question being asked, the type of science being applied and the laws that apply to that particular branch of science, you may need to modify the method and alter or remove one or several of the steps. 1. Ask Questions A scientific investigation typically begins with observations. Observations often lead to questions. This question will include one of the key starters, which are, how, what, when, why, where, who or which. The question you ask should also be measurable and answerable through experimentation. It is often something that can be measured with a numerical result, although behavioral results are part of the scientific method as well. 2. Perform Background Research With your question formulated, conduct preliminary background research to prepare yourself for the experiment. You can find information through online searches or in your local library, depending on the question you are asking and the nature of the background data. You may also find previous studies and experiments that can help with your process and conclusions. 3. Establish your Hypothesis Based on the data that were gathered, the researcher formulated a hypothesis. A hypothesis is a tentative explanation for a set of observations. Your hypothes should also include your predictions that you can measure through experimentation and research.A hypothesis must be based on scientific knowledge, and it must be logical. 17 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 4. Test your Hypothesis Next, test your hypothesis by conducting an experiment. Your experiment is a way to quantifiably test your predictions and should be able to be repeated by another scientist. Assess your scientific process and make sure that the conditions remain the same throughout all testing measures. If you change any factors in your experiment, keep all others the same to maintain fairness. After you complete the experiment, repeat it a few more times to make sure the results are accurate. 5. Analyze the Results and Draw a Conclusion You can now take your experiment findings and analyze them to determine if they support your hypothesis or not. Drawing a conclusion means determining whether what you believed would happen actually happened. If it did not happen, you can create a new hypothesis and return to step three, then conduct a new experiment to prove your new theory. If what you hypothesized happened during the experimentation phase, the final step is putting together your findings and presenting them to others. 6. Communicating Results The last step in a scientific investigation is communicating what you have learned with others. This is a very important step because it allows others to verify your methods and results. If other researchers get the same results as yours, the hypothesis becomes stronger. However, if they get different results, they may not support the hypothesis. When scientists share their results, they should describe their methods and point out any possible problems with the investigation. Finally, communicating results can be done in a variety of ways including scientific papers, blogs, news, articles, conferences, etc. 18 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Figure 1.9 Steps in Scientific Method Example1.9: Simple experiment with candle that shows the necessary of air for burning. Consider how the scientific method applies in this simple experiment with air necessary for burning under two different conditions. 1. Ask Question: Is air necessary for burning? 2. Do back ground Research: From different literatures ‘‘air is necessary for burning.’’ 3. Formulate Hypothesis: The null hypothesis is that there wil be no air needs for burning. The alternative hypothesis is that there will be air needs for burning. 4. Test Hypothesis by Experiment and Collect Data:Take a candle and fix it on a table. Light the candle. The candle will continue to burn due to continuously available fresh air providing the required oxygen for combustion.Now cover the burning candle by putting an inverted gas jar over it. After a short time, the candle stops burning and gets extinguished. 5. Analyze the Results and Draw Conclusion: When the burning candle is covered with gas jar, then the candle takes away the oxygen necessary for burning from 19 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook the air enclosed in the gas jar. After some time, when all the oxygen of air inside the gas jar is used up, then the burning candle gets extinguished. This proves that air is necessary for combustion or burning of substances. Figure 1.10 a) Burning of candle b) Candle stops burning 6. Communicate Results: Report your findings in the form of a written report as an oral presentation. Air is necessary for burning. Activity 1.7 Form groups and conduct investigations on activities listed below. After investigation present your findings to the class. a. What is the effect of sunlight on the growth of bean plant? b. Does a coiled nail act like a magnet? c. How do plants store their food in their leaf? Exercise 1.13 Describe the components of a scientific investigation. Project 1.3 Conduct some investigations (for example, making injera) using local materials and methods (procedures) in groups by reading different reference books or asking a person who is knowledgeable and experienced in the area. 20 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Figure 1.11Injera Figure 1.12 Injera being cooked on a griddle Key Terms: -Fundamental quantity, -Derived quantity, -Fundamental unit, -Derived unit, -Prefix, Accuracy -Precision, and -Scientific method. 21 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Summary Measurement is the process of obtaining the magnitude of a quantity relative to an agreed standard. Indigenous units of measurement for length: cubit, span, digit, foot and pace, for mass weqet and quntal, for time length of a shadow are used. Fundamental quantities are a set of physical quantities which cannot be expressed in terms of any other quantities. Their corresponding units are called “Fundamental units”. The physical quantities which can be obtained by mathematically combining (i.e., multiplying and dividing) the fundamental quantities are known as “Derived quantities”. Their corresponding units are called “Derived units”. Prefixes are a short hand form of writing very large or very small numbers. Accuracy refers to how close a measurement is to the accepted value while precision refers to how close measurements are to each other. Scientific method is the process by which scientists approach their work. 22 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Review Exercise I. Choose the correct answer from the given alternative 1. Which of the following quantities is a fundamental quantity? a) Area b) volume c) temperature d) force 2. The difference between fundamental and derived unit is a) Fundamental units are big in value but derived units are small in value. b) Fundamental units are derived from derived units. c) Derived units are derived from fundamental units. d) There is no difference between them. 3. Which of the following is a derived quantity? a) mass b) area c) time d) length 4. The SI unit of density is a) kg/m2 b) kg/m3 c) kg/m d) g/m3 1 5. The prefix that represents is__________. 1000 a) kilo b) mega c) centi d) milli II. Fill in the blank spaces with an appropriate word. 1. Length, mass, time and temperature are ________quantities. 2. Area, volume, density and force are ___________ quantities 3. One million centimeter is equal to _____________ meter. 4. The prefix for a number 0.01 is _______________. 5. The SI unit of volume is ____________________. 23 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook III. Match the quantities in column-I to their units in column-II: Column I Column II 1 Area (a) K 2 Temperature (b)m3 3 Density (c ) m2 4 Volume (d)kg 5 Mass ( e) kg/m3 IV. Give short answer 1. Write four fundamental quantities with their units. 2. Write four derived quantities with their units. 3. Write the measurement 0.005 m using prefix. 4. Convert 1000 cm to kilometer. 5. The value of acceleration due to gravity on the surface of Earth is known to be 9.81 m/s2. In an experiment students have found the following results. 12.2 m/s2, 12.3 m/s2, 12.1 m/s2 and 12.08 m/s2. Is this measurement accurate or precise? 6. List the steps used in scientific method. 24 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook  Narrate the historical development of the atomic nature of substances; UNIT TWO  Appreciate that atoms are the building blocks which make up all COMPOSITION OF MATTER substances; Learning Outcomes:  Demonstrate understanding of the idea that the identity of a  Narrate the historical development of the atomic nature of substance is determined by its atomic structure; At the end of this unit, you will be able to: substances;  Differentiate molecules of elements from molecules of  narrate the historical Appreciate that atomsdevelopment of blocks are the building the atomic whichnature of all make up compounds; substances; substances; appreciate that atoms Demonstrate are the scientific building inquiry blocks skills whichthis along makeunit: up allDemonstrate substances; understanding of the idea that the identity of a communicating, asking questions, drawing conclusions, demonstrate substance is understanding determined by itsofatomic the idea that the identity structure; applying concepts. of a substance is determined by its atomic structure; Differentiate molecules of elements from molecules of  contents Main differentiate molecules of elements from molecules ofcompounds; 2.1 Early compounds; thinking about the composition of matter 2.2 Inside demonstrate Demonstrate scientific of an atom inquiry scientific skills skills inquiry along this unit:this unit: along communicating, asking questions, drawing conclusions,  communicating, asking and Parts of an atom (nucleus questions, drawing conclusions, electron Shells) applying concepts.  applying concepts. The Subatomic Particles of the atom Main  contents Relative mass, the charge and location of sub-atomic particles 2.1 Early thinking  Atomic aboutand number themass composition number of matter 2.2 Inside of an atom of the electrons, protons and neutrons  Determination 2.3  Parts of an atom (nucleus and electron Shells) Molecules  The Subatomic Molecules Particles of the atom of elements  Relative Moleculesmass, the charge and location of sub-atomic particles of Compounds  Atomic number and mass number  Determination of the electrons, protons and neutrons 38 2.3 Molecules  Molecules of elements  Molecules of Compounds 25 38 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 2.1 Early Thinking about the Composition of Matter At the end of this section, you will be able to: Give a short history of the concept of the atom; Compare and contrast the continuity and discreteness (discontinuity) theory of matter; Compare earlier conceptions of the structure of matter with their conceptions. Activity 2.1 Form groups and discuss the following and present your opinion to the class. 1. What is matter? 2. What do you think matters made up of? The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers, the scientists of their day. Some of them argued that matter is continuous i.e., it could be divided endlessly into smaller pieces. Others believed that matter is discrete; i.e., it cannot be infinitely divided. Democritus (460 - 370 B.C) expressed the belief that all matter consists of very small, indivisible particles, which he named atomos (meaning uncuttable or indivisible). He thought of atoms as moving particles that differed in shape and size which could join together. According to Democritus matter is discrete. Aristotle (384 – 322 B.C) argued that matter is divided into smaller and smaller parts, the division continuous forever without any limit. He did not believe in microscopic building particles of matter. Therefore, according to Aristotle, matter is continuous and he believed that matter consisted of the combinations of fire, earth, air, and water. 26  consistedofofthe consisted thecombinations combinationsofoffire, fire,earth, earth,air, air,and andwater. water. Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 2.2 8 Activity2.2 Activity Student TextBook Formtwo Form twogroups groupsand anddebate debateononone oneofofthe thefollowing followingideas ideasassigned assignedtotoyour your group.After Afterdiscussion group. Activity 2.2 discussionpresent presentyour yourreasons reasonstotothe theclass. class. Form1.1.IfIfmatter two matter groups isisdivided divided and andon and debate subdivided subdivided one of the again again andagain, and following again, what what ideas would would assigned to yourultimately group. After ultimately discussion present your reasons to the class. bebeobtained? obtained? 1. Ifa.a. matter Groupis Group 1:1:divided and According According subdivided totoAristotle‘s again and again, what would believe Aristotle‘sbelieve ultimately Group2:be b.b.Group obtained? 2:According According totoDemocritus‘s Democritus‘sbelieve believe a. Group 1: According to Aristotle’s believe b. Group 2: According to Democritus’s believe Table2.1Comparison Table 2.1Comparisonbetween betweenthe thediscrete discreteand andcontinuous continuoustheory theory ofofmatter matter DiscretenessTheory Discreteness Theory ContinuousTheory Continuous Theory ProposedbybyDemocritus Proposed Democritus ProposedbybyAristotle Proposed Aristotle Thereisisa alimit There limittotowhich whichmatter matterisisbroken broken Matterisisinfinitely Matter infinitelydivisible divisible Believedininthe Believed theexistence existenceofofatoms atoms Rejectedthe Rejected theidea ideaofofatoms atoms Exercise 4040 2.1 1. Compare and contrast the continuity and discreteness theory of matter. 2.2 Inside of an Atom At the end of this section, you will be able to: describe the structure of an atom as a nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels); state the relative charge and approximate relative mass of a proton, a neutron and an electron; draw hydrogen atoms, including the location of the protons and electrons, with respect to the nucleus; differentiate between mass number and atomic number; determine the number of protons, neutrons, and electrons in an atom. 27 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook What are the two parts of atom? An atom consists of a tiny dense nucleus surrounded by electrons. The nucleus contains positively charged protons and neutral neutrons, so it is positively charged. The electrons are negatively charged. Protons and neutrons have approximately the same mass and are about 1800 times more massive than an electron. This means that most of the mass of an atom is in its nucleus. However, most of the volume of an atom is occupied by its electrons. Figure 2.1 Diagrammatic representation of the atom The subatomic particles Activity 2.3 Draw a simple sketch of hydrogen atom model on your exercise book by using coloured pen following the instructions listed below. i. Draw a small circle labeled ‘‘nucleus’’. ii. Add a smaller circle labeled ‘‘proton’’ inside the nucleus. iii. Add another circle around the nucleus and add a symbol such as a dot for the electron Atoms possess internal structure; that is, they are made up of even smaller particles, which are called subatomic particles. A subatomic particle is a very small particle that is a building block for atoms. 28 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook An atom contains three fundamental sub atomic particles: proton, electron and neutron. An atom has a definite number of protons, electrons and neutrons. The structure of the atom describes how these particles The relativeare arranged charge to ismake of a proton +1. Theanelectron atom.is assigned a charge The of −1.relative The neutron charge of azero is assigned proton charge.isSince +1.anTheatom electron has equal is assigned a charge of −1. The neutron is assigned zero charge. Since an atom number of protons and electrons, it is electrically neutral. has equal number of protons and electrons, it is electrically neutral. A proton has a mass of 1.673 × 10–24 g, and –24 a neutron has a mass of A proton has a mass of 1.673 × 10 g, and a neutron has a mass of –24 1.675 ××1010–24 1.675 g. Thus, a proton g. Thus, and a neutron a proton and ahave almost the neutron havesamealmost the same –28 mass. Since mass. Since the the massmass of an iselectron of an electron very small,is9.109 very× 10small,g, its9.109 × 10–28 g, its mass mass is assumed is assumed to beornegligible to be negligible approximatelyorzeroapproximately because it is zero because it2000 is times ≈ 2000 times less heavy thanless heavier both the than proton and both the proton and neutron. neutron. Table 2.2Nature and location of sub-atomic particles Particle‘ Location Actual Mass Relative Actual Charge Relative Relative charge Name (g) Mass (amu) (C) Charge ( C) Proton Nucleus 1.673 10-24 1.00728 1 +1.60218 10-19 +1 +1 Electron Outside nucleus 9.109 10-28 0.00055 0 -1.60218 10-19 -1 -1 (shell) Neutron Nucleus 1.675 10-24 1.00866 1 0 0 Project Project Work 2.1 Work Prepare Prepare hydrogen hydrogen model bymodel by using using locally locally available available materials in groups materials in groups and present and present your model your to the rest ofmodel class.. to the rest of class. Atomic Number and Mass Number Atomic Number and Mass Number Activity 2.4 Activity 2.4 Form groups and discuss the following. Share your opinion with Form groups and discuss the following activity. Share your opinion your group members and present your group opinion‘s to the class. with your group members and present your group opinion’s to the class. Determine atomic numbers and mass numbers of common elements Determine atomic numbers and mass numbers of common elements by using periodic table. by using periodic table. 43 29 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook All atoms can be identified by the number of protons and neutrons they contain. The atomic number (Z) of an atom equals the number of protons in its nucleus. The atomic number is also the number of electrons that surround the nucleus of a neutral atom. Atomic number (Z) = Number of protons= number of electrons Mass number (A) is the sum of the number of protons and the number of neutrons in the nucleus of an atom.Except for the most common form of hydrogen, which has one proton and no neutrons, all atomic nuclei contain both protons and neutrons. Mass number (A) = Number of protons + Number of neutrons. = Atomic number + Number of neutrons. The mass and atomic numbers of a given atom are often specified using the notation: Mass number Atomic number A Z x Symbol of element 12 Example: 6 C , mass number = 12, atomic number = 6, and C is the symbol of carbon. Determination of the electrons, protons and neutrons Activity 2.5 Form groups and discuss the following activity. Share your opinion with your group members. 1. Use a periodic table to tell the atomic number, mass number, proton numbers, neutron numbers and electron numbers of the first 10 elements. Proton is equal to the atomic number of atoms. Number of protons = atomic number (Z) Electron: The atom is neutral therefore the number of electrons is equal to the number of protons. Number of electrons = atomic number (Z) = number of protons The number of neutrons in an atom is equal to the difference between the mass number and the atomic number or proton number. 30  equal to the Fetena.net : number of protons. No#1 Educational Resource Ethiopian General Science GRADE 8 Number of electrons = atomic number (Z) = number of protons Student TextBook The number of neutrons in an atom is equal to the difference between the mass number and the atomic number or proton number. Number of neutrons = Mass number (A) - Number of protons Number of neutrons = Mass number (A) - Number of protons Exercise 2.2 Exercise 2.2 Give Give the appropriate the appropriateanswers answers for thefollowing for the following questions. questions. 1. Complete thethe 1. Complete following following table. table. Particle Location Actual Mass (g) Relative Relative Mass (amu) Charge Proton Electron Neutron 2. A nucleus consists of 9 protons and 10 neutrons. Determine: 2. A nucleus consists of 9 protons and 10 neutrons. Determine: i.i. TheThe element by referring periodic table element by referring periodic table ii. Mass number ii. Mass number 3.3.How Howmany neutrons,protons many neutrons, protons andand electrons electrons are inthere are there in an an atom of theofelement 14 14 atom 7 N? the element 7N ? 2.3 Molecules 45 At the end of this section, you will be able to: define molecules; give examples of monatomic, diatomic and polyatomic molecules; use models or particles model diagram to represent molecules of elements and compounds. Activity 2.6 Form groups and discuss the following activiy. Share your opinion with your group members. After discussion present your findings to the class. 1. What is molecule? 2. Mention some examples of monoatomic, diatomic and poly atomic molecules. 31 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Molecules of Elements A molecule of an element consists of only one type of an atom. Molecules of elements can be classified as monoatomic, diatomic and polyatomic. 1. Monoatomic molecules are molecules that contain one atom of the element. Examples: He, Ne, Ar, Kr, Xe and Rn are monoatomic molecules 2. Diatomic molecules are molecules that contain two atoms of the element. Examples: O2, H2, F2, Cl2, I2 are diatomic molecules. Figure 2.2 Diagrammatical representations of Ne and H2. 3. Polyatomic molecules are molecules that contain more than three atoms of the element. Examples: O3, P4, S8 are polyatomic molecules. Molecules of compounds A molecule of a compound always contains two or more atoms of different elements combined chemically. Water (H2O), ammonia (NH3), carbon dioxide (CO2), etc. are some examples of molecules of compounds. Exercise 2.3 Give the appropriate answers for the following questions. 1. What is a molecule? 2. Classify the following molecules as monoatomic, diatomic or polyatomic? a. Ar d. O3 b. N2 e. He c. S8 f. Br2 32 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 3. Draw the diagram representation of ozone (O3) molecule. 4. Which of the following molecules are molecules of elements? Which of them are molecules of compounds? a. Ne d. Br2 b. H2O e. NH3 c. HCl f. P4 Key Terms Atom Electron shell Atomic nucleus Mass number Atomic number Molecule Continuous theory Monoatomic molecule Diatomic molecule Neutron Discreteness theory Polyatomic molecule Electron Proton 33 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Summary Democritus (460-370 BC) introduced the idea that matter consists of very small indivisible particles called “atoms”. The three fundamental subatomic particles are protons, neutrons and electrons. Protons are positively charged. Neutrons are chargeless. Electrons are negatively charged. A proton and a neutron have approximately the same mass; but the mass of an electron is negligible. The atomic number of an element is the number of protons in the nucleus of an atom of the element. An atom is electrically neutral because the amount of positive charge on a proton equals the amount of negative charge on an electron. The mass number is the sum of the number of protons and the number of neutrons in the nucleus of an atom. The number of neutrons in an atom is equal to the difference between the mass number and the atomic number or proton number. An atom is represented by the notation, AZ x in which X is the symbol of an element Z is the atomic number, and A is the mass number. A molecule is the smallest particle of an element or a compound that can exist freely in nature. Molecules of elements consist of only one type of atoms and can be classified as monoatomic, diatomic or polyatomic. Molecules of compounds consist of two or more different type of atoms. 34 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Review Exercise I. Write ‘‘True’’ if the statement is correct and write ‘‘False’’ if the statement is incorrect. 1. Nucleus consists of protons and neutrons. 2. Atomic number is the number of protons in the nucleus. 3. Molecules of elements consist of two or more different type of atoms. 4. Proton and electron have approximately the same mass. 5. Different elements have the same number of protons. II. Choose the correct answer from the given alternatives. 6. The idea that matter is ‘continuous’ was proposed by A. Democritus B. Aristotle C. Dalton D. None 7. The idea of ‘atoms’ first proposed by the Greek philosopher---- A. Aristotle B. Plato C. Dalton D. Democritus 8. Which of the following particles located in the nucleus of an atom? A. Proton and electron C. Electron and neutron B. Neutron and proton D. Proton, electron and neutron 9. The sum of the number of protons and neutrons in an atom is known as A. Atomic number B. Atomic mass C. Mass number D. Number of electron 35 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 24 10. The number of neutrons in 12Mg are A. 12 B. 11 C. 24 D. 13 11.Which of the following statements concerning the nucleus of an atom is correct? A. Contains only neutrons B. Contains all protons and all electrons C. Is always positively charged D. Accounts for most of the total volume of an atom 12. Which of the following molecule is diatomic molecule? A. O2 B. O3 C. P4 D. S8 13. Which of the following statement is false? A. Molecules of elements consist of only one type of atoms. B. Nucleus is positively charged. C. Molecules of compounds consist of only one type of atoms. D. Neutrons have no charge. 14. Which of the following molecule is molecule of elements? A. H2O B. NH3 C. H2 D. HCl 36 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook III. Give short answers for the following questions. 15. What are the two main parts of an atom? 16. What are the fundamental sub-atomic particles? 17. Determine the atomic number, number of protons, number of 16 neutrons, number of electrons and mass number for 8 O. 37 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook UNIT THREE CLASSIFICATION OF COMPOUNDS Learning Outcomes: At the end of this unit, you will able to: explain the classification of compounds into organic and inorganic; write the formulas and names the first ten alkanes, alkenes alkynes and list the uses some important common organic compounds; classify oxides into different groups and give examples of each group; develop skills in identifying acidic, basic and neutral solutions; define, and apply the concept of neutralization; explain the safety precautions while working with acids and bases; demonstrate scientific inquiry skills along this unit: observing, classifying, comparing and contrasting, communicating, asking questions, designing experiment, drawing conclusion, applying concepts and problem solving. Main contents 3.1 Introduction 3.2 Organic compounds 3.3 Inorganic compounds 3.4 Neutralization reaction and salts 38 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook 3.1 Introduction At the end of this section, you will be able to: define organic compounds as carbon containing compounds and give examples; After completing this section, you will be able to: define inorganic compounds as compounds of elements other Define  than organic compounds as carbon containing compounds and give examples carbon.  Define inorganic compounds as compounds of elements other than carbon. Activity 3.1 Activity 3.1 Form groups and discuss the following activity. After the group dis- Form groups cussion, choose and discuss representative a group the following. After the group to present thediscussion, choose a group group’s opin- ionrepresentative to the class.to present the group’s opinion to the class. 1. 1. StateState earlier definitions earlier definitionsofoforganic organicand and inorganic compounds. inorganic compounds. 2. Do you agree with the notion that says: “organic compounds can be 2. Do you agree with the notion that says: “organic compounds can be synthesized only from animals and plants”? 3. synthesized State modern onlydefinitions from animals ofand plants”? organic and inorganic compounds. 3. State modern definitions of organic and inorganic compounds.During the During the latter part of the eighteenth century and the early part latter part of the eighteenth century and the early part of the nineteenth century, chemists of the nineteenth century, chemists began to categorize compounds began to categorize compounds into two types: organic and inorganic. Compounds into two types: organic and inorganic. Compounds obtained from living organisms obtained wereorganisms from living called organic were called compounds, and compounds organic compounds, and compounds obtained obtainedfrom from mineral mineral constituents constituents ofofthethe earth earth were were called called inorganic inorganic compounds. compounds. During During this early thischemists period, early period, believedchemists believed that a special that a special ―vital force “vital force” supplied by a living organism was necessary for the ‖ supplied by a living organism was necessary for the formation of an organic formation of an organic compound. This concept was disproved in compound. 1828 by theThis concept chemist German was provedFriedrich incorrect inWöhler. 1828 by the Germanprepared Wohler chemist Friedrich urea, an organic Wöhler. compound, Wöhler heated from an aqueous the ofreaction solution between two inorganic solutions compounds, ammonium of chloride inorganic compounds and silver ammonium cyanate, and obtained ureachloride andofsilver (a component urine). cyanate. NH4Cl (aq) + AgCNO (aq) NH4CNO (aq) + AgCl (s) Ammonium chloride Silver cyanate Ammonium cyanate Silver chloride O NH4CNO (aq) Heat (NH2)2CO (s) Or Urea H2N C NH2 Soon Soonother otherchemists chemists had successfully had successfully synthesized synthesized organic organic compounds compounds from inorganic from inorganic starting materials. As a result, the vital-force theory starting materials. As a result, the vital-force theory was completely abandoned. was completely abandoned. 54 39 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook The terms organic and inorganic continue to be used in classifying compounds, but the definitions of these terms no longer reflect their historical origins. All organic compounds contain carbon and hydrogen, along with other possible elements such as oxygen, nitrogen, sulphur, halogens and phosphorus except the oxides of carbon, carbonates, hydrogen carbonates, cyanides and cyanates. Inorganic compounds are the compounds consisting of mineral constituents of the earth or generally found in non-living things. The term inorganic compound refers to all compounds that do not contain carbon. Although, carbon dioxide, carbon monoxide, carbonates and hydrogen carbonates are carbon-containing compounds, which are classified as inorganic compounds. Exercise 3.1 Classify each of the following compounds as organic or inorganic. a. C12H22O11 d. C2H5OH b. NaCl e. CH3Cl c. CaO f. C2H4 3. 2 Organic Compounds At the end of this section, you will be able to: Define hydrocarbons and mention at least one source of hydrocarbons; Write the general formula of alkanes, alkenes and alkynes; Write the specific chemical formulas of the first ten members of alkanes, alkenes and alkynes; Describe a homologous series and its general characteristics; Name the first eight members of alkanes, alkenes and alkynes; Identify some common uses of organic compounds. 40 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Hydrocarbons Activity 3.2 Form a group and perform the following activity. Share your opinion with your group members. 1.What is hydrocarbon? 2. List the sources of hydrocarbons and indicate their location in Ethiopia. A hydrocarbon is a compound that contains only carbon atoms and hydrogenatoms.Hydrocarbons divided into three large classes:alkanes, alkenes and alkynes. Alkanes Alkanes are hydrocarbons that have the general formula CnH2n+2, where, n is the number of carbon atoms present, n = 1, 2, 3….. For example, the molecular formulas of the first four alkanes are C1H2×1+2 = CH4, C2H2×2 + 2 = C2H6, C3H2×3 + 2 = C3H8, and C4H2×4 + 2 = C4H10, respectively. When we compare the formulas of CH4 and C2H6 or C2H6 and C3H8, they differ by one carbon and two hydrogen atoms or – CH2 – group called the methylene group. A family of compounds in which each member differs from the next by one methylene (-CH2-) groupis called homologous series (homo is Greek for “the same as). The members of a homologous series are called homologues. Exercise 3.2 1. Write the formulas of alkanes that contain 5, 7 and 9 carbon atoms. Alkenes Alkenes are hydrocarbons that have the general formula CnH2n, where, n is the number of carbon atoms present, n = 2, 3….. For example, the molecular formulas of the first three alkenes are C2H2×2 = C2H4, C3H2×3 = C3H6, and C4H2×4 = C4H8, respectively. Exercise 3.3 1. Write the formulas of the alkenes that contain 6, 8 and 10 carbon atoms. 41 Fetena.net : Ethiopian No#1 Educational Resource General Science GRADE 8 Student TextBook Alkynes Alkynes Writeare 1.1. Write the hydrocarbons theformulas formulasofofthe thatthat thealkenes alkenes have that the6,6,general contain contain 88and and10 formula 10carbon carbon CnH2n-2, atoms. atoms. where Alkynesn = 2, 3, 4, etc. For example, the formulas of the first three Alkynes alkynes Alkynesare Alkynes are are C2H2×2-2 =that hydrocarbons hydrocarbons Chave that H2, the 2have C Hgeneral the3general =formula C3H4C,Cnand 2×3-2 formula nH H ,C 2n-2 2n-2 H nn===2,2,C ,where where 4 2×4-2 3,3,4H ,etc. 4,4,6etc. respectively. Forexample, For example,the theformulas formulasofofthe thefirst firstthree threealkynes alkynesare 2H areCC2H 2×2-2==CC 2×2-2 2H2H 2,2,CC 3H3H2×3-2==CC 2×3-2 3H3H 4,4, and 4H andCC4H 2×4-2==CC 2×4-2 4H 4H6,6,respectively. respectively. Exercise 3.4 1.Exercise Write3.4 Exercise the 3.4 formulas of the alkynes that contain five-eight carbon atoms. 1. Write 1. Writethe theformulas formulasofofthe thealkynes alkynesthat thatcontain containfive-eight five-eightcarbon carbonatoms. atoms. Nomenclature(Naming) Nomenclature (Naming)ofofHydrocarbons Hydrocarbons Nomenclature (Naming) of Hydrocarbons Activity3.3 Activity Activity 3.33.3 Form Formaaagroup Form group group and and and perform perform perform the the following thefollowing following activity. activity.Share activity. ShareyourShare your your opinion opinion withopinion with yourgroup your group with your members. members. group members. 1.1. 1.How How How do dowe do we wegive give specific givespecific specific nametotoname name to a hydrocarbon? aahydrocarbon? hydrocarbon? 2. Are hydrocarbons named based on certain rules or randomly? 2.2. Arehydrocarbons Are hydrocarbonsnamed namedbased basedon oncertain certainrules rulesororrandomly? randomly? The name of hydrocarbons is derived from the number of carbon atoms The present Thename name (prefix)isis ofofhydrocarbons hydrocarbons and the from derived derived ending fromthe itnumber contains thenumber (suffix).The ofofcarbon carbon names atomspresent atoms present (prefix) (prefix) of andalkanes, and the endingalkenes theending and