Grade 8 General Science Textbook PDF

General Science General Science General Science Student Textbook Student Textbook...

General Science General Science General Science Student Textbook Student Textbook Grade 8 Grade 8 Student textbook Grade 8 ISBN No - Amhara National Regional State Education Bureau Amhara National Regional State Education Bureau Price -  General Science Student Textbook Grade 8 Authors: Minbale Admas (Ph.D.) Dawud Yimer (M.Sc.) Getachew Tarekegn (Ph.D.) Editors: Agmas Amare (M.Sc.) Kindu Nibret (M.Sc.) Hassen Worku (Ph.D.) Team Leader Ehtegebreal Aregehagn (M.Ed.) Illustrator Belayhun Tsegaye Designer Atersaw Tigyhun (M.Sc.) Amhara National Regional State Education Bureau The textbook is prepared by Amhara National Regional State Education Bureau and the Scholars Council with budget allocated by the Amhara National Regional State Government. ©2015 Amhara National Regional State Education Bureau. All rights reserved. Scholars Council Amhara National Regional State Education Bureau  Table of Contents Introduction..................................................................................................... v Unit 1 Basics of Scientific Investigation 1.1 Doing scientific investigation.................................................................. 1 1.2 Semi-guided investigations..................................................................... 10 1.3 Conducting fair test................................................................................ 12 Unit 2 The Composition of Matter 2.1 Early thinking about the composition of matter..................................... 15 2.2. Inside of an atom.................................................................................... 17 2.3 Molecules................................................................................................ 24 Unit 3 Classification of Compounds 3.1 Organic and inorganic compounds......................................................... 28 3.2 Acids and bases....................................................................................... 36 3.3 Neutralization reaction and salts............................................................. 45 Unit 4 Human Body Systems and Health 4.1 Integumentary system............................................................................. 55 4.2 Muscular system..................................................................................... 64 4.3 Skeletal system....................................................................................... 70 4.4 Digestive system.................................................................................... 80 4.5 Respiratory system................................................................................. 90 4.6 Circulatory system................................................................................. 99 4.7 Reproductive system.............................................................................111 Unit 5 Ecosystem and Conservation of Natural Resources 5.1 Ecosystem and interactions................................................................... 123 5.2 Conservation of natural resources........................................................ 135 Grade 8 - Student Textbook General Science iii  Unit 6 The Solar System 6.1 Formation of the solar system.............................................................. 154 6.2 Family of the solar system................................................................... 157 Unit 7 Physical Phenomena in the Surrounding 7.1 Phenomena of light.............................................................................. 180 7.2 Sound................................................................................................... 190 7.3 Heat...................................................................................................... 200 7.4 Simple circuit........................................................................................ 209 7.5 Magnetism............................................................................................ 217 iv Grade 8 - Student Textbook General Science  Introduction Rationale for the Curriculum Reform Different studies have been conducted to improve the relevance and quality of Ethiopian general education. Worth mentioning are the Education Road-map (2018) and Cambridge Assessment Studies (2019). On top of these, the Ministry of Education and Amhara National Regional State Education Bureau carried out repeated monitoring and assessment of the school curriculum. The study and assessment reports consistently reiterated that the previous general education curriculum had weaknesses. Some of the weaknesses were: The curriculum focused on low-level cognitive domain of learning; the presentation of some contents in the curriculum did not consider students’ age and maturity level, and the curriculum did not acknowledge indigenous knowledge systems. Furthermore, the curriculum did not adequately encourage students to develop scientific thinking skills such as observing, classifying, inferring, measuring, communicating, predicting, identifying variables, constructing hypotheses, tabulating and graphing data, defining variables, designing investigations, and experimenting. It also did not provide adequate opportunity for students to develop 21st-century skills such as critical thinking, problem-solving, global and cultural awareness, digital literacy, oral and written communication, creativity, collaboration, decision making, and the like. To alleviate these shortcomings, a new curriculum framework and syllabus have been developed. General science textbooks are developed on the bases of the new curriculum framework and grades 7 and 8 general science syllabus. General science curriculum Based on the new curriculum framework, general science textbooks are prepared for middle school students (grades 7 and 8). General Science education includes physics, chemistry, and biology subjects. General Science education aims to equip students with foundational science knowledge and skill that serve as a base for secondary education. It intends to nurture scientific inquiry skills which students use on their day to day lives and in learning science. In addition, it aims to cultivate among students science processes and 21st-century skills that can be used in their day-to-day life and academic career. Grade 8 - Student Textbook General Science v To achieve these major goals of general science education, the textbooks are prepared based on the principles of inquiry-based, problem-based and context- based learning. Inquiry is the intentional process of identifying problems, critiquing experiments, distinguishing alternatives, planning investigations, researching conjectures, searching for information, constructing models, debating with peers, and forming coherent arguments. Inquiry-based learning involves posing questions, making observations, reading books to find out what others have learned, planning investigations, gathering, and analyzing information, reflecting on what was learned in the light of new evidence, and proposing explanations and predictions. It encourages students to use critical thinking skills that include designing and carrying out investigations, interpreting data as evidence, creating arguments, building models, and communicating findings to deepen students understanding through logic and evidence. Problem-based learning allows students to become the drivers of their learning. Problem-based learning uses complex, real-world issues as the classroom’s subject matter, encourage students to develop problem-solving skills and learn concepts instead of just absorbing facts. It promotes students’ conceptual learning and skill development. It helps students to acquire knowledge and skill in the context of real-world problems. It engages the students in solving meaningful problems. Context-based learning is using students’ prior knowledge, experience, and context as a base for the new knowledge and skill development. It encourages students to recall, relate, describe, or apply knowledge from relevant prior experience that can be used as a foundation for the new knowledge. Teachers, students, and parents will take part in the knowledge construction process. It is important that teachers, students, and parents engage in observations, experiments, and construction of knowledge. Hence, the textbooks are intended not only to provide adequate knowledge and skill but also develop among students learning to learn skills. vi Grade 8 - Student Textbook General Science  Implementation of the new learning and teaching techniques and methods General science textbooks include many activities that are helpful to put into practice the aforementioned learning-teaching methods. The activities are designed based on students’ prior knowledge, skill, and experiences. They are aimed to connect students’ life experience with classroom science, to develop students’ science process, inquiry, and 21st-century skills. To achieve the aims of these activities, students should prepare themselves before class, and should be active participants in the classroom. Teachers should encourage students to work on the activities before class. During the teaching-learning process, the teachers are not expected to provide answers for each activity before the students work on it. The students should be given enough time, needed materials, and clues while they are working on the activities. The teachers should lead students’ work very closely and scaffold them when necessary. Contents in the textbooks require appropriate utilization of instructional time and extending learning to home and libraries. It is also indispensable that every student brings his or her textbook to class. Parents should also assist students in carrying out different activities. It is critical that teachers ensure that all activities and contents in textbooks are properly covered and learned by students. Dear students, please take good care of the textbook! Learning requires effort, experimenting, and exercise! We wish you a successful academic year Grade 8 - Student Textbook General Science vii  n Unit 1 : U it Basics of Scientific Investigation 1 Basics of Scientific Investigation Learning outcomes: At the end of this unit, you will be able to:  Describe the components of a scientific investigation  Describe the meanings of some basic scientific concepts such as observation, hypothesis and variables  Differentiate the dependent and independent variables  Demonstrate ability to work effectively and Keywords: respectfully with others in performing fair  Scientific method testing  Observation 1.1 Doing scientific investigation  Hypothesis  Variables As you have learnt about the nature of science in your  Fair-test grade 7 general science, science is the process of gaining knowledge by asking questions and seeking answers  Ethical rules to these questions. To answer questions, scientists use scientific methods. They include identifying a question, forming and testing a hypothesis, analyzing results, and drawing conclusions. Grade 8 - Student Textbook General Science 1  Unit 1 : Basics of Scientific Investigation 1.1.1 Scientific methods Activity 1.1 After a heavy rainy season in an area, the soil turned into mud. Many life creatures such as worms and snails crawl on top of the mud. These creatures were not seen before. 1. Where do these creatures come from? 2. How can you investigate this phenomenon? 3. Briefly outline the steps that you will follow during your investigation. 4. What does the study of fungi have in common with the study of atoms, and motion? How is research in a chemistry laboratory similar to research in a biology or physics laboratory? Scientific investigation is the way in which scientists use a systematic process to answer questions about the world around us. It is a way of finding the answer to a question using carefully arranged steps. In other words, the scientific method is a process by which observations are questioned, hypotheses are formulated and tested, and the results are analyzed. There are common steps, in which a scientific method can be implemented. 1.1.2 Steps of scientific methods Step 1: Making A scientific investigation often starts when someone observes an observation an event in nature, laboratory, or day to day experiences and wonders why or how it occurs. Observation can be qualitative or quantitative. Qualitative observations describe properties or types. For example, consider the statements shown below. All describe the properties or types of substances or events. 1. Outside air temperature varies in seasons. 2. Table salt is a crystal solid. 3. Sugar dissolves in water. 4. A ripen strawberry has red color. 2 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation Quantitative observations are measurements, which are described by numbers and units. In other words, quantitative observations describe the amount of something present in the study. For example, consider the following statements which are expressed in numbers and units. 1. The melting point of sulfur crystal is 115.210 Celsius. 2. 100 gram of water can dissolve 36 gram of table salt at 200 Celsius. Step 2: Ask A critical observer creates a scientific question from his observation a question that can be answered with the time and resources available. It is the basic step in a scientific investigation. From your day to day activities, you can raise many questions. Example: a. Why does an iron nail rusts? b. Why is black carbon obtained when sugar is heated? c. Why do plants change the color of their leaves? Step 3: A hypothesis is a tentative explanation that can be tested with a Develop a scientific investigation. It uses prior knowledge and observations hypothesis to predict what will happen and why. A hypothesis may not be correct, but it helps to understand the system being studied into a form that can be tested. Consider the following scientific questions and the hypothesis developed for each question. 1. Scientific question: What is the cause of overweight of people? Hypothesis: Daily intake of sugary drinks leads to overweight. 2. Scientific question: What is the cause of lung cancer? Hypothesis: Smoking cigarettes causes lung cancer. It is also possible to develop a hypothesis from day to day observation. For example, the observation that we experience alternating periods of light and darkness corresponding to observed movements of the sun, moon, clouds, and shadows is consistent with one of the following hypotheses: Grade 8 - Student Textbook General Science 3  Unit 1 : Basics of Scientific Investigation 1. Earth rotates on its axis every 24 hours, alternately exposing one side to the sun. 2. The sun revolves around the Earth every 24 hours. Suitable experiments can be designed to choose between these two alternatives. Some hypothesis cannot be answered through direct testing. For example, one of the hypothesis for the disappearance or extinction of the dinosaurs is the impact of a large extraterrestrial object. Unfortunately (or perhaps fortunately), this hypothesis does not lend itself to direct testing by any obvious experiment, but scientists can collect additional data that either support or refute it. You will learn about these methods in higher grades. Step 4: Since a hypothesis is a tentative explanation, it should be tested Testing the whether it is valid or not. To test the hypothesis, you need to hypothesis design your experiments and collect data from the experiments. /Design After a hypothesis has been formulated, researchers conduct experiment experiments to test its validity. Experiments are systematic observations or measurements to get qualitative or quantitative information or data. Experiments should be conducted preferably under controlled conditions in which a single variable changes. Step 5: The data obtained from the experiment should be analyzed and Interpret interpreted. It helps to decide whether the hypothesis is valid and analyze (correct) or invalid (wrong). If the result obtained from the the result experiment shows the hypothesis is invalid (wrong), it must start at step 3 or restate the hypothesis and do all the steps again. If the hypothesis is valid, the researcher can move to the final step. Describe the trends that can be seen from the data and compare it with related studies and finally draw a conclusion. 4 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation Additional experimental data are then collected and analyzed, at which point a researcher may begin to think that the results are sufficiently reproducible (i.e., dependable) to merit being summarized in a law, a verbal or mathematical description of a phenomenon that allows for general predictions. A law simply states what happens; it does not address the question of why. Consider the following laws: 1. The law of conservation of energy (energy neither created nor destroyed) 2. The law of conservation of mass (mass neither created nor destroyed) 3. The law of definite proportion of mass (a chemical substance always contains the same of elements by mass) Another verified explanations or statements about a phenomenon is a theory, it attempts to explain why nature behaves as it does. Laws are unlikely to change greatly over time unless a major experimental error is discovered. In contrast, a theory, by definition, is incomplete and imperfect, evolving with time to explain new facts as they are discovered. Sometimes, a researcher may start with a hypothesis formed by reading about work done by others before in the field, rather than by making direct observations. Example: Assume you have two plants grown in your garden. One of the plants looks green while the other is turning brown. Both plants were grown on the same type of soil and obtained the same amount of water. From the given information, it is possible to apply the basic steps of scientific methods as follows. Step-1: Observation  In this garden, plants have different leaf colors. Step-2: Asking questions  Why plants that grow in the same garden have different leaf colors? Step -3: Develop a hypothesis  The plant with green leaves might receive more sunlight. Or the plant with brown leaves might receive less sunlight. Grade 8 - Student Textbook General Science 5  Unit 1 : Basics of Scientific Investigation Step 4: Design experiments Grow two plants with controlling sunlight. Place one of the plants in full/direct sunlight and another one in a place where there is limited/little sunlight and carefully observe the difference between the two plants. The result of your experiment may or may not support your hypothesis. If the result of your experiment supports your hypothesis, your hypothesis is valid. If the result of your experiment does not support your hypothesis, it does not mean not a final failure. You can improve the hypothesis and try again..Activity 1.2 Classify the following statements as an observation, a hypothesis, an experiment, a law, or a theory a. Ice always floats on liquid water. b. Birds evolved from dinosaurs. c. Hot air is less dense than cold air. d. When 10 g of ice were added to 100 ml of water at 25°C, the temperature of the water decreased to 15.5°C after the ice melted. e. The ingredients of Diana /Repi soap were analyzed to see whether it really is 99.44% pure, as advertised. In scientific investigations, we often want to study the effect of one variable on another variable. A variable is a condition or factor in scientific experiments. Variables are an important part of any scientific investigations. To test the hypothesis, an experiment with a variable is crucial. There are three common types of variables. These are independent variables, dependent variables, and controlled variables. 6 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation Make an observation Ask question Develop a hypothesis Testing the hypothesis Analyze the result Valid Hypothesis Invalid Hypothesis Figure 1.1: Basic steps of scientific methods I. Independent The independent variable in an experiment is a condition Variable that is manipulated, or changed, by a scientist. The effects of manipulating an independent variable are measured by changes in a dependent variable. This variable does not rely on any other variables. Being able to modify or manipulate this variable is important to study the effects. II. Dependent Dependent variables are observed and measured during Variable an experiment; they are the experimental data. Changes in dependent variables “depend upon” the manipulation of the independent variable. Suppose a scientist is testing medications to treat high blood pressure. The independent variable is the dose of medication. The dependent variable is blood pressure. Grade 8 - Student Textbook General Science 7  Unit 1 : Basics of Scientific Investigation III. Control In scientific investigations, some variables need to remain Variable constant. These are your control variables. By keeping a variable constant, you ensure your results remain accurate. If you have an experiment without control, then you cannot be completely sure of the cause and effect. Ideally, only one independent variable should be tested in an experiment. Thus, all of the other conditions have to stay the same. The conditions that do not change during an experiment are called constants. To study the effects of an independent variable, a scientist uses a control group or control condition. Subjects in a control group are treated exactly like experimental subjects except for the independent variable being studied. The independent variable is manipulated in experimental groups or experimental conditions. Activity 1.3 For the following scientific questions, identify the independent and dependent variables Scientific questions Independent Dependent variable (s) variable (s) 1 Does temperature affect solubility of salt? 2 Does temperature affect leaf color in plants? 3 How is enzyme activity affected by different pH values? 4 What is the effect of soft drink on blood sugar levels? In scientific investigations, one of the most common methods is designing a controlled experiment, which has at least two groups (a control group and an experimental group). The experimental group is used to study the effect of a change in the independent variable on the dependent variable. The control group contains the same factors as the experimental group, but the independent variable is not changed. Without a control, it is impossible to know if your experimental 8 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation observations result from the variable you are testing or some other factor. It is impossible to control two variables at a time. Activity 1.4: What do plants need to prepare their food? 1. Formulate testable hypotheses. 2. Design an experiment with experimental and control groups and identify the variables (independent, dependent, and control variables) [Hint: Plants need CO2 to prepare their own food. What other raw materials do they need? Test one variable at a time]. 3. Using bean seeds try out this investigation at home on your own. Suppose a scientist wants to develop a drug for a human disease. The scientist, then, needs to design an experiment to verify whether an agent can be a cure for a specific disease. Do you think that this experiment could be directly conducted on humans? Are there any other options for conducting an experiment? What should we consider while conducting scientific investigation? Any scientific investigation should be guided by ethical values. Ethics are moral principles that govern a person’s behavior and the things that are considered right or wrong. In other words, ethics can be described as distinguishing between acceptable and unacceptable behaviors. Particularly, scientific investigations, which are conducted on human beings and animals, should be guided by ethical values such as honesty, transparency, objectivity, safety, and harms. Activity 1.5 1. Why are mice considered model animals to study drugs? Since, the entire goal of any scientific investigation and science education is to find the truth, and researchers should be honest, transparent and objective about their findings and data they obtained. Throughout the process of investigation, no harm is expected either to the researcher or the subject that the research conducts on it. Grade 8 - Student Textbook General Science 9  Unit 1 : Basics of Scientific Investigation 1.2 Semi-guided investigations 1.2.1 Does a coiled nail act like a magnet? Activity 1.6 Materials needed: Copper wire; nail; batteries; tape; paper clips Procedure: 1. Wrap the wire tightly around the whole length of the nail. Do not overlap the wire. Do not wrap the wire at each end of the nail. 2. Try to pick up the paper clips by touching them with the nail. What happens? 3. Connect the two ends of the wire with the batteries at its two ends. 4. Try again to pick up the paper clips. What happens? Why? 5. Now disconnect the battery and try to pick up the paper clips again. Does the same thing happen as in step 2? What has happened to the nail? 6. How many paper clips can your electromagnet pick up? 7. How could you make it stronger? 1.2.2 How do plants store their food in their leaf? How do plants store their food in their leaf? Plants make food (glucose) during photosynthesis. When a plant produces glucose in excess, it can be converted into starch, lipids (oils) and proteins and stored. The storage areas are usually plant parts modified as under - ground storage organs, but other plant parts above ground can also act as storage organs. Storage in plants occurs in vegetative organs (roots, stem and leaves) and reproductive structures (fruits and seeds). In this section, you are going to see leaves as storage organs of food in plants. 10 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation Experiment 1.1 All plants store food temporarily in their leaves. Most store starch. How do you know that plants store food temporarily in their leaves? Now you are going to conduct a simple experiment to verify this. Laboratory activity: Test for starch in leaves Materials required: Iodine solution, plant leaves, ethanol alcohol, heat source (Bunsen burner or sprite lamp), forceps, beakers, and water. Procedures: 1. Collect freshly detached plant leaves. 2. Closely observe the collected plant leaves’ physical features. 3. Boil a plant leaf in water for 30 seconds. 4. Then boil the leaf in ethanol alcohol for a few minutes. Ethanol is flammable, so there should not be a direct heat contact with it! 5. Wash the boiled leaf with tap water. 6. Spread the leaf out on a flat area/container. 7. Add certain drops of iodine solution onto the leaf using a pipette and watch. Questions: 1. What did you observe? Note the color change. 2. Why boiling the leaf in water is important? Why do you wash it? 3. What is the purpose of ethanol? 4. What can you conclude from this simple experiment? ( Hint: formation of dark green color with iodine indicates the presence of starch ) Grade 8 - Student Textbook General Science 11  Unit 1 : Basics of Scientific Investigation 1.2.3. Is air necessary for burning? Activity 1.7 Take 2 cm of candle, a water glass, and a match. Put the candle under the glass. Take another 2cm candle and put it on a table in the open air. Burn both candles with a match. 1. What did you observe? 2. Why did the candle under the glass turn off but the one in the open air did not? 1.3 Conducting fair test Any scientific investigation should be fair. In other words, it is important for an experiment to be a fair test. You can conduct a fair test by making sure that you change only one factor (variable) at a time while keeping all other conditions the same (constant). Activity 1.8: Conducting Fair Tests 1. If you want to study the effect of fertilizers on the growth of a bean plant, what do you do? Conducting a fair experiment (test) is one of the most important features of scientific investigation and makes the test valuable. To say that your experiment is a fair test, you must change only one variable at a time while keeping all other factors, which can affect your experimental result constant. 12 Grade 8 - Student Textbook General Science Unit 1 : Basics of Scientific Investigation Summary  Science is not simply a collection of facts. Important theories are created with the idea of explaining observations. To be accepted, theories are tested by comparing their predictions with the results of actual experiments. Note that, in general, a theory cannot be “proved” in an absolute sense.  The steps of scientific method includes making critical observation, asking scientific questions, formulating hypothesis, testing hypothesis through experiments, and analyzing data and interpreting the results.  A scientific theory is usually deeper and more complex than a scientific law. A scientific law is a concise statement, often expressed in the form of an equation, which quantitatively describes a wide range of phenomena. Grade 8 - Student Textbook General Science 13  Unit 1 : Basics of Scientific Investigation Review Questions I. Fill in the blank space 1. ________________is a plan for asking questions and testing possible answers in order to advance scientific knowledge. 2. Conducting a fair test is measuring the effect of _________________. II. Multiple choice questions 1. A person takes a painkiller “Aspirin” whenever he feels headache. The independent variable is A. Headache C. Pain killer B. Aspirin D. The person 2. Foods like milk have high calcium content and are good for the bone strength of a child. What is the dependent variable? A. Milk C. Any food B. Bone strength D. Child III. Investigative Questions 1. Describe the main steps of a scientific method. 2. What is the source of any hypothesis? 3. In conducting a fair test, the effect of one variable should be measured keeping other conditions constant, why? 14 Grade 8 - Student Textbook General Science  Unit Unit 2 : The Composition of Matter 2 The Composition of Matter Learning outcomes: At the end of this unit, you will be able to:  Narrate the historical development of the atomic nature of substances  Describe that atoms are the building blocks which make up all substances  Demonstrate the idea that the identity of a substance is determined by its atomic structure  Differentiate molecules of elements from molecules of compounds.  Differentiate monatomic, diatomic and polyatomic molecules. Keywords:  Demonstrate scientific inquiry skills along this unit: communicating, asking questions, drawing  Matter, conclusions and applying concepts.  Atom, 2.1 Early thinking about the composi-  Nucleus, tion of matter  Proton, Activity 2.1  Neutron, In your home, so many things are available such as tables,  Electron, chairs, clothes, books, jewelleries, and stones. The tables  Molecules, and and chairs are made of wood. The clothes are made of  Compounds. cotton, wool and other synthetic polymers. Grade 8 - Student Textbook General Science 15  Unit 2 : The Composition of Matter The books are made of paper and an endless array of things is made of plastic The jewelleries are also made of gold, silver, copper and any other precious metals. 1. Where do all these things come from? 2. What do you think about the composition of all these things? 3. Is the composition of all these things similar or different? 4. Is the property of all these things similar or different? 5. How do you classify the above-mentioned things? 6. What is the smallest particle of all these things? 7. Are they all matter? What is matter and what is not? You are all completely surrounded by matter. You need to get a basic understanding of the types, composition and properties of matter. Democritus (460–370 B.C.) and Aristotle (384-322 B.C.) proposed different theories about the types, composition and properties of matter. Democritus developed a theory of matter known as discontinuous theory. Based on Democritus theory, if you took a stone and cut it in half, each half had the same properties as the original stone. He reasoned that if you continue to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. Democritus called these very small pieces of matter atoms (Greek word which means atomos). Democritus used the word “atomos” meaning “indivisible” or “uncuttable” to describe the ultimate building blocks of matter. He suggested that atoms were eternal and could not be destroyed. Democritus theorized that atoms were specific to the material that they made up meaning that the atoms of stone were unique to stone and different from the atoms of other materials, such as fur. This was a remarkable theory that attempted to explain the whole physical world in terms of a small number of ideas. Democritus’s ideas were based on reasoning rather than experiment. On the other hand, philosophers such as Plato and Aristotle argued for the continuous theory of matter. They believed that matter could be continuously divided without end (the “continuous” idea of matter). According to them, it was proposed that matter was continuous, infinite, present in every form, and always all around us. It 16 Grade 8 - Student Textbook General Science  Unit 2 : The Composition of Matter was thought that matter could be divided and subdivided into smaller and smaller pieces without limit. Aristotle strongly argued that all matter is made from four natural elements (earth, water, air, and fire). This concept was called the continuous theory of matter. The continuous theory of matter received widespread support until the 1800’s when John Dalton refreshed the atom concept to explain certain aspects of chemical reactions. Activity 2.2 1. Be in groups and debate on the ideas of Discontinuous and Continuous theory of matter. Which theory do you support and why? Argue for or against each theory? According to the current understanding, the world we live in is made of matter. Matter is anything that has mass and takes up space. Things we can see, such as tables, chairs, clothes, books, jewelleries, stones and things that we cannot see, such as air, are also matter. Matter is made up of atoms. 2.2. Inside of an atom Activity 2.3 Around your surroundings there are so many materials. Consider buildings, humans, a fly, pinhead, amoeba, molecules, atoms and atomic nuclei. From the above-mentioned things: 1. Which one is the largest? 2. Which one is the smallest? 3. How can you see small size things such as amoeba, molecules, and atoms? Grade 8 - Student Textbook General Science 17  Unit 2 : The Composition of Matter 2.2.1 Observing an atom Recently, powerful new instruments such as transmission electron microscope (TEM), atomic force microscope (AFM) and Scanning Tunneling Microscope (STM) were invented to see individual atoms of a given matter with higher magnification. To see inside an atom, we need instruments with more magnification power. Figure 2.1: Scanning Tunneling Microscope (STM) Image of Gold and Copper atoms. 2.2.2 The subatomic particles An atom consists of subatomic particles. It can be divided into internal part (nucleus) and external part (electron). Experimental evidence indicated that most parts of the atom are empty space. The rest consists of a positively charged nucleus (protons) surrounded by a cloud of negatively charged electrons. The nucleus is small and dense Figure 2.2 Subatomic particles compared with the electrons, which are the lightest charged particles in nature. Electrons are attracted to any positive charge by their electrostatic force, bind the electrons to the nucleus of an atom. The structure of an atom describes how these particles are arranged 18 Grade 8 - Student Textbook General Science Unit 2 : The Composition of Matter 2.2.3 Relative mass, charge and location of subatomic particles Activity 2.4 Suppose you have one cup of sugar, add one granule of sugar in a cup, can you notice the added mass of granulate sugar? Relate the granulated sugar with subatomic particle electrons? Think over it and tell your conclusion for your classmate. Atoms and their subatomic particles (protons, neutrons and electrons) that compose them are extremely small. The subatomic particles have mass and charge. The masses of subatomic particles are also very small. Compared to protons and neutrons, the mass of an electron is negligible. For example, a carbon atom weighs less than 2 × 10−23 g, and an electron has a charge of less than 2 × 10−19 C (coulomb). The properties of tiny objects are described using small units of measurements such as the atomic mass unit (amu) and the fundamental unit of charge (e). The amu was originally defined based on hydrogen, the lightest element, then later in terms of oxygen. Since 1961, it has been defined with regard to the mass of carbon-12, meaning the mass of Carbon-12 is exactly 12 amu. An atomic mass unit is equal to one twelfth ( 1/12 ) of the mass of an atom of carbon-12, 1 amu = 1.66 × 10−24 g. The fundamental unit of charge (also called the elementary charge) equals the magnitude of the charge of an electron (e) with e = 1.602 × 10−19 C. Table 2.1: Properties of Subatomic Particles Subatomic Symbol Location Charge Unit Actual Mass Relative Particles (C) Charge (g) mass Mass (amu) Proton P+ Nucleus 1.6 x 10-19 +1 1.673x 10-24 1 Neutron N0 Nucleus 0 0 1.675 x 10-24 1 Electron e- Outside 1.6 x 10-19 -1 9.09 x 10-28 0 nucleus Therefore, a proton has a mass of 1.0073 amu and a charge of +1. A neutron is a slightly heavier particle with a mass 1.0087 amu and a charge of zero; as its name Grade 8 - Student Textbook General Science 19  Unit 2 : The Composition of Matter suggests, it is neutral. The electron has a charge of −1 and is a much lighter particle with a mass of about 0.00055 amu (it would take about 1800 electrons to equal the mass of one proton) as summarized in Table 2.1. 2.2.4 Atomic number and mass number 1. Atomic Number (Z) Atomic number is the number of protons contained in the nucleus of an atom. The identity of an element arises from the number of protons. Each element has a unique atomic number and chemical symbol. Figure 2.3: The number of electrons, protons, and neutrons of a Carbon atom. Example: Carbon (C) has 6 protons = atomic number = 6 Sodium (Na) has 11 protons = atomic number = 11 Copper (Cu) has 29 protons = atomic number = 29 2. Mass Number (A) The sum of the number of protons and neutrons in an atom. Mathematically it can be expressed as Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N) A=Z+N Symbolic representation of an element (X) with atomic number (Z) and Mass Number (A) Mass Number A Atomic Number Z X Atomic Symbol of an Element 20 Grade 8 - Student Textbook General Science Unit 2 : The Composition of Matter Examples: 23 31 32 39 Na P S K 11 15 16 19 Example: Subatomic Particle How many protons, electrons, and neutrons are there in an atom of Nitrogen, 14 N? 7 1. Analyze based on the given information: Numbers of protons, electrons, and neutrons are required. 2. Plan: Atomic number = number of protons = number of electrons Mass number = Number of neutrons + number of protons 3. Solve: The atomic number of Nitrogen is 7; hence, Nitrogen has seven protons and seven electrons. Therefore, Number of neutrons = mass number - atomic number = 14 - 7 = 7 neutron 4. Check your work: The number of protons in a neutral atom equals the number of electrons. The sum of the protons and neutrons equals the given mass number (7 + 7 = 14) Practical Problem 1. How many protons, electrons, and neutrons make up an atom of Chlorine, 35 Cl ? 17 3. Isotope and Atomic Mass The nucleus of an atom contains protons and neutrons. Unlike protons, the neutrons is not absolutely fixed for most elements. Atoms that have the same number of protons and hence the same atomic number, but different number of neutrons are said to be isotopes. The atomic mass of an element is the average mass of its naturally occurring isotopes. To determine average atomic mass, each exact atomic mass is multiplied by its percent of natural abundance and the sum of the resulting number in appropriate significant figures. The relative masses of atoms are reported in the atomic mass unit (amu). Atomic mass unit (amu) is defined as 1/12 of the mass of one atom of carbon-12, with 6 protons, 6 neutrons and 6 electrons. Carbon Grade 8 - Student Textbook General Science 21  Unit 2 : The Composition of Matter has three isotopes (12C, 13C and 14C). The relative atomic mass of carbon, which is the average masses of the three isotopes of carbon, is 12.01 amu. Example 1. Magnesium (Mg) has three isotopes (24Mg, 25Mg and 26Mg) with mass and percent of natural abundance of 23.98504 and 78.70 %, 24.98584 and 10.13 %, and 25.98259 and 11.17 % respectively. Calculate the atomic mass of Mg. Solution: To calculate the atomic mass of Mg from the masses and percent of natural abundance of the three isotopes is : amu of Mg = (23.98504 x 0.7870) + (24.98584 x 0. 1013) + (25.98259 x 0.1117) amu of Mg = 24.31 amu Hydrogen has three isotopes: protium (1H), deuterium (2H) and tritium (3H). Mass number is always greater than atomic number except hydrogen (protium), which 1 has equal number of mass number and atomic number ( H , A=Z). 1 Activity 2.5 In the following table, elements with atomic number 1 to 10 are given. Write the number of electrons, neutrons, and protons on the space given in the table. Remember you should know every information of elements listed in this table for this grade 8 general science lesson. Number Number Number Atomic of Elec- Symbol of Neu- Proton ber(Z) Num- ber of Name Mass Num- tron tron (A) Hydrogen H 1 1 Helium He 2 4 Lithium Li 3 7 Beryllium Be 4 9 Boron B 5 11 Carbon C 6 12 Nitrogen N 7 14 Oxygen O 8 16 Fluorine F 9 19 Neon Ne 10 20 22 Grade 8 - Student Textbook General Science Unit 2 : The Composition of Matter Activity 2.6 (Model Construction) In most atoms of an element, the number of neutrons in the nucleus is the same as the number of protons. The number of electrons can change, but for now, we are going to make models of neutral atoms. Therefore, there must be the same number of electrons as protons. Choose an element from the first 10 elements (from activity 2.5). Your teacher will guide you to get the information. Build a model of the element you have chosen. Materials: Glue, paper plate, play dough, dried lentils or peas, and Marker Procedure: 1. Prepare the paper plate into a circle shape. 2. Using a marker, mark the center of the circular paper. 3. Draw other circles next to the center with equal distance. 4. Take a dried lentil or pea grain and put each grain on the nucleus and around the nucleus. 5 Stick the grains with glue. Remember the grains represent the protons, electrons, and neutrons. The circular line represents the shells on electrons. Reflective Activities 1. What is the name of your element? 2. What is the atomic number of your element? 3. How many protons will you need to make your atom? 4. After you have built your model, draw a model of your atom. Provide labels. These are both models of your atom. N.B: If play dough is available at your school, use it instead of lentils or peas. Grade 8 - Student Textbook General Science 23  Unit 2 : The Composition of Matter 2.3 Molecules Activity 2.7 The air you breathe is a mixture of oxygen, carbon dioxide, nitrogen, neon, etc. Which of the above mentioned substances consist of a single atom? A molecule is the smallest stable particle of an element or a compound that can exist by itself. Molecules can be classified into molecules of elements and molecules of compounds. Molecules of elements can be further classified into three (Mono atomic, Diatomic and Polyatomic) molecules. 1. Monatomic Molecules – A single atom of an element can exist in stable form. All six noble gases or inert gases are monatomic molecules; Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). 2. Diatomic Molecules – Two atoms of a given element can exist in stable form. There are seven common Diatomic molecules such as Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2) and Iodine (I2). 3. Polyatomic Molecules – Three or more atoms of a given element can exist in stable form. Examples of polyatomic molecules are Ozone (O3), Phosphorus (P4) and Sulphur (S8) etc. Molecules of compounds are formed from two or more atoms of different elements. Examples of molecules of compounds – Hydrogen Chloride (HCl), Carbon Dioxide (CO2), Ammonia (NH3), Carbon tetrachloride (CCl4). O2 H2O N2 CO2 Figure 2.4: Model diagram of Diatomic and Polyatomic molecules 24 Grade 8 - Student Textbook General Science Unit 2 : The Composition of Matter Summary  Matters are made of tiny particles called atoms.  An atom has two regions: the atomic nucleus and electron shells.  The nucleus contains neutrons and protons.  The mass of an atom is concentrated in the nucleus.  Atomic number is the number of protons in the nucleus of an atom.  The mass number of an atom (A) is the sum of its number of protons (p) and number of neutrons (n); A = p + n.  In a neutral atom, there are equal number of protons and electrons.  A molecule is the smallest particle of an element or a compound that can exist in stable form.  A monatomic molecule contains a single atom in a molecule.  A diatomic molecule contains two atoms per molecule of the same element.  A polyatomic molecule contains more than two atoms per molecule of the same element.  Molecules of compounds formed from two or more atoms of different elements. Review questions I. Short answer questions 1. What are the basic building blocks of matter? 2. The simplest particles of iron are ____________ 3. Fill in the table below to compare the mass and the volume of a proton with the mass and the volume of an electron. Particle Mass Volume Proton Electron Grade 8 - Student Textbook General Science 25  Unit 2 : The Composition of Matter 4. If you were to cut a metal element over and over again, to the point that it is barely visible, would it still be considered as an element? Why? 5. Which is larger? A living cell or an atom of hydrogen? Explain your answer. II. Multiple choice questions (choose the correct answer) 6. An element is determined by the number of: A. Atoms B. Electrons C. Neutrons D. Protons 7. Which part of an atom takes up the most space? A. The nuclei B. The electrons C. The neutrons D. The protons 8. The nucleus of an atom consists of: A. Electrons C. Protons and neutrons B. Neutrons D. Protons, neutrons, and electrons 9. Which two particles of an atom attracted to each other? A. Electrons and neutrons B. electrons and protons C. Protons and neutrons D. All particles are attracted to each other III. Investigative Questions 10. Describe the composition of a matter. 11. If scientists found a new particle that they thought was the smallest part of an atom, how could they be sure that there wasn’t anything smaller? Can you think of anything that is not made of atoms? 12. How do you know that the air you breathe is not a single substance? 13. Gallium (Ga) with atomic mass 69.72 amu has two isotopes, 69Ga and 71Ga. 69 Ga has atomic mass of 68.9257 amu. What is the mass of 71Ga, if the natural abundance of the two isotopes (69Ga and 71Ga) are 60 % and 40 % respectively? 26 Grade 8 - Student Textbook General Science  Unit Unit 3 : Classification of Compounds 3 Classification of Compounds Learning outcomes: At the end of this unit, you will be able to:  Explain the classification of compounds into organic and inorganic  Write the formula and names of the first ten alkanes, alkenes, alkynes  List the uses of some important common organic compounds  Classify oxides into different groups and give examples of each group  Develop skills in identifying acidic, basic and Keywords: neutral solutions  Compounds  Define, and apply the concept of neutralization  Organic  Explain the safety precautions while working  Inorganic with acids and bases  Oxides  Demonstrate scientific inquiry skills along this  Acids unit: Observing, classifying, comparing and contrasting, communicating, asking questions,  Bases designing experiment, drawing conclusion,  Salts and applying concepts and problem solving  Neutralization Grade 8 - Student Textbook General Science 27  Unit 3 : Classification of Compounds Activity 3.1 In our daily life, we use different compounds such as water (H2O), table salt (NaCl), table sugar (C12H22O11)). calcium carbonate (CaCO3), sodium hydrogen carbonate (NaHCO3), methane (CH4) and sodium hypochlorite (NaOCl). 1. What do we mean by a compound? 2. Which compounds are found in foods we eat? 3. Which one is found in common drinks? 4. Which one is found in chalk, baking powder, soap, marsh gas, vinegar and bleaching powder? 5. Would you describe the use of those compounds in your daily life? 6. Would you classify these compounds as organic and inorganic compounds? And what is the main difference between them? There are more than 100 known elements that combine in a multitude of ways to produce compounds. Compounds are pure substances that are formed by the chemical combination of two or more atoms. Compounds can be classified into two main groups called organic compounds and inorganic compounds. 3.1 Organic and inorganic compounds Organic compounds are compounds that contain carbon and hydrogen, along with other possible elements such as oxygen and nitrogen. But some compounds containing carbon such as carbonates, hydrogen carbonates, carbon monoxide, carbon dioxide, carbides and cyanides are not organic compounds. These are inorganic compounds. Inorganic compounds are commonly known as minerals. Inorganic compounds are typically lack carbon–hydrogen bonds. Most inorganic compounds are minerals, which are found in the earth crust. Inorganic chemistry is the study of gases, rocks, minerals and the compounds that could be made from them. 28 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds 3.1.1 Organic compounds Activity 3.2 Compounds which are used as fuel for household activities, vehicles and for making plastics are organic compounds. 1. Mention examples of organic compounds that you know. 2. Why are they used as fuel? Organic compounds are compounds that contain carbon and hydrogen. Organic molecules of various sizes, shapes, and chemical properties are based on carbon. Except water and salts, most compounds in living cells are organic compounds. The skin, hair, horns, and tissues of the animals are made of organic compounds (proteins). An organic compound is a compound that contains the element carbon. Carbon is a unique element because it can join with itself and other elements. Carbon can form short chains, long chains, branched chains, and ring structures. A. Hydrocarbons and its source Hydrocarbons are organic compounds whose molecules are composed of hydrogen and carbon only. Hydrocarbons are organic compounds whose molecules are composed of Alkanes, Alkenes and Alkynes. There are three groups of hydrocarbons such as alkanes, alkenes and alkynes. All members of Alkanes can be represented by a general formula, CnH2n+2, where n is the number of carbon atoms. Alkenes have the general formulas, CnH2n, and Alkynes have also the general formula, CnH2n-2. The family of alkanes, alkenes and alkynes are called homologous series (homos is a Greek word meaning “the same as”). A homologous series is a family of compounds in which each member differs from the next by one methylene, “CH2” group. Each member of the homologous series is said to be homologs. For example methane and ethane are homologs of alkane, ethane and propane are also homologs of alkane. Methane and ethane differ one another by one methylene, “CH2” group. Grade 8 - Student Textbook General Science 29  Unit 3 : Classification of Compounds Nomenclature The name of a hydrocarbon is derived from the number of carbon atoms present in hydrocarbons which is expressed by prefix, and the ending of the name contains a suffix. The prefixes used for hydrocarbons containing upto 10 carbon atoms are listed below: Table 3.1: the basic prefix in naming hydrocarbons Prefix Number of carbon Prefix Number of carbon Meth 1 Hex 6 Eth 2 Hept 7 Prop 3 Oct 8 But 4 Non 9 Pent 5 Dec 10 The suffixes for the three groups of hydrocarbons are The suffix –ane for alkanes The suffix – ene for alkenes The suffix –yne for alkynes Example: Butane is an alkane containing 4 carbon atoms; the name of an alkene containing 7 carbon atoms is heptene. Since, hept indicates 7 carbon atoms and –ene is for alkene. The molecular formula of a hydrocarbon can be deduced from its name. 30 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds Activity 3.3 The numbers of carbon atoms of Alkanes, Alkenes, and Alkynes are given in the following Table. Write the name and molecular formula of the homologous series of hydrocarbons. Table 3.2: The Homologous Series of Alkanes, Alkenes and Alkynes Number Alkanes (CnH2n+2 ) Alkenes (CnH2n ) Alkynes(CnH2n-2 ) of Carbon Name Molecular Name Molecular Name Molecular Atoms Formula Formula Formula 1 2 3 4 5 6 7 8 9 10 B. Uses of some hydrocarbons Activity 3.4 (Project) Hydrocarbons are good sources of energy. One technology that is used at different zones and woredas in the Amhara region is biogas. Develop a small- scale device that can generate electricity from biogas. Present your work for your class. 1. Have you ever heard about biogas? 2. What are the components of biogas? 3. What can you say about its advantages? 4. What do you suggest to the local people in your area? Biogas is a renewable fuel produced by the breakdown of organic matter such as food and animal wastes. It can be used in a variety of ways including as vehicle fuel and for heating and electricity generation. Grade 8 - Student Textbook General Science 31  Unit 3 : Classification of Compounds There are useful hydrocarbons for different purposes. The known use of common hydrocarbons is listed below as an example  Methane used for fuel gas.  Butane is used in lighter  Propane and butane are used for bottled gas (buta gas).  Octane is used as a component of petrol (fuel for engines).  Decane is one of the components of kerosene (for cooking and lighting)  Ethene and propene are feedstock for polymers (starting material for plastics).  Ethyne is used at high temperatures for cutting and welding metals like Aluminium. Activity 3.5 In our country, Ethiopia, there are known areas of hydrocarbons. In groups, ask your geography teachers or other experts and write your report for the class. 3.1.2 Inorganic compounds Activity 3.6 Substance Taste (Sour, In our daily life, we use a large number of substances. bitter or Recall tastes of some edible substances listed in other) the table below. If you have not tasted any of these Lemon juice substances yet, taste it, if they are available around Orange juice your home and write the result in the table. Vinegar Sugar Observation and analysis Common salt 1. Do these substances have the same taste? Baking soda 2. Do you think the substances belong to the Grape same category? Mango Coffee 3. What do you conclude about their properties? Tea 32 Grade 8 - Student Textbook General Science  Unit 3 : Classification of Compounds Based on their properties and compositions, inorganic compounds can be classified into four groups: Oxides, Acids, Bases and Salts. Oxides Activity 3.7 The rusting of iron made materials such as bridges, buildings, motor vehicles, tools, fences, and other structures is a major economic problem throughout the world. 1. Have you ever seen a used nail or piece of metal in your surrounding? What kind of color do they have? Is it similar to the original color? Why? 2. You have learned in previous classes about physical and chemical change. Has the above used nail or metal went through chemical or physical change? Why? 3. What is rusting? Oxygen reacts with most elements to form oxides. Oxides are binary compounds of oxygen. Oxides are loosely classified into metallic and non-metallic oxides. Metallic oxides are formed when a metal reacts with oxygen, whereas non-metallic oxides are formed when a non- metal reacts with oxygen. Common oxides are water (hydrogen oxide), carbon dioxide, rust (Iron (III) oxide), and lime (calcium oxides). Oxides can be further classified into five classes, these are Basic oxides, Acid oxides, Amphoteric oxides, Neutral oxides and Peroxides. Each group of oxide has its own specific properties. Now you are going to learn about basic oxides and acidic oxides. 1. Basic oxides are mostly metallic oxides and can react with acid. Example: Na2O, MgO, and CaO … etc. 2. Acidic oxides are mostly non-metallic oxides and can react with bases. Example: NO2, N2O5, P2O5, SO2, and SO3 … etc. Grade 8 - Student Textbook General Science 33  Unit 3 : Classification of Compounds Properties of oxides There are different properties of oxides which can help to differentiate the two main types of oxides such as acidic and basic oxides. Acidic oxides are oxides of non-metals (acidic anhydride) and give acids in water. SO3 + H2O H2SO4 (Sulfuric acid) Experiment 3.1 Title: Preparation of sulphur dioxide. Objective: To prepare sulphur dioxide and test whether it is an acidic oxide or a basic oxide. Materials: litmus paper (blue and red), gas jar, Bunsen burner, deflagrating spoon. Chemicals: sulphur, water. Procedure: 1. Put some powdered sulphur in a deflagrating spoon and ignite it. 2. When it starts burning, put it into a gas jar. 3. When the burning stops, add 5mL of water into the gas jar and shake. 4. Put blue and red litmus paper, one after the other, in the jar. 5. Record your observations. Observation and Analysis 1. What is the color of the flame when sulphur burns in air? 2. What is the color change of blue and red litmus paper? 3. Classify the oxide as acidic or basic? Why? CO2 + H2O H2CO3 (Carbonic acid) Basic oxides (basic anhydride) are oxides of metals and give bases with water. CaO + H2O Ca(OH)2 (Calcium hydroxide) MgO + H2O Mg(OH)2 (Magnesium hydroxide) 34 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds Na2O + H2O 2NaOH (Sodium hydroxide) The term anhydride “without water” refers to compounds that give either an acid or a base upon the addition of water. Experiment 3.2 Preparation of magnesium oxide Objective: To prepare magnesium oxide and test whether it is an acidic or a basic oxide. Materials: Red and blue litmus paper, Bunsen burner, tongs, crucible. Chemicals: Magnesium ribbon Procedure: 1. Cut about 2 cm of magnesium ribbon. 2. Hold the ribbon with a tong and burn it over a flame from Bunsen burner. 3. When it starts burning, put the burning metal into a crucible and collect the product. 4. Add a small amount of water to the resulting powder in the crucible and shake it. 5. Rub the resulting substance between your fingers. 6. Test the solution with blue and red litmus paper. Observation and Analysis 1. What is the color of the flame produced when magnesium burns in air? 2. What happens to the color of red and litmus papers? 3. Is the resulting solution basic or acidic? Why? Grade 8 - Student Textbook General Science 35  Unit 3 : Classification of Compounds 3.2 Acids and bases Activity 3.8 Suppose you are playing in a field together with your friends; unfortunately, you are stung by a type of insect called wasp and one of your friends is bitten by an ant. 1. Do you feel any pain? 2. What do you think about the cause of the pain? 3. What do you do for you and your friend to get relief from the pain? Acids cause the sour taste of fruits and other foods; citric acid makes lemons and oranges taste sour and vinegar is sour because it contains acetic acid. Acids are substances that release hydrogen ion or proton (H+) or hydronium ion (H3O+) in water. For example hydrochloric acid (HCl) is an acid, which releases hydronium + ion (H3O+) in water (HCl + H2O H3O + Cl-). Acids neutralize bases and basic oxides. Bases are substances that release hydroxide ions (OH-) in water. Soluble bases are called alkalis. For example sodium hydroxide (NaOH) a base + - (NaOH + H2O Na + OH ) Bases have a bitter taste. Bases neutralize acids and acidic oxides. Activity 3.9 From your previous reading and experience, mostly the sour and bitter tastes of food are due to acids and bases present in them. If someone in the family is suffering from a problem of acidity after overeating, 1. Which of the following would you suggest as a remedy? A. Lemon juice B. vinegar C. Baking soda solution 2. Which property do you look for while choosing the remedy? 3. What do you conclude about the reaction of the remedy? 36 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds 3.2.1 Naming and writing formula of acids and bases Names and formula of acids Since all acids contain hydrogen, the name of an acid is based on the non-metal (negative) ion that goes with it. This negative ion can either be monatomic or polyatomic. The three different suffixes that are possible for the negative ion lead to the three rules below. 1. When the negative ion ends in –ide, the acid name begins with the prefix hydro-. The root of the negative ion followed by the suffix–ic. Example: HCl is hydrochloric acid because Cl- is the chloride ion. 2. When the polyatomic ion ends in –ate, the name of the acid is the root name of polyatomic ion followed by the suffix –ic. There is no prefix. Example: H2SO4 is sulfuric acid (not sulfic) because SO42- is the sulfate ion. 3. When the anion ends in –ite, the name of the acid is the root of the anion followed by the suffix –ous. Again, there is no prefix. Example: HNO2 is nitrous acid because NO2- is the nitrite ion. Activity 3.10 Formula of Acids Name Give the name of the HBr following acids HF HI H2SO3 HNO3 H2CO3 Like other compounds that you have studied, acids are electrically neutral. Therefore, the charge of the negative ion part of the formula must be exactly balanced out by the H+ ions. Since H+ ions carry a single positive charge, the number of H+ ions in the formula is equal to the quantity of the negative charge of monoatomic or Grade 8 - Student Textbook General Science 37  Unit 3 : Classification of Compounds polyatomic ions. From your grade 7 textbook, you have read how to write chemical formulas of compounds using valence numbers. Therefore, use the following steps of writing the formula of acids: H1+ SO42- Step1: Write the valance number of negative ions followed by H+ ion. Step 2: Crisscross the two opposite charges and put it as a H1+ SO42- subscript number. H2SO4 Example: The sulfate ion carries a −2 charge, so two H+ ions are needed in the formula of the acid (H2SO4). Names and formulas of bases There is no special system for naming bases. Since they all contain the OH- ion, names of bases end in hydroxide. The positive ion is simply named first. The number of hydroxides in the formula does not affect the name. The compound must be neutral, so the charges of the ions are balanced just as acids. For example, Sodium ion (Na+) requires one OH- ion to balance the charge, so the formula is NaOH. Calcium ion (Ca2+) requires two OH- ions to balance the charge, so the formula is Ca(OH)2. Hydroxide ion is a polyatomic ion and must be put in parentheses when there are more than one in a formula. Activity 3.11 Name Formula of bases Give the formula of the Magnesium hydroxide Ammonium hydroxide following bases Potassium hydroxide 3.2.2 Acid-base indicators and pH Activity 3.12 In your home, you may find different substances such as vinegar, baking soda, and lemon; how do you identify the acidity or basicity of the substances? 38 Grade 8 - Student Textbook General Science  Unit 3 : Classification of Compounds Special type of substance is used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their color when added to a solution containing an acidic, a basic or a neutral substance. Turmeric, litmus plant, and plant petals, etc., are some of the naturally occurring indicators. The most commonly used natural indicator is litmus. Litmus is a natural dye; it is extracted from lichens. It has a purple color in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper. Figure 3.1: Red and Blue litmus paper pH is a measure of hydrogen ion concentration, a measure of the acidity or alkalinity of a solution. The pH has a numerical scale usually ranges from 0 to 14. A solution with pH equal to 7 (pH = 7) is neutral, pH less than 7 (pH < 7) is acidic and pH greater than 7 (pH > 7) is basic. Figure 3.2: The pH scale Grade 8 - Student Textbook General Science 39  Unit 3 : Classification of Compounds Experiment 3.3: Effect of acids on indicators. Objective: To investigate the effect of dilute hydrochloric acid and sulphuric acid on the colors of litmus paper, phenolphthalein, universal indicator and methyl orange. Materials: blue and red litmus paper, test tube, test tube rack. Chemicals: phenolphthalein, methyl orange, universal indicator, sulphuric acid and hydrochloric acid. Procedure: 1. Pour about 5 mL of dilute HCl into four test tubes. 2. Hold the first test tube in an inclined position and put red and blue litmus papers turn by turn into it and see if there is any color change. 3. Add a few drops of phenolphthalein into the second test tube and few drops of methyl orange in to the third, few drops of universal indicator in the fourth and observe if there is color change. 4. Repeat the above procedure using a dilute H2SO4 solution. Observation and conclusion Record your finding in table1. Table 3.3: Effect of acids on acid base indicators. Acid Colour of the indicators in the acid solution Litmus Phenolphthalein Methyl orange Universal Indicator Lemon Juice Dilute HCl Dilute H2SO4 3.2.3 Investigating properties of acids and bases Properties of acids Acids are easily recognized by their sour taste, but they can also be identified by using indicators. A common indicator used to test acids is litmus paper. Acids turn blue litmus paper to red. Acid can react with different substances. 40 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds 1. Acids neutralize bases and basic oxides. 2. Acid can react with some metals such as iron, zinc, aluminum and magnesium to give salt and hydrogen gas. 3. Acids react with carbonates or bicarbonates to give salt, water and carbon dioxide. 4. Aqueous solutions of acids are electrolytes (conduct electrical current). Experiment 3.4: Reaction of an acid with a metal. Objective: To investigate the reaction of zinc with hydrochloric acid. Materials: test tube, test tube rack, rubber stopper, match, steel wool and wooden splint. Chemistry: Zinc and dilute HCl. Procedure: 1. Pour about 5mL of dilute HCl into a test tube. 2. Clean a piece of zinc with the steel wool until it is shiny. 3. Add zinc to the test tube containing dilute HCl, close the test tube with a rubber stopper and record your observation. 4. Ignite a wooden splint using a match, remove the rubber stopper and introduce the lighted splint into the mouth of the test tube. Observation and Analysis: a. Why do you clean the piece of zinc with steel wool? b. What happens when you drop zinc metal into the test tube containing dilute HCl? c. How do you know that a gas is produced in the reaction? d. What is the color of the gas? e. What happens when the lighted splint is held inside the mouth of the test tube? Grade 8 - Student Textbook General Science 41  Unit 3 : Classification of Compounds Uses of acids Acids are very important in our life. Our stomach contains an acid that helps break down the food we eat. Amino acids are parts of protein that make up our body tissues. Carbonic acid and phosphoric acid maintain the acidity or basicity balance in our blood. Acids are also important in our diet. For example, vitamin C, which is found in lemon, orange, and other foods, is ascorbic acid. Serious health problems can result from a lack of vitamin C. Acids are also used in making many products. Manufacturers use sulphuric acid in a wide variety of products, including fertilizers, detergents, plastics, and pesticides. Sulphuric acid is used in car batteries as electrolyte. Hydrochloric acid, commonly called muriatic acid, is used as a strong cleaner for bricks and concrete. Like sulphuric acid, hydrochloric acid is used in manufacturing products ranging from rubber to medicine. Properties of bases Like acids, bases have a common set of properties. In aqueous solutions, they feel slippery on our skin, you experience this when you use soap, which are made from bases. If you have ever forgotten soap in your mouth, it tastes bitter. A bitter taste is another physical property of bases. Solutions of bases cause red litmus paper to turn blue. Like solutions of acids, solutions of bases contain ions, so they can conduct electricity. Bases neutralize acids and acidic oxides by forming salts and water. 42 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds Experiment 3.5 Title: The effect of base on an indicator Objective: To study the effect of a base on an indicator Materials: red and blue litmus paper, test tubes, test tube holder and test tube rack Chemicals: ammonia solution, methyl orange and phenolphthalein Procedure: Take four clean test tubes. Add about 5mL of NH3 solution in each of the tubes and level the test tube 1, 2, 3 and 4. Put red litmus paper, blue litmus paper, 2 drops of phenolphthalein solution and 2 drops of methyl orange solution in test tubes 1, 2, 3, and 4 respectively. Observe the colour change and record your observation. Observation and analysis: a. What is the color of ammonia solution, phenolphthalein and methyl orange before the experiment b. What happens to the color of red and blue litmus paper, phenolphthalein and methyl orange solution after the addition of ammonia solution. Uses of bases Bases are important in life; Magnesium hydroxide is found in milk of magnesia, a medicine used to relieve stomach distress. Another base, baking soda, is used to make biscuits and breads. Gardeners use bases to make acidic soil neutral. Strong bases, such as sodium hydroxide, are used for cleaning because they are able to eliminate grease. Bases are also used to produce new products. For example, sodium hydroxide is used to manufacture soap, rayon, and paper. Calcium hydroxide is used to make Grade 8 - Student Textbook General Science 43  Unit 3 : Classification of Compounds plaster and mortar. Investigation using local indicators Activity 3.13 Take a small amount of baking powder in a spoon and dissolve it with enough water in a beaker or a glass. Then take a sheet of paper and drop a small amount of baking powder solution on the paper and rub the paper with freshly cut beetroot. Observation and analysis a. What happens to the paper? Explain your observation? b. Would you mention any parts of plants such as roots, leaves, flowers or fruits, which can be used as indicators? Using synthetic indicators have various effects on human beings; mainly chemical hazardous and pollution effects. To overcome the above mentioned negative effects of synthetic indicators, currently people focus on preparing indicators from locally available natural sources or bio-sources. Local indicators are prepared from natural sources (plants). They have an advantage like easily accessible, less toxic, and environmentally friendly (Eco- friendly). Local indicators can be prepared by extracting from different parts of plants such as roots, leaves, flowers or fruits. Consider the figures shown below, both Petunia flower and Beetroot can be used as a natural indicator. Petunia Flower Beetroot Figure 3.3: Plant beetroot and flowers used as indicators. 44 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds 3.2.4 Precautions in working with acids and bases Care must be taken whenever you are working with strong acids and strong bases. Strong acids and strong bases are corrosive in nature. Common acids such as hydrochloric acid, sulfuric acid, nitric acid and common bases such as potassium hydroxide (caustic potash) and sodium hydroxide (caustic soda) are corrosive. The followings are some of the precautions whenever working with strong acids and bases 1. Wear acid-resistant gloves when handling strong acids and bases. 2. Wash with water during splashing of acids and bases, and 3. Safe handling of strong acids and bases. 3.3 Neutralization reaction and salts Figure 3.4: Neutralization reaction 3.3.1 Neutralization reaction Activity 3.14 1. Why is taking too much soft drinks, candy and citrus fruits are unsafe for your teeth? 2. How do anti- acids, such as milk of magnesia, relieve gastritis (excess stomach acid)? Remember, from your grade 7 chemistry, you have learnt about chemical reactions. Neutralization reaction is a type of chemical reaction between an acid and a base that produces salt and water. Consider the reaction between an acid (HCl) and a base (NaOH), Grade 8 - Student Textbook General Science 45  Unit 3 : Classification of Compounds HCl + NaOH NaCl + H2O Acid Base Salt Water The products, sodium chloride and water, are neutral. The resulting solution has a pH of 7, and so it is neutral. In other words, a neutral solution (pH = 7) has no effect of acidity or basicity. Neutralization reactions have different practical applications in home, health, waste water treatment and agriculture. Some of the applications of neutralization reactions in our daily life are the following. 1. Waste water treatment Acidic gases such as sulphur dioxide (SO2) can be emited from industries form acidic effluents in the waste water. To reuse the waste water after treatment for drinking or any other purpose, the treatment should include neutralization of the acid component. 2. Controlling soil pH Soil pH is very important for crop cultivation. Plants Crop best pH range grow on a soil which has a particular pH value. High Potatoes 5.5 – 6.5 acidity of soil affects the yield of crop production. Oats 5.5 – 7.0 Different crops require different nutrients in different Beans 6.0 – 7.5 proportions and, as a consequence, each crop grows best in soil within a particular pH range. When organic materials, such as dead leaves, animal wastes etc. decay, organic acids are produced and these lower the pH of the soil so, over the years, soil naturally becomes more acidic. In order to increase the yield of crop production, it is often necessary to control the soil pH by adding alkaline substances such as wood ash, limestone (CaCO3) and quick lime (CaO). They can help to neutralize the acidity of soil and increase the crop yield. 3.3.2 Salts Activity 3.15 1. What is meant by a salt? 2. Do you think that salt refers to NaCl only? 46 Grade 8 - Student Textbook General Science  Unit 3 : Classification of Compounds Salts are compounds formed by the reaction of acids and bases. They contain negative ions of acids and positive ions of bases. Different salts naturally occur as rocks (carbonates, Sulfates, phosphates, etc). Naming of salts has two parts. The first part (positive ion) comes from a base or a metal oxide and the second part (negative ion) comes from an acid or non metal oxide. For example, if we consider the salt Sodium Chloride (NaCl), the sodium ion (Na+) may come from a base (NaOH) or an oxide (Na2O) and the chloride ion (Cl-) from an acid (HCl). Classification of salts Based on the number of elements they contain, salts can be classified into two, binary salts and ternary salts. a. Binary salts contain two elements. Example: Sodium chloride contains sodium and chlorine. b. Ternary salts contain three elements. Example: Sodium carbonate contains sodium, carbon and oxygen. Preparation of salts Salts can be prepared by using different methods. Neutralization reaction is the most common and useful method of salt preparation. It is the reaction involving an acid and a base to give salt and water. (Acid + Base Salt + Water). For example: HCl + NaOH NaCl + H2O. Reaction of basic oxide and acid also gives salt and water. Acid + Basic oxide Salt + Water. For example: 2HCl + CaO CaCl2 + H2O Grade 8 - Student Textbook General Science 47  Unit 3 : Classification of Compounds Experiment 3.6: Preparation of salt by the reaction of acid and base Objective: To prepare sodium chloride by the reaction of sodium hydroxide and hydrochloric acid. Materials: beaker, dropper, glass rod, evaporating dish, wire gauze, tripod, Bunsen burner, red and blue litmus paper Chemicals: sodium hydroxide solution and dilute hydrochloric acid Procedure: 1. Measure 15 mL of sodium hydroxide and pour into a beaker 2. Measure 20 mL of dilute hydrochloric acid solution and pour about 13 mL of it into the beaker containing sodium hydroxide solution 3. Stir the mixed solution with a glass rod and test with red and blue litmus paper 4. Then continue adding the acid solution using a dropper and testing the solution using blue and red litmus paper until the color of the blue and red litmus does not change. 5. When the color of red and blue litmus remains the same pour the solution into an evaporating dish and heat the evaporating dish. Observation and Analysis 1. Which color of the litmus paper changed? 2. What volume of hydrochloric acid was added till the colors of red and blue litmus paper changes? 3. When do the colors of blue and red litmus paper remain the same? 4. What is left in the evaporating dish after evaporation? Name the compound? 48 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds 3.3.3 Naming and writing formula of salts Naming of salts has two parts. The name of the positive ion (cation) comes first followed by the name of the negative ion (anion). Examples of cations are sodium, potassium, ammonium, etc. and anions are chloride, sulfate, nitrate, etc. For example, if you consider the salt sodium chloride (NaCl); the cation, sodium ion (Na+) comes first followed by anion, chloride ion (Cl-). Activity 3.16 Formula of Salts Name Give names for the following salts CaCl2 Mg(NO3)2 Al2(SO4)3 BaSO4 Writing formula of salts In writing the formula of salts, the positive ion comes first followed by the negative ion. For example, in writing the formula of the salt sodium chloride (NaCl), the positive ion (Na+) comes first and then the negative ion (Cl-). Activity 3.17 Name Formula of Salts Give formula for the following Potassium bicarbonate salts Magnesium sulfate Sodium nitrate Ammonium sulfate Uses of salts Activity 3.18 In your home or local area, people use different salts for different purposes. In groups, mention the salts and discuss the uses of the salts you have mentioned. Salts have different uses. In ancient times, salts have been used for food preservation and flavouring. In addition to this, salts have also been used in chemical industries such as soap production, tanning, dyeing, etc. Currently, salts have different applications. The applications of some salts are listed in the table below. Grade 8 - Student Textbook General Science 49  Unit 3 : Classification of Compounds Table 3.4: Formula, name and major uses of some salts Formula of Name Main Uses Salts NaCl Sodium chloride In food cooking, preservatives, in production of soap, etc. CaCO3 Calcium carbonate An ant-acid, fillers in cosmetics, disinfectant agent and as pH corrector NaHCO3 Sodium bicarbonate An antacid and making baking powder CaSO4 Calcium sulfate Setting fractured bones, surface smoothing, fire- proofing material KNO3 Potassium nitrate For fertilizer (NH4)2HPO4 Diammonium For fertilizer phosphate Activity 3.19 (project) In group, collect different types of rocks such as limestone, marble, etc. from your local area. Using vinegar or lemon juice test whether the rocks contain carbonate or not. After testing write a report to your teacher with describing the industrial importance of carbonate rocks. 50 Grade 8 - Student Textbook General Science Unit 3 : Classification of Compounds Summary  Hydrocarbons are organic compounds, which contain mainly two elements carbon and hydrogen.  There are three groups of hydrocarbons such as alkanes, alkenes and alkynes.  All members of hydrocarbons such as alkanes, alkenes and alkynes can be represented by a general formula of CnH2n+2, CnH2n and CnH2n-2 respectively.  Oxides are binary compounds which contain elements and oxygen.  Oxides are mainly metallic and non-metallic oxides; most metallic oxides are basic oxides, whereas most non-metallic oxides are acidic oxides.  Basic oxides behave like a base and react with acids to form salts.  An acid is a substance that forms hydrogen (H+) ions when dissolved in water.  Base is a substance that forms hydroxide (OH-) ions when dissolved in water.  Acid-base indicators are weak organic acids; they change from one colour to another within a particular pH range.  pH is the measure of acidity or basicity of a substance.  Salt is a substance formed by the reaction of acid and base by neutralization reaction. Review Questions I. Short answer questions 1. What is a compound? 2. What are the two main classes of compounds? 3. Hydrocarbons are organic compounds, which are composed of two chemical elements _________________and_________________. Grade 8 - Student Textbook General Science 51  Unit 3 : Classification of Compounds 4. __________ neutralize acids and acidic oxides. 5. The reaction of acid and base gives _________ and ________. The reaction is called _______. II. Multiple choice questions (choose the correct answer) 6. Which formula represents the hydrocarbon octene? A. C8H14 B. C8H12 C. C8H18 D. C8H16 7. Which organic compound, is an alkyne?. A. Acetylene B. Urea C. Aspirin D. Ethene 8. Which one of the following is an inorganic compound? A. Marsh gas B. Water C. Sugar D. Vinegar 9. All metallic oxides are basic oxides. A) True B) False 10. Which one of the following is the chemical formula for Ammonium phosphate? A. NH4PO4 B. (NH3)4PO4 C. (NH3)3PO4 D. (NH4)3PO4 11. Identify the nonmetallic oxide among the followings A. Al2O3 B. MgO C. CO2 D. BaO 12. A student burned a certain substance in air and put the product into water; the solution turned red litmus paper to blue. The product obtained by the student was ___________. A. A basic oxide C. An acidic oxide B. An acid D. A neutral oxide 13. One of the following is not the products when an acid reacts with carbonates? A. Carbon dioxide gas C. Salt B. Oxygen gas D. Water 14. Which one of the following is a binary salt? A. MgSO4 C. Al2(SO4) B.NaH2PO4 D. FeS 52 Grade 8 -