Grade 7 General Science PDF

General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Unit 3 ELEMENTS,COMPOUND AND CHEMICAL REACTION Learning Outcome At...

General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Unit 3 ELEMENTS,COMPOUND AND CHEMICAL REACTION Learning Outcome At the end of this unit, you will be able to: compare elements to compounds and how they are represented by symbols and formulae; identify and write symbols of common elements or compounds; name compounds given their formula and write formula given the name of the compound; use symbols and chemical formulae as a way of communicating information about elements and compounds; state and apply the law of conservation of mass to writing balanced equations; interpret chemical formulae of,compounds in terms of the elements present and the ratios of their atoms. Main Contents 3.1. Elements and their Representation 3.2 Compounds and their Representation 3.3 Simple Chemical Reactions and Equations 3.4 Uses of Chemical Reactions in every day Situation 67  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Introduction Pure substance, whether an element or compound, has its own unique name, symbol or formula. Scientists use chemical symbols in place of the names of the elements because it helps for scientists in writing chemical formulas and equations. The symbols and formulas are designed in such a way that they are internationally accepted. Therefore, they enable all scientists in the world to communicate easily. Symbols and formulas of elements or compounds are used in certain combination-ratios as a short hand representation of chemical reaction and these short hand languages is known as chemical equation. 3.1. Elements and Their Representation After completing this section, you will be able to: define element; identify symbols of some common elements; write chemical symbols for common elements. Activity 3.1 Form a group and discuss the following questions and share your ideas with the rest of the class. 1. From your previous knowledge, what is an element? 2. List some common elements you are familiar with. Try to classify them as a. metal b. non -metal 3.1.1. Common Elements An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. An element compose of only one kind of matter (atoms).There are 118 known elements.92 out of 118 elements are naturally occurring elements. As you have learnt in unit two, elements are classified as metal and nonmetals. Oxygen, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen, nitrogen, gold, silver, copper, sulfur, and chlorine are some common elements. 68  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.1.2 Chemical Symbols Activity 3.2 Perform the following activities in group. Then present your opinion to the whole class. 1. What is an atomic symbol? 2. Explain why do some symbols for examples He, Cl and Si have two letters? Scientists use symbols as abbreviation of names of an elements. An atomic symbol is defined as shorthand way of representing elements or atoms of an element. Every element has its own symbol. No two elements can have the same symbol. How to write symbols of elements? Chemists use chemical symbols in place of the names of the elements because they are much easier and quicker to write a symbol. A symbol for an element is taken from the first letter or the first letter plus another letters of the common name or Latin/Greek name of the element. If a symbol has one letter, it is written in capital, letter besides if it has two letters, the first is in capital and the second is in small letter. For example, S stands for sulfur, O stands for oxygen, and K represents potassium. In the case of potassium, the symbol is derived from the Latin name, Kalium. Why are not all elements symbolized by the first letter of their names? The names of some elements such as carbon and calcium begin with the same letter “C”. Therefore we cannot use the letter “C” as a sym- bol for both elements. Hence two letters are used for other elements except one. The first letter “C” is assigned as a symbol for carbon. The other element calcium is represented by two letter symbols Ca The same thing is true for hydrogen and helium. The first letter “H” is assigned as a symbol for hydrogen while “He” symbol stands for element helium. 69  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook.The same things true for hydrogen and helium. The first letter ―H‖ is assigned as a symbol for hydrogen while ―He‖ symbol stands for element helium. Table 3.1: Name and symbols of some elements Name of elements Symbol Name of Symbol elements Hydrogen H Magnesium Mg Helium. He Aluminum Al Lithium Li Silicon Si Beryllium Be Phosphorus P Boron B Sulfur S Carbon C Chlorine Cl Nitrogen N Argon Ar Oxygen O Calcium Ca Fluorine F Zinc Zn Neon Ne Bromine Br Magnesium Mg Iodine I Symbols of element derived from their Latin names are listed below. Table 3.2: Symbols of element derived from Latin names Table 3.2: English nameSymbols of name Latin element derived from Latin names Symbol Sodium English name Natrium Latin name Na Symbol Potassium Kalium K Sodium Natrium Na Iron Potassium FerrumKalium Fe K Iron Copper CuprumFerrum Cu Fe Copper Cuprum Cu Silver Argentum Ag Silver Argentum Ag Gold Aurum Au 75 Lead Plumbum Pb Tin Stannum Sn Mercury Hydrargyrum Hg 70  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Exercise 3.1 I. Write true for correct statement and false for wrong statement 1.Ca is the symbol of sodium. 2.Water is not an element. 3.Elements are pure substances. Choose the correct answer from the given alternatives. 4.Which of the following is the correct chemical symbol for silicon? A. S B. Si C. SI D. Sl 5.“C” stands for_____ A. calcium B. Chlorine C. Carbon D. Copper III. Fill the missing symbols and names of the elements in the following table Name of element Symbol Name of element Symbol Potassium Iodine He B Chlorine calcium Cu Nikel Gold H Li silver 3.2. Compounds and their Representation After completing this section, you will be able to define compound as a substance formed when two or more elements chemically combined together. define valence numbers as the combining power of an atom. write the formulae of simple binary compounds using symbols and valences. name binary compounds. describe polyatomic ion. write the chemical formulas of common compounds that contain polyatomic ions. 71  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Name compounds containing polyatomic ions. Identify the elements and number of atoms, and give a chemical formula. 3.2.1. Compounds Activity 3.3 Form a group and discuss the following questions and share your ideas with the rest of the class. 1. From your previous knowledge, what is a compound? 2. Copy the table on your exercise book and classify the substance as an element or, a compound. Substance Element Compound Sodium chloride (table salt) Water Gold As you have learnt in unit two, a compound is a pure substance consists of two or more elements which have been chemically combined. For example, water is a compound of hydrogen and oxygen. Each of its molecules contains two hydrogen atoms and one oxygen atom. There are many different compounds. Some examples of compounds are sodium chloride, iron sulfide, carbon dioxide, sugar, calcium carbonate, calcium oxide, etc. 3.2.2. Chemical Formulas It is the symbolic representationof an element or a compound. Chemical formulas can be classified as formulas of elements and formulas of compounds. Formulas of Elements The formula of an element consists of one kind of symbol. A molecule is the smallest particle of an element or a compound that has a stable,independent existence. 72  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook The elements helium,neon,argon,krypton,xenon and radon are collectively known The elements helium,neon,argon,krypton,xenon and radon are as noble gas.Because collectively knownthey exist uncombined as noble as single atoms,they gas, or monoatomic gases are also known Because they exist uncombined as monoatomic gases.asTheir single atoms,they formula are also are the same known as their as monoatomic symbols.Example He for gases. Helium,Their Ne forformula Neon Aris forthe same as their symbol.Example He for Argon. Helium, Ne for Neon, Ar for Argon. Some nonmetallic elements exist as molecules containing two, four, or eight Some nonmetallic elements exist as molecules containing two, four, or atoms.atoms. eight Hydrogen, nitrogen, nitrogen, Hydrogen, oxygen, fluorine, oxygen,chlorine, bromine fluorine, and iodine chlorine, are bromine and iodine found are found as diatomic as diatomic molecules. molecules. Table 3.3 symbols and formulas of diatomic elements. Name Symbol Formula Hydrogen H H2 Nitrogen N N2 Oxygen O O2 Fluorine F F2 Chlorine Cl Cl2 Bromine Br Br2 Iodine I I2 Elemental formula Elemental formulaalso found also in homo found polyatomic in homo molecules polyatomic that contain molecules thatmore contain than twomore atoms.than two atoms. Examples Ozone-O3, Phosphorus-P4 Examples and Sulfur-S8 Ozone-O 3, Phosphorus-P4 and Sulfur-S8 Formulas of Compounds Formulas of compound Elements combine to form compounds. Just as symbol is a shorthand Elements way combine to an of representing form compounds. element, a chemical symbol iscomprising Just as formula a shorthand two wayor of more different representing symbols, element, is a short a chemical hand representation formula comprising two of or amore compound. different Insymbols, formulas is a of a compound, short the following hand representation pointsInare of a compound. noticed. formulas of compound, theInfollowing each formula, the symbol of elements which form the compound points are noticed. are given. Each symbol is immediately followed by a subscript  In each formula, the symbol of elements which form the compound are given. Each showing the number of atoms of that element. Chemical symbol formulas is immediately indicate followed by the relative a subscript number showing the of atomsof of number eachof atoms thatelement element. present in the compound. For example, water (H2O) is a compound of hydrogen and oxygen. Each of its molecules contains 78 two hydrogen atoms (2H) and one oxygen atom (O). 73  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Exercise 3.2 I.Choose the correct answer from the given alternatives. 1. Elements exists as a diatomic and polyatomic molecular form except_______. A. phosphorus B Nitrogen C Oxygen D Neon 2. For which of the following do the atom and molecule have different formula? A. Helium B. Argon C. Nitrogen D. Neon 3.2.3. Valence Number Activity 3.4 Discuss in groups and share your ideas to the class What is a valence number? Elements combine in accordance with the laws of nature at atomic levels. Each element in a formula of a compound has a combining power. The combining power of an element is called valence. If we know the combining power (valence number) of the elements, it is easy to write the formula of a compound. Most common elements have valence 1, 2, or 3. Some elements have more than one valence number, which is different combining powers under different conditions. Common examples of these elements that have variable valence are iron, copper, lead and tin.Ions are atoms that have positive or negative charge. The number of negative or positive charge an ions carries is equal to the valence number of the ion.Thus, the valences of Cl-, O2- and Al3+ are 1, 2 and 3 respectively. The following table shows the combining power of some common elements. 74  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook valences of of Cl-, O2- and Al3+= are 1, 2 and 3 respectively. The following table shows the combining power of some common elements. Table 3.4: V valences of some common elements. Elements Valence 1 Valence 2Valence 2 Valence 3 Valence 3 Name symbol Name symbol Name symbol Metals Lithium Li Magnesium Mg Aluminum Al Sodium Na Calcium Ca Iron(III) Fe Potassium K Iron(II) Fe Copper(I) Cu Zinc Zn Silver Ag Lead(II) Pb Non-metal Chlorine Cl Oxygen O Nitrogen N Bromine Br Sulfur S Iodine I Fluorine F 3.2.4. 3.2.4. Formulas Formulas of Compounds of Binary Binary Compounds Binary compounds Binary arecompounds compounds are compounds formed formed from from two two types different different types of elements. of elements. To write To write formulas formulas of binary of binary compounds, compounds, follow the follow following simple rule the i. following Write thesimple symbol of rule the elements ii. i. Write Write the symbol the valence of the number above theelements. symbol ii. Write the iii. Criss-cross thevalence valence number numbers above charge to conserve the symbol. or to become the compound iii. Criss-cross electrically the write neutral and valence belownumbers toIfconserve the symbols. the valencecharge orone number is to omit become the compound electrically neutral and write below the the subscript. symbols. Examples Ifwrite the valence the formulanumber for is one, omit the subscript. Examples a. Write the chemical formula for Potassium iodide a. Write the chemical formula for Potassium iodide Solution Solution Step 1: K I Step 1: K I 1 1 Step 2: K I 1 1 1 Step12: Step 3: K I = KK 1I1 I 1 1 Step 3: K80 I = K1I1 K I Since the subscript is 1 we omit and the chemical formula for Potassium iodide is KI. b. Write the chemical formula for calcium chloride 75  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Solution 2 1 Step 1 Ca Cl Step 2: Ca Cl 1 2 Step 3 = CaCl2 Ca Cl Therefore, the chemical formula of calcium chloride is CaCl2 Exercise 3.3 I. Give short answers 1. Write the chemical formula for A.Copper (II) oxide B. Magnesium nitride C. sodium chloride D. Aluminum Oxide E. Iron (III) Oxide F. Iron (II) bromide G. Silver Oxide H. Calcium fluoride 2. Write the chemical name for A. MgO B. FeS C. AgCl II. Choose the correct answer from the given alternatives. 3. Which of the following is the chemical formula of aluminum nitride? A. Al5N3 B. Al3N2 C. AlN3 D. AlN 4. How many valence numbers does an Aluminium have? A. 1 B. 2 C. 3 D.5 3.2.5. Naming Binary Compounds Activity:3.5 Perform the following activities. 3 A student wrote this name for a compound made of calcium and sulfur: Sulfur calcium. What is wrong with this name? Write the correct name for the compound. In naming a compound, the positive ion (metal) mentioned first followed by the negative ion (nonmetal). Binary compound is a compound that is made of only two different elements in a certain whole number ratio. 76  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Rules for naming simple binary compounds. 1.If the binary compounds consists of metal and non -metal, the name of the metal named by its elemental name while the last letters of the non-metal is replaced by the suffix-ide. Table 3.5 Names of nonmetallic elements in binary compounds Nonmetallic Name in Binary Non-metallic Name in Binary Element compounds Element compounds Nitrogen Nitride Bromine Bromide Oxygen Oxide Iodine Iodide Fluorine Fluoride Phosphorous Phosphide Chlorine Chloride Sulfur Sulfide 2.There are metals that form more than one positive ions. In naming compounds of metals with more than one valence number, state valence with Roman number in bracket to indicate positive charge. Thus Fe2+ is Iron (II) (read as ‘’ iron two ‘’) and Fe3+ is iron (III) (read as “iron three”) 3.2.6. Polyatomic Ions Ions are atoms or a group of atoms that have positive or negative charges. They can be simple ions as Cl-, O2- and Al3+ or polyatomic ions as NH4+,OH-,NO3-, SO42- and PO43- A polyatomic ion, also called compound ion is positively or negatively charged group of atoms. The following tables give the valence number of some polyatomic ions. Table 3.6 some common valence of polyatomic ions Valence 1 Valence 2 Valence 3 Ammonium ion (NH4+) Sulfate ion(SO42-) Phosphate ion (PO43-) Hydroxide ion (OH-) Carbonate ion(CO32-) Phosphite ion(PO33-) Nitrate ion (NO3-) Sulfite ion (SO32-) Nitrite (NO2-) Hydrogen carbonate (HCO3-) Hydrogen sulfate ion (HSO4-) 77  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook In writing chemical formulas of compounds that contain polyatomic ions, follow the same steps you used for writing formulas of binary compounds and use bracket if the valence number is different from 1 and not simplified. Examples 1. Write the formula for ammonium chloride Step 1 NH4+ Cl- Step 2 NH4+ Cl- Step 3 NH4+ Cl- So the molecular formula for ammonium chloride is =NH4Cl 2. write the formula ammonium sulfate Step 1 NH4+ SO42- Step 2 NH4+ SO42- Step 3 NH4+ SO4 Cl- 2- So the molecular formula for ammonium sulfate is (NH4)2SO4 In naming compounds containing polyatomic ions, the name of metals and ammonium ion are written first followed by the name of the polyatomic ions. Examples: NH4Cl (ammonium chloride), Al2(SO4)3 (aluminum sulfate) and FeCl3 Iron (III)chloride, Exercise 3.4 1. Fill in the blank by writing the formula of a compound Ions Nitrate Sulfate Carbonate Phosphate Na _________ ________ + __________ ___________ Ca _________ ________ 2+ __________ ___________ Al _________ ________ 3+ __________ ___________ NH4 _________ ________ + __________ ___________ Fe 3+ _________ ________ __________ ___________ 2. Name the following compounds. A. NH4Cl C. NaHCO3 B. Cu(NO3)2 D. FePO4 3. Which three elements are combine in magnesium carbonate? 4. Which four elements are combine in ammonium sulfate? 78  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.2.7 Interpreting Formula Activity 3.6 Perform the following activities. 1. What information is obtained from the coefficient and subscript in a formula? When a formula is interpreted, it will give qualitative and quantitative meanings. Chemical symbols and formulas with numbers around them at particular positions give specific information. Thus, symbols and formulas of elements have qualitative and quantitative meaning. Qualitatively : A symbol represents the identity (kind) of the element. A formula represents the kinds or types of element involved in forming a compound. For example: Cl2 qualitatively the subscript 2 shows a chlorine molecule. The symbol O represents an atom of oxygen. No other element can be represented by the symbol O. Fe stands for iron metal CaO is qualitatively stands for calcium oxide made from one atom of calcium and one atom of oxygen. Ca (NO3)2 qualitatively stands for calcium nitrate made from one atom of calcium and two nitrate groups. Quantitatively: a symbol represents the number of atoms of the elements. A formula stands for one molecule or for formula unit of an element or a compound. In Cl2 quantitatively 2 shows there are two atoms in chlorine molecule Number preceding symbols, called coefficient, indicates the number of atoms of the element in a formula. 2Fe stands for two atoms of iron (The number 2gives a quantitative meaning while Fe itself gives a qualitative meaning. A subscript written after a symbol (to the right) indicates that the element is in molecular form. For example, Cl2 a chlorine molecule and O2 is oxygen molecule The coefficient of a molecule or formula unit indicates the number of molecules or formula unit of that substance. 79  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3H2O;the coefficient three shows that there are 3 molecules of water 2CO2 the coefficient 2 shows that there are 2 molecules of carbon dioxide 4NaCl the coefficient 4 shows that there are 4 formula unit of sodium chloride. CaO quantitatively it shows one formula unit of CaO Ca(NO3)2 quantitatively it shows one formula unit of Ca(NO3)2 Exercise 3.5 1. What does 3H2O represents? A.3H2O atoms B. 6 H molecule C.3 H2O molecule D. 3O molecule 2 write the qualitative meaning for A. 2Fe B. CO2 C. O2 3. Write the quantitative meaning for the following. A. 3H2 B. 4H2O C. 2NaCl Project Work Write and interpret formulae of common compounds By using reference materials, such as a Science books and/or the Internet, try to discover the formulae of common compounds such as baking soda, Vinegar (acetic acid), lime ,sugar(sucrose),chalk, milk of magnesia etc. and interpret them in terms of the elements present and the ratios of their atoms 3.3. Simple chemical Reactions and Equations After completing this section, you will be able to define chemical reaction and give examples; describe evidences that show chemical reaction has occurred; state the law of conservation of mass; conduct an experiment in a group to show simple chemical reaction; write a chemical equation; balance simple chemical equation by inspection; create and use models of particles to demonstrate balanced equations. 80  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.3.1. Simple chemical Reaction Activity 3.7 Discuss in groups and share your ideas with the rest of the class 1.Give some examples of chemical changes that takes place in your home or school. 2. What kind of chemical changes occurred when you coo food? 3. Imagine that you drop a glass beaker and it breaks down. a. Is a new substance formed? b. Is this a physical change or chemical change? The starting materials in chemical reaction called reactants, react alone or with each other to produce one or more new substances, called products. A chemical reaction involves the transformation of reactants into products. Reactants Products An arrow ( ) separates the two side and can be read as ‘produce’, ‘give’,form’,’yield’. Reactants are always written on the left hand side of the arrow while product is/are written on the right hand side by putting “+” sign read as ‘and’ if there are two or more products. The “+” sign means “com- bines with” or “reacts with”. For example when magnesium is in its metal form it will burn very easily in air. In burning of magnesium, the reactants are magnesium and oxygen while the product is the white ash known as magnesium oxide. 2Mg + O2 2MgO Similarly in the reaction between iron and sulfur, the iron and sulfur atoms are reactants where as the formed new substance Iron sulfide is the product. By chemical reaction, some of the common examples of changes brought about Rusting of iron, Fermentation and Digestion of food. 81  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.3.2 Evidences that Show Chemical Reaction has Occurred Activity Activity 3.83.8 Form a group and perform the following activity.Then present your finding to the rest of the class. Record and describe the various chemical changes that occur in your daily lives (e.g cooking food, etc.) and describe the evidence you use to determine that chemical reaction occurred. In a chemical reaction, new products are formed from the reactants. How can you tell this happened? There are few signs that indicate a chemical reaction has occurred. These are: 1. Color changes Gently heating black copper oxide with sulfuric acid produce a blue solution of copper sulfate. Copper oxide + sulfuric acid copper sulfate + water (Black) (Blue) Figure 3.1: blue copper sulfate solution 2. Evolution of a gas (formation of bubbles) When magnesium is placed in hydrochloric acid, bubble of hydrogen gas are given off. Magnesium + hydrochloric acid magnesium chloride + hydrogen 3.Change of temperature (heat change):-either endothermic or exothermic When potassium is placed in water, hydrogen gas is given off. The reaction produces so much heat that the gas burns. Potassium + water potassium hydroxide + hydrogen 82  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 4. Precipitate (formation of a solid) If you mix solutions of silver nitrate and sodium chloride, a chemi- cal reaction takes place. In the reaction insoluble solids are formed. This is called a precipitate. The solid is silver chloride. Silver nitrate + sodium chloride silver chloride + sodium nitrat Figure 3.2 white precipitate of AgCl 3.3.3. Law of Conservation of Mass Activity 3.9 Form a group and perform the following activity. Then present your opinion to the class. When we burn something it gets lighter or, in other words, it loses mass. For example when paper burn, the solid ash left over is lighter than the original paper. Does it mean that mass is not conserved? Discuss in groups and present your ideas to the whole class? The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction. It means that the mass of reactants is exactly equal to the mass of the products. 83  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.3.4. Investigating Chemical Reaction Expriment 3.1 Title: burning of Magnesium ribbon Objective: to investigate the chemical reaction Apparatus: Burner, crucible, a pair of tongs Chemicals: Magnesium ribbon Procedure 1. Take about 5 cm of magnesium ribbon. Rub its surface gently with an abrasive. Notice its color and hardness. 2. Hold it by a pair of tongs and burn it. Hazards!!! In addition to being extremely bright, burning magnesium produces some ultraviolet light; avoid looking directly at it. The burning magnesium is very hot; do not touch it or let it come in contact with other flammable materials. Figure 3.3: burning of magnesium 3.Collect the substance formed.Then add in a crucible and examine it carefully. Feel it. Notice its color. Observation and analysis 1. What is the reactant materials? 2. Does it bend? It is shiny? Will it burn if heated again? Does it have any resemblance to the magnesium ribbon you started with? 3. Is chemical (change) reaction occur? 84  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 3.3.5. Writing and Balancing Simple Chemical Equation Activity 3.10 Perform the following activity 1. What is chemical equation? A chemical equation is a short hand expression of a chemical chanes (chemical reaction) through symbols and formulas. In general, to write a chemical equation for a given reaction one can follow the following three steps. Step 1: Write a word equation for the reaction. Step 2: Change the word equation to a chemical equation i.e., write the correct symbol or formula for each reactant and product. Step 3: Balance the equation so that it obeys the law of conservation of mass. Example: the reaction between hydrogen and oxygen to give water. Step1: Hydrogen + Oxygen → Water Step 2: H2 + O2 → H2O Step 3: 2H2 + O2 → 2H2O 3.3.6. Balancing Chemical equation Activity 3.11 Perform the following activity Why should the chemical equation be balanced? 85  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Chemical equation is balanced in order to obey the law of conservation of mass.So a balanced chemical equation is an equation in which the total number of atoms on the left hand side are equal to the total number of atoms on the right hand side. When we balance a Chemical equation, we have to change the coefficients not subscripts. This is because, changing subscripts changes the identity of substances. In balanced chemical equation: Mass reactants = mass of products Number of atoms in reactant side = Number of atoms in product side There are many methods of balancing chemical equations. Only 2 methods of balancing chemical equations are discussed in this grade level, namely 1. The inspection method 2. Least common multiple (LCM) method 1. The inspection method- is trial and error method. It involves examining the equation and adjusting the coefficients until each kind of atoms are equal on the reactant and product sides. For example, to balance the equation when nitrogen react with hydrogen to give ammonia N2 + H2 NH3 Balance nitrogen by placing 2 before ammonia N2 + H2 2NH3 Now you have 6 hydrogen atoms on the product side. To balance hydrogen write a coefficient 3 before H2 N2 + 3 H2 2NH3 Finally check whether the equation balanced or not 86  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Reactants Products N (2) N (2) H (6) H (6) Therefore, the equation is balanced 2. Least common multiple (LCM) method The steps used in this method are shown by the following examples. Consider the reaction between aluminum and oxygen to form aluminum oxide. Step 1: Represent the reaction by word equation Aluminum + oxygen aluminum oxide Step 2: Write the correct formula for each of the reactants and products Al + O2 Al2O3 Step 3: Find the total valence number (subscript time’s valence number) and place above each symbol and formula 3 4 6 6 Al + O2 Al2O3 Step: 4 Find the LCM of total valence number and place it above the arrow. The LCM of 3, 4, and 6 is 12 3 4 12 6 6 Al + O2 Al2O3 Step 5: Divide the LCM by each total valence number to obtain the coefficients for each of the reactants and products. Place the coefficients thus obtained in front of the respective formula. 4Al + 3O2 2 Al2O3 Check Reactants Products Al (4) Al (4 O (6) O (6) So the equation is balanced 87  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Exercise 3.6 1. Balance the following by inspection a. CaCO3 (s) CaO(s) + CO2(g) b. H2 + I2 HI c. C2H2(g)+O2(g) CO2(g)+H2O(l) d. Ca + H2O Ca(OH)2 + H2 e. Fe2O3 +CO Fe + CO2 2. Balance the following chemical equations a. Fe + O2 Fe2 O3 b. Cu + H2SO4 CuSO4 +SO2 + H2O 3.4 Use of Chemical Reactions in Every Day Situation After completing this section, you will be able to: describe the uses of chemical reactions in everyday situations. Activity 3.12 Discuss in groups and share your ideas with the rest of the class. 1. How do the local people in Ethiopia prepare alcoholic beverages like “Tella”? What raw materials are used? Is the process a chemical change? 2. Give some examples of useful chemical reactions such as fermentation in brewing which produces carbon dioxide and ethanol/ 3.4.1. Uses of Chemical Reaction Chemical reaction happen everywhere. It happen inside your body to keep you alive. For example, reactions to digest food i.e. the breakdown of large molecules (protein, starch and fats) into smaller ones, so that they can be absorbed. Chemical reactions are an integral part of technology of culture, and indeed of life itself. Burning fuels, smelting iron, making glass and pottery, brewing beer, and making wine and cheese are among many examples of activities incorporating chemical reactions that have been known. 88  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Some Important Chemical Reactions 1.Synthesis of ammonia: Reaction: 3H2(g) + N2(g) 2NH3(g) Hydrogen gas and nitrogen gas are combined in the presence of a catalyst at high temperature and pressure to produce ammonia gas. Significance: Synthesis of ammonia leads to the production of fertilizer (ammonium nitrate) and to the production of ammunitions. 2. Combustion of hydrogen 2H2(g) + O2(g) 2H2O(l) Hydrogen gas and oxygen combine to produce liquid water. Significance: In the forward direction this is a spontaneous reaction that explosively oxidized hydrogen to water. 3. Combustion of methane (hydrocarbons) Reaction:CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Methane gas and oxygen gas combine exothermically to produce carbon dioxide gas and water vapor. Significance: Methane is the simplest of the hydrocarbons, all of which combine with oxygen and undergo oxidation. If the oxidation is complete the products are carbon dioxide (a greenhouse gas) and water. 4. Photosynthesis Reaction: 6CO2(g) + 6H2O(l) 6O2(g) + C6H12O6(aq) Carbon dioxide and water combine in the presence of sunlight (and many biologically catalyzed reactions) to produce oxygen and glucose (sugar). Significance: Our atmosphere is 21% oxygen - in spite of the tendency of oxygen to react with so many substances. The constant level of oxygen is maintained by the many plants that inhabit our planet through the reaction of photosynthesis. This is truly solar energy at its most efficient and productive. 5. Synthesis of sulfuric acid Reaction: 2S(s) +3O2(g) + H2O(l) 2H2SO4(aq) Sulfur is first oxidized to sulfur dioxide and then to sulfur trioxide. This gas is bubbled through water to produce sulfuric acid. Significance: Sulfuric acid is a very important chemical and an indicator of a nation’s industrial strength. 89  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook Key terms Element Balanced Chemical Equation Reactants Chemical Symbol Valence Products Chemical Formula Binary Compounds Chemical Equation Molecule Poly Atomic Ion Law Of Conservation Of Mass Chemical Reaction Inspection Method Lcm Method Unit summary An atomic symbol is defined as shorthand way of representing elements or atoms of an element. A compound is a pure substance consists of two or more elements which have been chemically combined. Chemical formula is the symbolic representation of an element or a compound. A molecule is the smallest particle of an element or a compound that has a stable, independent existence. The combining power of an element is called valence. Most common elements have valence 1, 2, or 3. Binary compounds are compounds formed from two different types of elements. In naming a compound, the positive ion (metal) mention first followed by the negative ion (nonmetal). A polyatomic ion, also called compound ion is positively or negatively charged group of atoms. A chemical reaction is a process in which some substances is changed into one or more different new substances. The starting materials in chemical reaction called reactants, react alone or with each other to produce one or more new substances, called products. There are few signs that indicate a chemical reaction has occurred. These are: color change, evolution of gas, heat change and formation of precipitate. 90  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction. A chemical equation is shorthand expression of a chemical changes (chemical reaction) through symbols and formulas. There are many methods of balancing chemical equations. Some of them are inspection method, Least common multiple (LCM) method. Review Exercise I Write ‘True’ for the Correct Statements and ‘False’ for the Wrong Statements. 1. A compound is pure substance. 2. In a chemical reaction atoms are neither created nor destroyed. 3. O2 and 2O have the same meaning. 4. The symbol copper denoted by Co. 5. When we balance chemical equation, we change the subscript but not the coefficient. 6. Respiration is a chemical change (chemical reaction). 7. The combining power of an element is called valence. 8. The formula and symbol of nitrogen is the same. 9. A number in front of a symbol or formula is coefficient 10. A chemical symbol is a short hand notation for the chemical name of an element. II. Choose the correct answer from the given alternatives. 11. The Latin name of silver is A. Argentum B. Kalium C. Natrium D. Cuprum 12. In 4O3 the coefficient and subscript respectively A.3, 4 B.4,3 C.7,3 D.3,7 13. The formula of nitrate ion is A. NO B. NO3- C. NO2- D. N2- 14. All of the following elements can exist as diatomic molecules EXCEPT A Hydrogen B Oxygen C Sodium D Chlorine 91  General Science Fetena.net Grade 7 StudentResource : Ethiopian No#1 Educational Textbook 15. What is the chemical formula for Iron (III) chloride? A. FeCl2 B.FeCl C.FeCl3 D. Fe2Cl3 16. The chemical symbol for Helium is A. H B. Hl C. He D.HE 17. Which of the following is the correct name of MgO? A. Magnesium oxygen B. oxygen magnesium C. Magnesium oxide D. molybdenum oxide III. Give short answers 18. Write the symbol of a. Zinc b. phosphorous c. Mercury d. calcium e. Tin 19. Balance the following chemical equations a. Al(OH)3 Al2O3 +H2O b. NH3 + O2 NO +H2O c. Ca(OH)2 + H3PO4 Ca3(PO4)2 +H2O 20. Write chemical equations for the following reactions and balance them. a. Aluminum + bromine Aluminum bromide b. Sodium + Nitrogen Sodium nitride c. Potassium + Water Potassium hydroxide + Hydrogen 92