Class 10 Chemistry Lakhmir Singh Manjit Kaur PDF

 This book has been revised according to the CCE pattern of school education based on NCERT syllabus prescribed by the Central Board of Secondary Education (CBSE) for Class X SCIENCE FOR TENTH CLASS (Part – 2) Chemistry As per...

 This book has been revised according to the CCE pattern of school education based on NCERT syllabus prescribed by the Central Board of Secondary Education (CBSE) for Class X SCIENCE FOR TENTH CLASS (Part – 2) Chemistry As per NCERT/CBSE Syllabus (Based on CCE Pattern of School Education) t aining RT LAKHMIR SINGH n Co to NCE And ers ons answ k questi sed MANJIT KAUR boo alue-ba v and uestions q  S. CHAND SCHOOL BOOKS (An imprint of S. Chand Publishing) A Division of S. Chand And Company Pvt. Ltd. (An ISO 9001 : 2008 Company) 7361, Ram Nagar, Qutab Road, New Delhi-110055 Phone: 23672080-81-82, 9899107446, 9911310888; Fax: 91-11-23677446 www.schandpublishing.com; e-mail : [email protected] Branches : Ahmedabad : Ph: 27541965, 27542369, [email protected] Bengaluru : Ph: 22268048, 22354008, [email protected] Bhopal : Ph: 4274723, 4209587, [email protected] Chandigarh : Ph: 2725443, 2725446, [email protected] Chennai : Ph: 28410027, 28410058, [email protected] Coimbatore : Ph: 2323620, 4217136, [email protected] (Marketing Office) Cuttack : Ph: 2332580; 2332581, [email protected] Dehradun : Ph: 2711101, 2710861, [email protected] Guwahati : Ph: 2738811, 2735640, [email protected] Hyderabad : Ph: 27550194, 27550195, [email protected] Jaipur : Ph: 2219175, 2219176, [email protected] Jalandhar : Ph: 2401630, 5000630, [email protected] Kochi : Ph: 2378740, 2378207-08, [email protected] Kolkata : Ph: 22367459, 22373914, [email protected] Lucknow : Ph: 4026791, 4065646, [email protected] Mumbai : Ph: 22690881, 22610885, [email protected] Nagpur : Ph: 6451311, 2720523, 2777666, [email protected] Patna : Ph: 2300489, 2302100, [email protected] Pune : Ph: 64017298, [email protected] Raipur : Ph: 2443142, [email protected] (Marketing Office) Ranchi : Ph: 2361178, [email protected] Siliguri : Ph: 2520750, [email protected] (Marketing Office) Visakhapatnam : Ph: 2782609, [email protected] (Marketing Office) © 1980, Lakhmir Singh & Manjit Kaur All rights reserved. No part of this publication may be reproduced or copied in any material form (including photocopying or storing it in any medium in form of graphics, electronic or mechanical means and whether or not transient or incidental to some other use of this publication) without written permission of the publisher. Any breach of this will entail legal action and prosecution without further notice. Jurisdiction : All disputes with respect to this publication shall be subject to the jurisdiction of the Courts, Tribunals and Forums of New Delhi, India only. S. CHAND’S Seal of Trust In our endeavour to protect you against counterfeit/fake books, we have pasted a holographic film over the cover of this book. The hologram displays the unique 3D multi-level, multi-colour effects of our logo from different angles when tilted or properly illuminated under a single source of light, such as 2D/3D depth effect, kinetic effect, gradient effect, trailing effect, emboss effect, glitter effect, randomly sparkling tiny dots, etc. A fake hologram does not display all these effects. First Published in 1980 Revised Edition 2014, 2016 Reprints 1981, 82, 83, 84, 85, 86, 87, 88, 89, 90 (Twice), 91 (Twice), 92 (Twice), 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2015, 2016 (Thrice) ISBN : 978-93-525-3029-8   Code : 1004H 268   Chemistry X: Lakhmir Singh  AN OPEN LETTER Dear Friend, We would like to talk to you for a few minutes, just to give you an idea of some of the special features of this book. Before we go further, let us tell you that this book has been revised according to the NCERT syllabus prescribed by the Central Board of Secondary Education (CBSE) based on new “Continuous and Comprehensive Evaluation” (CCE) pattern of school education. Just like ABOUT THE AUTHORS our earlier books, we have written this book in such a simple style that even the weak students will be able to LAKHMIR SINGH did his M.Sc. from Delhi understand chemistry very easily. Believe us, while writing University in 1969. Since then he has been this book, we have considered ourselves to be the teaching in Dyal Singh College of Delhi University, Delhi. He started writing books in students of Class X and tried to make things as simple 1980. Lakhmir Singh believes that book writing as possible. is just like classroom teaching. Though a book can never replace a teacher but it should make The most important feature of this revised edition of the book the student feel the presence of a teacher. is that we have included a large variety of different types of Keeping this in view, he writes books in such a questions as required by CCE for assessing the learning style that students never get bored reading his abilities of the students. This book contains : books. Lakhmir Singh has written more than 15 books so far on all the science subjects: Physics, (i) Very short answer type questions (including true-false Chemistry and Biology. He believes in writing type questions and fill in the blanks type questions), quality books. He does not believe in quantity. (ii) Short answer type questions, MANJIT KAUR did her B.Sc., B.Ed. from Delhi (iii) Long answer type questions (or Essay type questions), University in 1970. Since then she has been teaching in a reputed school of Directorate of (iv) Multiple choice questions (MCQs) based on theory, Education, Delhi. Manjit Kaur is such a popular (v) Questions based on high order thinking skills (HOTS), science teacher that all the students want to join those classes which she teaches in the school. She (vi) Multiple choice questions (MCQs) based on practical has a vast experience of teaching science to skills in science, school children, and she knows the problems faced by the children in the study of science. (vii) NCERT book questions and exercises (with answers), Manjit Kaur has put all her teaching experience and into the writing of science books. She has co- (viii) Value based questions (with answers). authored more than 15 books alongwith her husband, Lakhmir Singh. Please note that answers have also been given for the various types of questions, wherever required. All these features will It is the team-work of Lakhmir Singh and Manjit Kaur which has given some of the most popular make this book even more useful to the students as well as books in the history of science education in India. the teachers. “A picture can say a thousand words”. Keeping Lakhmir Singh and Manjit Kaur both write this in mind, a large number of coloured pictures and sketches exclusively for the most reputed, respected and of various scientific processes, procedures, appliances, largest publishing house of India : S.Chand and manufacturing plants and everyday situations involving Company Pvt. Ltd. principles of chemistry have been given in this revised edition of the book. This will help the students to understand the various concepts of chemistry clearly. It will also tell them how chemistry is applied in the real situations in homes, transport and industry.  We are sure you will agree with us that the facts and Other Books by Lakhmir Singh formulae of chemistry are just the same in all the and Manjit Kaur books, the difference lies in the method of presenting these facts to the students. In this book, the various 1. Awareness Science for Sixth Class topics of chemistry have been explained in such a simple way that while reading this book, a student will 2. Awareness Science for Seventh Class feel as if a teacher is sitting by his side and explaining the various things to him. We are sure that after 3. Awareness Science for Eighth Class reading this book, the students will develop a special interest in chemistry and they would like to study 4. Science for Ninth Class (Part 1) PHYSICS chemistry in higher classes as well. 5. Science for Ninth Class (Part 2) CHEMISTRY We think that the real judges of a book are the teachers concerned and the students for whom it is meant. So, 6. Science for Tenth Class (Part 1) PHYSICS we request our teacher friends as well as the students 7. Science for Tenth Class (Part 3) BIOLOGY to point out our mistakes, if any, and send their comments and suggestions for the further improvement 8. Rapid Revision in Science of this book. (A Question-Answer Book for Class X) Wishing you a great success, 9. Science for Ninth Class (J & K Edition) Yours sincerely, 10. Science for Tenth Class (J & K Edition) 11. Science for Ninth Class (Hindi Edition) : PHYSICS and CHEMISTRY 12. Science for Tenth Class (Hindi Edition) : 396, Nilgiri Apartments, PHYSICS, CHEMISTRY and BIOLOGY Alaknanda, New Delhi-110019 E-mail : [email protected] 13. Saral Vigyan (A Question-Answer Science Book in Hindi for Class X) DISCLAIMER While the authors of this book have made every effort to avoid any mistake or omission and have used their skill, expertise and knowledge to the best of their capacity to provide accurate and updated information, the authors and the publisher do not give any representation or warranty with respect to the accuracy or completeness of the contents of this publication and are selling this publication on the condition and understanding that they shall not be made liable in any manner whatsoever. The publisher and the authors expressly disclaim all and any liability/responsibility to any person, whether a purchaser or reader of this publication or not, in respect of anything and everything forming part of the contents of this publication. The publisher and authors shall not be responsible for any errors, omissions or damages arising out of the use of the information contained in this publication. Further, the appearance of the personal name, location, place and incidence, if any; in the illustrations used herein is purely coincidental and work of imagination. Thus the same should in no manner be termed as defamatory to any individual.  CONTENTS FIRST TERM 1. CHEMICAL REACTIONS AND EQUATIONS 1 – 51 Chemical Reactions Involve Chemical Changes ; Formation of New Substances by the Rearrangement of Atoms ; Reactants and Products ; Characteristics of Chemical Reactions : Evolution of a Gas, Formation of a Precipitate, Change in Colour, Change in Temperature and Change in State ; Chemical Equations : Short-Hand Method of Representing a Chemical Reaction ; Balanced Chemical Equations and Unbalanced Chemical Equations ; Balancing of Chemical Equations to Satisfy the Law of Conservation of Mass in Chemical Reactions ; To Make Chemical Equations More Informative : By Indicating the Physical States of Reactants and Products in the Equation (Solid, Liquid, Aqueous Solution and Gas), By Indicating the Heat Changes in the Equation (Exothermic Reactions and Endothermic Reactions), and by Indicating the Conditions Under Which the Reaction Takes Place (Heat, Catalyst, Pressure and Temperature) ; Important Examples on Writing of Balanced Chemical Equations ; Types of Chemical Reactions : Combination Reactions, Decomposition Reactions, Displacement Reactions, Double Displacement Reactions, and Oxidation and Reduction Reactions ; Oxidising Agents and Reducing Agents ; Uses of Decomposition Reactions ; Decomposition Reactions in Our Body ; Effects of Oxidation Reactions in Everyday Life ; Corrosion of Metals and Rancidity of Food ; Prevention of Rancidity of Food : Adding Anti-Oxidants, Packaging in Nitrogen Gas, Keeping in a Refrigerator, Storing in Air-Tight Containers, and Away From Light. 2. ACIDS, BASES AND SALTS 52 – 101 Types of Compounds : Acids, Bases and Salts ; Indicators for Testing Acids and Bases : Dyes Which Change Colour in Acids and Bases ; Acid-Base Indicators : Litmus, Methyl Orange, Phenolphthalein, Turmeric, Red Cabbage Extract and Olfactory Indicators (Onion and Vanilla Extract) ; Acids : Mineral Acids and Organic Acids ; Strong Acids and Weak Acids; Concentrated Acids and Dilute Acids ; Diluting Acids ; Properties of Acids ; What do All Acids Have in Common ; To Show That All the Compounds Containing Hydrogen are Not Acids ; Acids do Not Show Acidic Behaviour in the Absence of Water ; Uses of Mineral Acids in Industry ; Bases ; Water Soluble Bases : Alkalis ; What do All Bases Have in Common ; Strong Bases and Weak Bases ; Properties of Bases and Uses of Bases ; Strength of Acid and Base Solutions : pH Scale ; Measurement of pH of Solutions : Universal Indicator ; Importance of pH in Everyday Life : pH in our Digestive System, pH Change as the Cause of Tooth Decay , Plants and Animals are Sensitive to pH Changes , Self Defence by Animals and Plants Through Chemical Warfare ; Salts ; Family of Salts ; The pH of Salt Solutions ; Acidic, Basic and Neutral Salts ; Common Salt (Sodium Chloride) ; Chemicals from Common Salt : Sodium Hydroxide (Caustic Soda), Washing Soda (Sodium Carbonate), Baking Soda (Sodium Hydrogencarbonate) ; Bleaching Powder (Calcium Oxychloride) ; Plaster of Paris (Calcium Sulphate Hemihydrate) ; Water of Crystallisation ; Hydrated Salts ; Action of Heat on Hydrated Salts. 3. METALS AND NON-METALS 102—197 Main Groups of Elements : Metals and Non-Metals ; Physical Properties of Metals and Non-Metals ; Exceptions in Physical Properties of Metals and Non-Metals ; Chemical Properties of Metals ; Reactivity Series of Metals ; Chemical Properties of Non-Metals ; Comparison Among the Properties of Metals and Non-Metals ; Uses of Metals and Non-Metals ; How do Metals and Non-Metals React ; Inertness of Noble Gases ; Cause of Chemical Bonding (or Chemical Combination) ; Ions ; Formation of Positive Ions (Cations) and Negative Ions (Anions) ; Types of Chemical Bonds : Ionic Bond and Covalent Bond ; Formation of Ionic Bonds and Ionic Compounds ; Formation of Covalent Bonds : Single Bonds , Double Bonds and Triple Bonds ; Covalent Compounds ; Properties of Ionic Compounds and Covalent Compounds ; How to Distinguish Between Ionic Compounds and Covalent Compounds ; Occurrence of Metals ; Minerals and Ores ; Extraction of Metals ; Concentration of Ore (Enrichment of Ore) ; Conversion of Concentrated Ore into Metal ; Extraction of Highly Reactive Metals , Extraction of Moderately Reactive Metals and Extraction of Less Reactive Metals ; Refining of Metals (Purification of Metals) ; Corrosion of Metals ; Rusting of Iron ; Conditions Necessary for the Rusting of Iron : Presence of Air and Water ; Prevention of Rusting of Iron ; Corrosion of Aluminium, Copper and Silver Metals ; The Case of Gold and Platinum Metals, Alloys : Their Composition, Properties and Uses ; The Iron Pillar at Delhi SECOND TERM 4. CARBON AND ITS COMPOUNDS 198 – 267 Carbon : A Non-Metal Element ; Carbon Always Forms Covalent Bonds ; Carbon is Tetravalent ; Occurrence of Carbon : Free State (as Element) and Combined State (as Compounds) ; Allotropes of Carbon : Diamond, Graphite and Buckminsterfullerene ; Structure of Diamond and Graphite ; Uses of Diamond and Graphite ; Structure of Buckminsterfullerene ; Organic Compounds ; Reasons for the Large Number of Organic Compounds : Catenation (Self-Linking) and Tetravalency (Four Valency) of Carbon ; Types of Organic Compounds : Hydrocarbons , Haloalkanes (Halogenated Hydrocarbons), Alcohols, Aldehydes, Ketones and Carboxylic Acids (Organic Acids) ; Hydrocarbons : Saturated Hydrocarbons (Alkanes) and Unsaturated Hydocarbons (Alkenes and Alkynes) ; Alkyl Groups and Cyclic Hydrocarbons ; Naming of Hydrocarbons ; Isomers ; Homologous Series and its Characteristics ; Functional Groups in Organic Compounds : Halo Group, Alcohol Group, Aldehyde Group, Ketone Group, Carboxyl Group, Alkene Group and Alkyne Group ; Naming of Haloalkanes, Alcohols, Aldehydes, Ketones and Carboxylic Acids ; Coal and Petroleum ; Chemical Properties of Carbon Compounds : Combustion, Substitution Reactions and Addition Reactions ; Hydrogenation of Oils ; Some Important Organic Compounds ; Ethanol (Ethyl Alcohol) ; Harmful Effects of Drinking Alcohol ; Denatured Alcohol ; Ethanoic Acid (Acetic Acid) ; Soap : Manufacture of Soap and Preparation in the Laboratory ; Structure of Soap Molecule and Cleansing Action of Soap ; Limitations of Soap ; Detergents ; Differences Between Soaps and Detergents 5. PERIODIC CLASSIFICATION OF ELEMENTS 268 – 309 Dobereiner’s Triads ; Limitations of Dobereiner’s Classification of Elements ; Newlands’ Law of Octaves ; Limitations of Newlands’ Law of Octaves for the Classification of Elements; Mendeleev’s Periodic Table Based on Atomic Masses of Elements ; Merits of Mendeleev’s Classification of Elements : Prediction of Some Undiscovered Elements, Prediction of Properties of Elements and Accommodation of Noble Gases as a Separate Group of Elements ; Anomalies (or Limitations) of Mendeleev’s Classification of Elements : Position of Isotopes Could Not be Explained, Wrong Order of Atomic Masses of Some Elements Could Not be Explained and Correct Position Could Not be Assigned to Hydrogen in the Periodic Table ; Present Basis for the Classification of Elements : Atomic Numbers of Elements ; Explanation of the Anomalies of Mendeleev’s Classification of Elements ; Modern Periodic Law ; Explanation of Modern Periodic Law on the Basis of Electronic Configurations of Elements ; Modern Periodic Table (or Long Form of Periodic Table) ; Characteristics of the Periods and Groups of the Periodic Table : Variation in Valence Electrons (Outermost Electrons), Valency, Size of Atoms (Atomic Size), Metallic Character, Chemical Reactivity and Nature of Oxides of Elements in Moving From Left to Right in a Period and on Going Down in a Group of the Periodic Table ; Merits of the Modern Periodic Table and Advantages of Periodic Table ; Objective Type Questions Based on Periodic Table ; Periodic Table and Chemical Bonding Multiple Choice Questions (MCQs) 310 – 320 Based on Practical Skills in Science (Chemistry) NCERT Book Questions and Exercises (with answers) 321 – 344 Value Based Questions (with answers) 345 – 360 PHYSICS & BIOLOGY BY SAME AUTHORS Science for Tenth Class, Part 1 : PHYSICS 1.Electricity 2.Magnetic Effect of Electric Current 3.Sources of Energy 4.Reflection of Light 5.Refraction of Light 6.The Human Eye and the Colourful World Multiple Choice Questions (MCQs) Based on Practical Skills in Science (Physics) NCERT Book Questions and Exercises (with answers) Value Based Questions (with answers) Science for Tenth Class, Part 3 : BIOLOGY 1.Life Processes 2.Control and Coordination 3.How do Organisms Reproduce 4.Heredity and Evolution 5.Our Environment 6.Management of Natural Resources Multiple Choice Questions (MCQs) Based on Practical Skills in Science (Biology) NCERT Book Questions and Exercises (with answers) Value Based Questions (with answers)  LATEST CBSE SYLLABUS, CLASS 10 SCIENCE (CHEMISTRY PART) FIRST TERM (April to September) Theme : Materials UNIT : CHEMICAL SUBSTANCES — NATURE AND BEHAVIOUR Chemical reactions : Chemical equations, Balanced chemical equations, Implications of a balanced chemical equation, Types of chemical reactions : combination, decomposition, displacement, double displacement, precipitation, neutralisation, oxidation and reduction. Acids, bases and salts : Their definitions in terms of furnishing of H+ and OH– ions, General proper- ties, examples and uses, Concept of pH scale (Definition relating to logarithm not required), Importance of pH in everyday life, Preparation and uses of sodium hydroxide, washing soda, baking soda, bleaching powder and plaster of Paris. Metals and non-metals : Properties of metals and non-metals, Reactivity series, Formation and prop- erties of ionic compounds and covalent compounds, Basic metallurgical processes, Corrosion and its pre- vention. SECOND TERM (October to March) Carbon compounds : Covalent bonding in carbon compounds, Versatile nature of carbon, Homologous series, Nomenclature of carbon compounds containing functional groups (halogens, alcohols, aldehydes, ketones, carboxyl, alkanes, alkenes and alkynes), Difference between saturated hydrocarbons and unsaturated hydrocarbons, Chemical properties of carbon compounds (combustion, oxidation, substitution and addition reactions), Ethanol and ethanoic acid (only properties and uses), Soaps and detergents. Periodic classification of elements : Need for classification, Modern periodic table, Gradation in properties : valency, atomic number, metallic and non-metallic properties.  1 CHEMICAL REACTIONS AND EQUATIONS C hemical reactions are the processes in which new substances with new properties are formed. Chemical reactions involve chemical changes. During chemical reactions, a rearrangement of atoms takes place between the reacting substances to form new substances having entirely different properties. Chemical reactions involve breaking of old chemical bonds which exist between the atoms of reacting substances, and then making of new chemical bonds between the rearranged atoms of new substances. During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place in a chemical reaction. We will now discuss reactants and products of a chemical reaction. (i) The substances which take part in a chemical reaction are called reactants. (ii) The new substances produced as a result of chemical reaction are called products. In a chemical reaction, reactants are transformed into products. The products thus formed have properties which are entirely different from those of the reactants. We will now give an example of a chemical reaction. Before we do that please note that magnesium is a silvery-white metal. Magnesium metal is available in a science laboratory in the form of a magnesium ribbon (or magnesium wire). Let us study the chemical reaction of ‘magnesium’ with the ‘oxygen’ of air now. When a magnesium ribbon is heated, it burns in air with a dazzling white flame to form a white powder called magnesium oxide. Actually, on heating, magnesium combines with oxygen present in air to form magnesium oxide : Magnesium + Oxygen Heat Magnesium oxide  o (As ribbon) (From air) (White powder) The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction. In this chemical reaction there are two reactants ‘magnesium and oxygen’ but only one product ‘magnesium oxide’. The properties of the product magnesium oxide are entirely different from those of the reactants magnesium and oxygen.  2 SCIENCE FOR TENTH CLASS : CHEMISTRY The magnesium ribbon which we use usually has a coating of ‘magnesium oxide’ on its surface which is formed by the slow action of oxygen of air on it. So, before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of magnesium oxide from the surface of magnesium ribbon so that it may readily combine with the oxygen of air (on heating). Another point to be noted is that the dazzling (very bright) white light given out during the burning of magnesium ribbon is harmful to the eyes. So, the magnesium ribbon should be burned by keeping it as far as possible from the eyes. We can perform the chemical reaction involved in the burning of magnesium ribbon as follows : (i) Take about 2 cm long magnesium ribbon and clean it by rubbing its Figure 1. When magnesium surface with sand paper. burns in air, it combines with the oxygen of air to form (ii) Hold the magnesium ribbon with a pair of tongs at one end, and heat magnesium oxide. its other end over a burner [see Figure 2(a)] Pair of tongs Magnesium ribbon Burning Burner magnesium ribbon Magnesium Watch oxide glass (a) Magnesium ribbon being (b) Magnesium ribbon burns in air heated over a burner to form magnesium oxide Figure 2. (iii) The magnesium ribbon starts burning with a dazzling white flame. (iv) Hold the burning magnesium ribbon over a watch glass so that the magnesium oxide powder being formed collects in the watch glass [see Figure 2(b)]. It is not that chemical reactions can be carried out only in a science laboratory. A large number of chemical reactions keep on occurring in our daily life. Souring of milk (when left at room temperature during summer), Formation of curd from milk, Cooking of food, Digestion of food in our body, Process of respiration, Fermentation of grapes, Rusting of iron (when left exposed to humid atmosphere), Burning of fuels (like wood, coal, kerosene, petrol and LPG), Burning of candle wax, and Ripening of fruits, are all chemical changes which involve chemical reactions. In all these cases, the nature and identity of the initial substance changes because of the chemical reaction which takes place in it. An important question now arises : How do we come to know that a Figure 3. Ripening of fruits is chemical reaction has taken place ? This will become clear from the following a chemical change which discussion on the characteristics of chemical reactions. involves chemical reactions. CHEMICAL REACTIONS AND EQUATIONS 3 Characteristics of Chemical Reactions In a chemical reaction, the substances known as reactants are converted into new substances called products. The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be observed easily. The easily observable features (or changes) which take place as a result of chemical reactions are known as characteristics of chemical reactions. The important characteristics of chemical reactions are : (i) Evolution of a gas, (ii) Formation of a precipitate, (iii) Change in colour, (iv) Change in temperature, and (v) Change in state Any one of these general characteristics can tell us whether a chemical reaction has taken place or not. For example, if on mixing two substances a gas is evolved, then we can say that a chemical reaction has taken place. We will now give examples to show all the characteristics of chemical reactions, one by one. 1. Evolution of a Gas Some chemical reactions are characterised by the evolution of a gas. For example, when zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are produced. So, the chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas. (Please note that we can also use dilute hydrochloric acid in place of dilute sulphuric acid in this reaction.) We can perform this chemical reaction in the laboratory as follows : (i) Take some zinc granules in a conical flask (or a test-tube). Conical flask (ii) Add dilute sulphuric acid over zinc granules. (iii) We will see the bubbles of hydrogen gas being formed around Bubbles of zinc granules (see Figure 4). hydrogen gas (iv) If we touch the conical flask with our hand, we will find that it Dilute sulphuric acid is somewhat hot. So, a change in temperature (rise in Zinc granules temperature) also occurs in this chemical reaction. Figure 4. Dilute sulphuric acid reacts When magnesium reacts with a dilute acid (like dilute hydrochloric with zinc to evolve hydrogen gas. acid or dilute sulphuric acid), even then hydrogen gas is evolved (see Figure 5). Let us take another example of a chemical reaction in which a gas is evolved. When dilute hydrochloric acid is poured over sodium carbonate in a test-tube, then carbon dioxide gas is evolved. So, the chemical reaction between sodium carbonate and dilute hydrochloric acid is characterised by the evolution of carbon dioxide gas. Before we go further, we should know the meaning of the term ‘precipitate’. A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction. A precipitate can be formed by mixing aqueous solutions (water solutions) of reactants when one of the products is insoluble in water. A precipitate can also be formed by Figure 5. When magnesium reacts with a dilute acid, then hydrogen gas is evolved. passing a gas into an aqueous solution of a substance (like passing carbon dioxide gas into lime water). 2. Formation of a Precipitate Some chemical reactions are characterised by the formation of a precipitate. For example, when potassium iodide solution is added to a solution of lead nitrate, then a yellow precipitate of lead iodide is  4 SCIENCE FOR TENTH CLASS : CHEMISTRY formed (see Figure 6). Thus, the chemical reaction between potassium iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide. We can carry out this chemical reaction as follows : (i) Take some lead nitrate solution in a test-tube (or a beaker). (ii) Add potassium iodide solution to it. (iii) A yellow precipitate of lead iodide is formed at once. (iv) A change in colour (from colourless to yellow) also takes place in this chemical reaction. Let us take another example of a chemical reaction in which a precipitate is formed. When dilute sulphuric acid is added to barium chloride solution taken in a test-tube, then a white precipitate of barium sulphate is formed. Thus, the chemical reaction between sulphuric acid and barium chloride solution is Figure 6. Precipitation is characterised by the formation of a white precipitate of barium sulphate. the formation of an insoluble 3. Change in Colour product. When colourless solutions of lead nitrate and Some chemical reactions are characterised by a change in colour. For example, potassium iodide are mixed, when citric acid reacts with potassium permanganate solution, then the purple then a yellow precipitate of colour of potassium permanganate solution disappears (it becomes colourless). lead iodide is formed. So, the chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless. We can perform the reaction between citric acid and potassium permanganate solution as follows. (Lemon juice contains citric acid, so we will use lemon juice as a source of citric acid for carrying out this reaction). (i) Take some dilute potassium permanganate solution in a test-tube. It has purple colour. (ii) Add lemon juice to it dropwise with the help of a dropper and shake the test-tube. (iii) The purple colour of potassium permanganate solution goes on fading and ultimately it becomes colourless. Let us take another example of a chemical reaction in which a change in colour takes place. When sulphur dioxide gas is passed through acidified potassium dichromate solution, then the orange colour of potassium dichromate solution changes to green (see Figure 7). Thus, the chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterised by a change in colour from orange to green. Before we go further, we should know why temperature changes take place in chemical reactions. (a) Potassium dichromate (b) When sulphur dioxide gas Chemical reactions often produce heat energy. When a solution is orange in is passed through chemical reaction produces heat energy, then the colour. potassium dichromate temperature of reaction mixture rises (or increases) and solution, the orange colour it becomes hot. In some cases, however, chemical of potassium dichromate reactions absorb heat energy. When a chemical reaction solution changes to green. Figure 7. absorbs heat energy, then the temperature of reaction mixture falls (or decreases) and it becomes cold. So, when we talk of ‘change in temperature’ in a chemical reaction, it can be ‘rise in temperature’ or ‘fall in temperature’. Another point to be noted is that the compound ‘calcium oxide’ is known by two common names ‘lime’ as well as ‘quicklime’. And the compound ‘calcium CHEMICAL REACTIONS AND EQUATIONS 5 hydroxide’ is known as ‘slaked lime’. Keeping these points in mind, we will now describe the change in temperature in chemical reactions. 4. Change in Temperature Some chemical reactions are characterised by a change in temperature. For example, when quicklime reacts with water, then slaked lime is formed and a lot of heat energy is produced. This heat raises the temperature due to which the reaction mixture becomes hot. So, we can say that the chemical reaction between quicklime and water to form slaked lime is characterised by a change in temperature (which is rise in temperature). The reaction between quicklime and water to form slaked lime is an exothermic reaction (which means heat producing reaction). We can perform this chemical reaction as follows : (i) Take a little of quicklime in a hard-glass beaker [Figure 8(a)]. (ii) Add water to it slowly [Figure 8(b)]. Water Hard-glass beaker Quicklime Slaked lime (Calcium oxide) (Calcium hydroxide) (a) (b) Figure 8. Quicklime reacts with water to form slaked lime releasing a lot of heat. The beaker becomes hot. Its temperature rises. (iii) Touch the beaker carefully. (iv) The beaker feels to be quite hot (Its temperature is high). We have already studied the chemical reaction between zinc granules and dilute sulphuric acid to produce hydrogen gas. If we touch the conical flask containing zinc granules and dilute sulphuric acid, it is found to be warm (which means that the temperature rises during this reaction). Thus, the chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is rise in temperature). The chemical reaction in which carbon burns in air to form carbon dioxide also releases a lot of heat (see Figure 9). We will now give one example of a chemical reaction in which heat Figure 9. The chemical reaction in which energy is absorbed due to which the temperature falls. When barium carbon burns in air to form carbon dioxide hydroxide [Ba(OH)2] is added to ammonium chloride (NH4Cl) taken in releases a lot of heat. a test-tube and mixed with a glass rod, then barium chloride, ammonia and water are formed. A lot of heat energy is absorbed during this reaction due to which the temperature of reaction mixture falls and the bottom of test-tube becomes very cold. Thus, the chemical reaction between barium hydroxide and ammonium chloride to form barium chloride, ammonia and water is characterised by a change in temperature (which is fall in temperature). It is an endothermic reaction (which means heat absorbing reaction).  6 SCIENCE FOR TENTH CLASS : CHEMISTRY 5. Change in State Some chemical reactions are characterised by a change in state. For example, when wax is burned (in the form of a wax candle), then water and carbon dioxide are formed (see Figure 10). Now, wax is a solid, water is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas. Thus, the combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristics. For example, the chemical reaction between zinc granules and dilute sulphuric acid shows two characteristics : evolution of a gas (hydrogen gas) and change in temperature (rise in temperature). Similarly, the chemical reaction between potassium iodide solution and lead nitrate solution shows Figure 10. The combustion two characteristics : formation of a precipitate (lead iodide precipitate) and reaction of candle wax is characterised by a change in state. change in colour (from colourless to yellow). CHEMICAL EQUATIONS The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation. Let us take one example to understand the meaning of a chemical equation clearly. Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. This reaction can be written in words as : Zinc + Sulphuric acid  Zinc sulphate + Hydrogen This is known as the word equation. We can change this word equation into a chemical equation by writing the symbols and formulae of the various substances in place of their names. Now, Symbol of zinc is Zn Formula of sulphuric acid is H2SO4 Formula of zinc sulphate is ZnSO4 and, Formula of hydrogen is H2 So, putting the symbols and formulae of all the substances in the above word- Figure 11. Zinc metal reacts with dilute sulphuric acid to equation, we get the following chemical equation : form zinc sulphate solution and hydrogen gas. The substances which combine or react are known as reactants. Zinc and sulphuric acid are the reactants here. The reactants are always written on the left hand side in an equation with a plus sign (+) between them. The new substances produced in a reaction are known as products. Zinc sulphate and hydrogen are the products in this case. The products are always written on the right hand side in an equation with a plus sign (+) between them. The arrow sign (  ) pointing towards the right hand side is put between the reactants and products. This arrow indicates that the substances written on the left hand side are combining to give the substances CHEMICAL REACTIONS AND EQUATIONS 7 written on the right hand side in the equation. It should be clear by now that a chemical equation is a short-hand method of representing a chemical reaction. Balanced and Unbalanced Chemical Equations 1. A balanced chemical equation has an equal number of atoms of different elements in the reactants and products. In other words, a balanced equation has an equal number of atoms of the different elements on both the sides. This point will become more clear from the following example. Zinc reacts with dilute sulphuric acid to give zinc sulphate and hydrogen. This can be written in equation form as : Zn + H2SO4  o ZnSO4 + H2 Let us count the number of atoms of all the elements in the reactants and products separately. In reactants In products No. of Zn atoms : 1 1 No. of H atoms : 2 2 No. of S atoms : 1 1 No. of O atoms : 4 4 We find that the reactants contain 1 zinc atom and products also contain 1 zinc atom. Reactants contain 2 hydrogen atoms and products also contain 2 hydrogen atoms. Similarly, reactants contain 1 sulphur atom and products also contain 1 sulphur atom. And finally, reactants contain 4 oxygen atoms and the products also contain 4 oxygen atoms. Thus, there is an equal number of atoms of different elements in the reactants and products, so the above chemical equation is a balanced equation. Since the number of atoms of various elements in reactants and products is equal, we can say that a balanced chemical equation has equal masses of various elements in reactants and products. 2. An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products. In other words, an unbalanced equation has an unequal number of atoms of one or more elements on its two sides. This point will become more clear from the following example. Hydrogen reacts with oxygen to form water. This reaction can be written in an equation form as : H2 + O2  o H2O Let us count the number of hydrogen atoms and oxygen atoms in the reactants as well as product : In reactants In product No. of H atoms : 2 2 No. of O atoms : 2 1 In this equation, though the number of hydrogen atoms is equal in reactants and product (2 each), but the number of oxygen atoms is unequal. There are 2 oxygen atoms on the left side but only 1 oxygen atom on the right side. The above chemical equation contains an unequal number of oxygen atoms in reactants and product, so it is an unbalanced equation. Since the number of atoms of various elements in reactants and products is unequal, we can say that an Figure 12. Launch of space unbalanced equation has unequal masses of various elements in reactants and shuttle “Discoverer”. Liquid products. hydrogen burns in liquid The equation : H2 + O2 o H2O, contains 2 oxygen atoms in the reactants but oxygen to form water, providing a tremendous only 1 oxygen atom in the product. It appears as if 1 oxygen atom has been amount of energy to lift the destroyed in this chemical reaction. This, however, cannot happen because shuttle out of the earth’s according to the law of conservation of mass, “matter can neither be created nor gravitational field.  8 SCIENCE FOR TENTH CLASS : CHEMISTRY destroyed in a chemical reaction”. This means that the total mass of all the reactants must be equal to the total mass of the products. In other words we can say that, the number of various types of atoms in reactants must be equal to the number of same type of atoms in products. It is obvious that we have to make the number of different types of atoms equal on both the sides of a chemical equation. To make the number of different types of atoms equal in reactants and products is known as balancing of an equation. It should be noted that the chemical equations are balanced to satisfy the law of conservation of mass in chemical reactions. The reaction between hydrogen and oxygen to form water cannot be written as : H2 + O  o H2O because oxygen occurs in the form of O2 molecules and not as atoms O. All the substances have definite formulae which cannot be altered. So, we should never change the formula of an element or a compound to balance an equation. We can only multiply a symbol or a formula by figures like 2, 3, 4, etc. It will be good to note here that the elements which exist as diatomic molecules are oxygen, O2, hydrogen, H2, nitrogen, N2, fluorine, F2, chlorine, Cl2, bromine, Br2 and iodine, I2. All other elements are usually considered monoatomic in equation writing and represented by their symbols. We will now learn the balancing of chemical equations. Balancing of Chemical Equations The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of equation. The simple equations are balanced by hit and trial method. We will take one example to understand the balancing of equations by hit and trial method. Hydrogen burns in oxygen to form water. This reaction can be written in an equation form as : H2 + O2  o H2O In this reaction H2 and O2 are reactants whereas H2O is the product. Let us count the number of hydrogen atoms and oxygen atoms in reactants and product. In reactants In product No. of H atoms : 2 2 No. of O atoms : 2 1 The number of hydrogen atoms is equal on both the sides (2 each), but the number of oxygen atoms is unequal. There are 2 oxygen atoms on the left side but only 1 oxygen atom on the right side. To have 2 oxygen atoms on the right side, we multiply H2O by 2 and write 2H2O, so that : H2 + O2  o 2H2O Let us count the number of various atoms on both the sides again : In reactants In product No. of H atoms : 2 4 No. of O atoms : 2 2 Though the number of oxygen atoms has become equal (2 on both sides), but the number of hydrogen atoms has now become unequal. There are 2 hydrogen atoms on the left side but 4 hydrogen atoms on the right side. To have 4 hydrogen atoms on the left side, we multiply H2 by 2 and write 2H2, so that : 2H2 + O2  o 2H2O Let us count the number of various atoms on both the sides once again. In reactants In product No. of H atoms : 4 4 No. of O atoms : 2 2 CHEMICAL REACTIONS AND EQUATIONS 9 This chemical equation contains an equal number of atoms of hydrogen and oxygen on both the sides, so this is a balanced equation. After doing some more practice, we will find that there is no need to write so many steps to balance an equation. We will then be able to balance an equation in just one step. To Make Equations More Informative The equation which gives more information about the chemical reaction is known as more informative or information giving equation. The chemical equations can be made more informative in three ways : 1. By indicating the “physical states” of the reactants and products. 2. By indicating the “heat changes” taking place in the reaction. 3. By indicating the “conditions” under which the reaction takes place. We will discuss these three points in detail one by one. 1. To Indicate the Physical States of Reactants and Products in an Equation. There can be four physical states for the reactants and products of a chemical reaction : solid, liquid, aqueous solution and gas. Solid state is indicated by the symbol (s) Liquid state is indicated by the symbol (l) Aqueous solution (solution made in water) is indicated by the symbol (aq) Gaseous state is indicated by the symbol (g) The physical states of the reactants and products are shown by putting the above “state symbols” just after their symbols or formulae in an equation. This will become more clear from the following example. Zinc metal reacts with dilute sulphuric acid to form zinc sulphate solution and hydrogen gas. This can be written as : Figure 13. This picture shows Zn + H2SO4  o ZnSO4 + H2 zinc metal in the form of zinc Here, Zinc metal is a solid, so we write Zn (s) granules. Zinc metal is a solid, so we write Zn (s) for it in a Dilute sulphuric acid is an aqueous solution, so we write H2SO4 (aq) chemical equation. Zinc sulphate is also an aqueous solution, so we write ZnSO4 (aq) And, Hydrogen is a gas which is written as H2 (g) The above equation can now be written as : Zn (s) + H2SO4 (aq)  o ZnSO4 (aq) + H2 (g) This equation is more informative because it tells us the physical states of the various substances involved in it. It tells us that zinc is in the solid state, sulphuric acid is in the form of an aqueous solution, zinc sulphate is also an aqueous solution but hydrogen is in gaseous state. In some cases an insoluble product (called precipitate) is formed by the reaction between solutions of reactants (or a solution and a gas). Since the insoluble product (or precipitate) is a solid substance, its physical state is indicated in the equation by the symbol (s). For example, when calcium hydroxide solution (lime water) reacts with carbon dioxide gas, a white precipitate of calcium carbonate is formed alongwith water. This chemical reaction can be represented by the following chemical equation with state symbols of the reactants and products : Ca(OH)2 (aq) + CO2 (g)  o CaCO3 (s) + H2O (1) Calcium hydroxide Carbon dioxide Calcium carbonate Water (Lime water) (White ppt.) In this reaction, calcium carbonate is formed as a solid product (precipitate), so its physical state is indicated by the symbol (s). Please note that the word ‘precipitate’ is written in short form as ‘ppt’. Since water is a liquid, so its physical state has been indicated by the symbol (l). 10 SCIENCE FOR TENTH CLASS : CHEMISTRY 2. To Indicate the Heat Changes in an Equation. There are two types of reactions on the basis of heat changes involved : exothermic reactions and endothermic reactions. (i) Those reactions in which heat is evolved are known as exothermic reactions. For example, when carbon burns in oxygen to form carbon dioxide, a lot of heat is produced in this reaction : C (s) + O2 (g) o CO2 (g) + Heat Carbon Oxygen Carbon dioxide The burning of carbon in oxygen is an exothermic reaction because heat is evolved in this reaction. An exothermic reaction is indicated by writing “+ Heat” or “+ Heat energy” or just “+ Energy” on the products’ side of an equation (as shown in the above equation). So, whenever we are told that a particular reaction is an exothermic reaction, we should at once write “+ Heat” or “+ Heat energy” or just “+ Energy” on the right Figure 14. Carbon (in the form of side of the equation. coal) burns to produce heat. So, the burning of carbon is an exothermic Natural gas is mainly methane (CH4). When natural gas burns in the reaction. oxygen of air, it forms carbon dioxide and water vapour. A large amount of heat energy is also produced. This can be written as : CH4 (g) + 2O2 (g) o CO2 (g) + 2H2O (g) + Heat energy Methane Oxygen Carbon dioxide Water (Natural gas) (From air) The burning of natural gas is an exothermic reaction because heat is produced in this reaction. Please note that all the combustion reactions are exothermic reactions. For example, combustion of fuels such as wood, coal, kerosene, petrol and diesel, are all exothermic reactions (because all these reactions produce heat energy). Even the combustion of food (like glucose) in our body during respiration is an exothermic reaction. This is discussed below. We need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, the foods like chapatti (roti), bread, rice and potatoes, etc., contain mainly starch carbohydrate. During digestion, starch carbohydrate is broken down into a simple carbohydrate called glucose. This glucose then undergoes slow combustion by combining with oxygen in the cells of our body to produce energy in a process called respiration. In addition to other functions, this energy maintains our body heat. During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide and water alongwith the production of energy : C6H12O6 (aq) + 6O2 (g)  o 6CO2 (g) + 6H2O (l) + Energy Glucose Oxygen Carbon dioxide Water Respiration is an exothermic process because energy is produced during this process (as shown by the above equation). The burning of a magnesium wire in air to form magnesium oxide is an exothermic reaction because heat and light energy are given out during this reaction. The decomposition of vegetable matter into compost is also an example of exothermic process (because heat energy is evolved during this process). (ii) Those reactions in which heat is absorbed are known as endothermic reactions. For example, when nitrogen and oxygen are heated to a very high temperature (of about 3000°C) they combine to form nitrogen monoxide, and a lot of heat is absorbed in this reaction : N2 (g) + O2 (g) + Heat  o 2NO (g) Nitrogen Oxygen Nitrogen monoxide The reaction between nitrogen and oxygen to form nitrogen monoxide is an endothermic reaction because heat is absorbed in this reaction. An endothermic reaction is usually indicated by writing “+ Heat” or “+ Heat energy ” or just “+ Energy” on the reactants’ side of an equation (as shown in the CHEMICAL REACTIONS AND EQUATIONS 11 above equation). The reaction in which nitrogen and oxygen (of air) combine to form nitrogen monoxide takes place inside the engines of motor vehicles. All the decomposition reactions require energy (in the form of heat, light or electricity) to take place. So, all the decomposition reactions are endothermic reactions. For example, when calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide : CaCO3 (s) + Heat  CaO (s) + CO2 (g) Calcium carbonate Calcium oxide Carbon dioxide The decomposition of calcium carbonate is an endothermic reaction because heat energy is absorbed in this reaction. Photosynthesis is an endothermic reaction. This is because sunlight energy is absorbed during the process of photosynthesis by green plants. The electrolysis of water to form hydrogen and oxygen is also an endothermic reaction. This is because electric energy is absorbed during this reaction. It is clear from this discussion that energy can be given out or absorbed in chemical reactions in the form of heat, light or electricity. Figure 15. These white cliffs are made 3. To Indicate the Conditions Under Which the Reaction Takes Place. of calcium carbonate. If heat is required for a reaction to take place, then the heat sign delta () is put over the arrow of the equation. If the reaction takes place in the presence of a catalyst, then the symbol or formula of the catalyst is also written above or below the arrow sign in the equation. This will become more clear from the following example. When potassium chlorate (KClO3) is heated in the presence of manganese dioxide catalyst, it decomposes to form potassium chloride and oxygen gas. This can be written as :  2KClO3 (s)  2KCl (s) + 3O2 (g) MnO2 Potassium chlorate Potassium Oxygen chloride Here delta () stands for heat and MnO2 is the catalyst. So, the above equation shows the conditions under which the reaction takes place. The conditions of temperature and pressure at which the reaction takes place can also be indicated in an equation by writing their values above or below the arrow sign in the equation. This will become clear from the following examples. Figure 16. This is manganese dioxide Methanol (or Methyl alcohol) is manufactured from carbon monoxide (MnO2). It is used as a catalyst during and hydrogen. The mixture of carbon monoxide and hydrogen gases is the preparation of oxygen gas from compressed to 300 atmospheres pressure and then passed over a catalyst potassium chlorate. consisting of a mixture of zinc oxide and chromium oxide heated to a temperature of 300°C. So, the conditions for this reaction to take place are : a pressure of 300 atmospheres (written as 300 atm), a temperature of 300°C, and a catalyst which is a mixture of zinc oxide and chromium oxide (ZnO + CrO3). We can now write down a chemical equation for the reaction involved in the production of methanol alongwith conditions as follows : 300 atm; 300°C CO (g) + 2H2 (g)  CH3OH (l) ZnO + CrO3 Carbon monoxide Hydrogen Methanol (Methyl alcohol) The green plants make food by photosynthesis. During photosynthesis, carbon dioxide combines with water in the presence of ‘sunlight’ and the green pigment of leaves called ‘chlorophyll’ to make food like glucose and oxygen gas is given out. The conditions for the reaction of photosynthesis to take place are the presence of sunlight and chlorophyll. So, we can write a chemical equation for photosynthesis alongwith 12 SCIENCE FOR TENTH CLASS : CHEMISTRY conditions as follows : Sunlight 6CO2 (g) + 6H2O (l)  o C6H12O6 (aq) + 6O2 (g) Chlorophyll Carbon dioxide Water Glucose Oxygen Important Examples on Writing of Balanced Chemical Equations We should remember the following four steps for writing equations for the chemical reactions : First step : Write down the chemical reaction in the form of a word equation, keeping the reactants on the left side and products on the right side. Second step : Put the symbols and formulae of all the reactants and products in the word equation. Third step : Balance the equation by multiplying the symbols and formulae by the smallest possible figures (Do not change the formulae to balance the equation). Fourth step : If possible, make the equation more informative by indicating the physical states of reactants and products ; by indicating the heat changes, if any, taking place in the reaction ; and by indicating the conditions under which the reaction takes place. If, however, you do not have sufficient information regarding the physical states ; heat changes and conditions of the reaction, this step may be avoided. Keeping these points in mind, let us solve some problems now. Sample Problem 1. Write a balanced equation for the following reaction : Methane burns in oxygen to form carbon dioxide and water. Solution. This reaction can be written in the form of a word equation as : Methane + Oxygen  o Carbon dioxide + Water Now, Formula of methane is CH4 Formula of oxygen is O2 Formula of carbon dioxide is CO2 And, Formula of water is H2O Writing the formulae of all the substances in the above word equation, we get : CH4 + O2  o CO2 + H2O Let us count the number of various atoms in reactants and products : In reactants In products No. of C atoms : 1 1 No. of H atoms : 4 2 No. of O atoms : 2 3 The number of carbon atoms is equal on both the sides (1 each) but the number of hydrogen atoms and oxygen atoms is not equal. There are 4 hydrogen atoms on the left side but only 2 hydrogen atoms on the right side. To have 4 hydrogen atoms on the right side, we multiply H2O by 2 and write 2H2O. Thus, CH4 + O2  o CO2 + 2H2O Counting the number of various atoms on both the sides again, we get : In reactants In products No. of C atoms : 1 1 No. of H atoms : 4 4 No. of O atoms : 2 4 Only the number of oxygen atoms is unequal now. There are 2 oxygen atoms on the left side but 4 on the right side. To have 4 oxygen atoms on the left side, we multiply O2 by 2 and write 2O2 : CH4 + 2O2  o CO2 + 2H2O CHEMICAL REACTIONS AND EQUATIONS 13 Let us count the number of various atoms on the two sides once again : In reactants In products No. of C atoms : 1 1 No. of H atoms : 4 4 No. of O atoms : 4 4 This chemical equation contains an equal number of various types of atoms in the reactants and products, so this is a balanced equation. Discussion. The above equation can be made more informative by indicating the physical states of the reactants and products as well as the heat changes taking place in the reaction as discussed below : Methane is a gas, so we write CH4 (g) Oxygen is a gas, so we write O2 (g) Carbon dioxide is a gas, so we write CO2 (g) What about the physical state of H2O ? If a reaction takes place in the aqueous medium, then H2O is in the liquid state and we write, H2O (l) for it. If the reaction takes place in the vapour phase, then H2O is in the gaseous state and represented as H2O (g). In this case, methane gas burns in oxygen gas to form carbon dioxide gas and water vapour or steam. So, water is in the gaseous state here and we write H2O (g). If we put the Figure 17. When methane gas burns in oxygen physical states of all the reactants and products, then the above gas (of air), it forms carbon dioxide gas and water vapour. A lot of heat is also produced. equation can be written as : CH4 (g) + 2O2 (g)  o CO2 (g) + 2H2O (g) We will now discuss the heat changes taking place in this reaction. When methane burns in oxygen to form carbon dioxide and water, a lot of heat is also produced, so this is an exothermic reaction. An exothermic reaction is indicated by writing “+ Heat” sign on the products’ side. So, the above equation can finally be written as : CH4 (g) + 2O2 (g)  o CO2 (g) + 2H2O (g) + Heat This equation now gives the physical states of the reactants and products as well as the heat changes taking place in the reaction, so this is a more informative equation. Sample Problem 2. Convey the following information in the form of a balanced chemical equation : On adding an aqueous solution of sodium hydroxide to an aqueous solution of copper sulphate, copper hydroxide is precipitated and sodium sulphate remains in solution. Solution. In this reaction copper sulphate reacts with sodium hydroxide to form copper hydroxide and sodium sulphate. This can be written in the form of a word- equation as : Figure 18. When Copper sulphate + Sodium hydroxide  o Copper hydroxide + Sodium sulphate sodium hydroxide Now, Formula of copper sulphate is CuSO4 solution is added to Formula of sodium hydroxide is NaOH copper sulphate solution, a blue Formula of copper hydroxide is Cu(OH)2 precipitate of copper And, Formula of sodium sulphate is Na2SO4 hydroxide is formed alongwith sodium sulphate solution. 14 SCIENCE FOR TENTH CLASS : CHEMISTRY Putting these formulae in the above word-equation, we get : CuSO4 + NaOH  o Cu(OH)2 + Na2SO4 Let us count the number of various types of atoms in reactants as well as products. In reactants In products No. of Cu atoms : 1 1 No. of S atoms : 1 1 No. of O atoms : 5 6 No. of Na atoms : 1 2 No. of H atoms : 1 2 We find that the number of copper atoms and sulphur atoms is equal on both the sides (1 each), but the number of oxygen atoms, sodium atoms and hydrogen atoms is not equal. Let us take the oxygen atoms first. There are 5 oxygen atoms on left side but 6 oxygen atoms on the right side. To have 6 oxygen atoms on the left side, we multiply NaOH by 2 and write 2NaOH. Thus, CuSO4 + 2NaOH  o Cu(OH)2 + Na2SO4 Let us count the number of various types of atoms on both the sides once again. In reactants In products No. of Cu atoms : 1 1 No. of S atoms : 1 1 No. of O atoms : 6 6 No. of Na atoms : 2 2 No. of H atoms : 2 2 This equation contains an equal number of various types of atoms on both the sides, so this is a balanced equation. We will now indicate the physical states of the reactants and products which have been given to us in this problem. Copper sulphate is an aqueous solution, so we write CuSO4 (aq) Sodium hydroxide is also an aqueous solution, so we write NaOH (aq) Copper hydroxide is formed as a precipitate (solid), so we write Cu(OH)2 (s) Sodium sulphate is in solution, so we write Na2SO4 (aq) The above equation can now be written as : CuSO4 (aq) + 2NaOH (aq)  o Cu(OH)2 (s) + Na2SO4 (aq) Discussion. Before we answer the next question on the balancing of equations, we should know something about the various oxides of iron metal. Iron (Fe) forms two main oxides : (i) Iron (II) oxide, FeO. This is called iron (II) oxide because the valency of iron in it is II (two). The common name of iron (two) oxide, FeO, is ferrous oxide. (ii) Iron (III) oxide, Fe2O3. This is called iron (III) oxide because the valency of iron in it is III (three). The common name of iron (three) oxide, Fe2O3, is ferric oxide. Figure 19. This is iron (II, III) oxide. A third oxide of iron is Fe3O4. Actually, Fe3O4 is a mixture of iron When heated iron metal reacts with steam, (II) oxide FeO and iron (III) oxide, Fe2O3. So, Fe3O4 is named as iron it forms iron (II, III) oxide. (II, III) oxide (Fe3O4 = FeO + Fe2O3). The common name of Fe3O4 is magnetic iron oxide. Another point to remember is that steam is the gaseous form of water, so the formula of steam is the CHEMICAL REACTIONS AND EQUATIONS 15 same as that of water, which is H2O. It will now be easy for us to understand the next question on balancing of equations. Sample Problem 3. Write a balanced chemical equation with state symbols for the following reaction : Heated iron metal reacts with steam to form iron (II, III) oxide, (Fe3O4) and hydrogen. (NCERT Book Question) Solution. This reaction can be written in the form of a word equation as : Iron + Steam  o Iron (II, III) oxide + Hydrogen Now, Symbol of iron is Fe Formula of steam is H2O (It is the same as water) Formula of iron (II, III) oxide is Fe3O4 (Given) And, Formula of hydrogen is H2 By writing the symbols and formulae of all the substances in the above word equation, we get the following skeletal chemical equation : Fe + H2O  o Fe3O4 + H2 Here Fe and H2O are reactants whereas Fe3O4 and H2 are the products. Let us count the number of atoms of various types in the reactants and products : In reactants In products No. of Fe atoms : 1 3 No. of H atoms : 2 2 No. of O atoms : 1 4 We can see that the number of iron atoms on the left side of the equation is only 1 but there are 3 iron atoms on the right side. Now, to have 3 iron atoms on the left side, we multiply Fe by 3 and write it as 3Fe. Thus, 3Fe + H2O  o Fe3O4 + H2 Let us count the number of various types of atoms on both the sides again : In reactants In products No. of Fe atoms : 3 3 No. of H atoms : 2 2 No. of O atoms : 1 4 Now the number of iron atoms is equal on both sides (3 each) and the number of hydrogen atoms is also equal (2 each). But the number of oxygen atoms is not equal. There is only 1 oxygen atom on the left side but 4 oxygen atoms on the right side. So, to have 4 oxygen atoms on the left side, we multiply H2O by 4 and write it as 4H2O. This will give us : 3Fe + 4H2O  o Fe3O4 + H2 Let us count the number of various atoms on the two sides once again : In reactants In products No. of Fe atoms : 3 3 No. of H atoms : 8 2 No. of O atoms : 4 4 The number of hydrogen atoms now becomes unequal. There are 8 hydrogen atoms on the left side but only 2 hydrogen atoms on the right side. Now, to get 8 hydrogen atoms on the right side, we multiply H2 by 4 and write it as 4H2. This gives us the following equation : 3Fe + 4H2O  o Fe3O4 + 4H2 16 SCIENCE FOR TENTH CLASS : CHEMISTRY Let us count the number of various types of atoms on both the sides of this equation : In reactants In products No. of Fe atoms : 3 3 No. of H atoms : 8 8 No. of O atoms : 4 4 The above chemical equation contains an equal number of Fe, H and O atoms in the reactants and products, so this is a balanced equation. Iron (Fe) is a solid, steam (H2O) is a gas, iron (II, III) oxide (Fe3O4) is a solid and hydrogen (H2) is a gas. So, we can write the above chemical equation with state symbols as follows : 3Fe (s) + 4H2O (g)  o Fe3O4 (s) + 4H2 (g) Sample Problem 4. Write the balanced equation for the following chemical reaction : Hydrogen + Chlorine  o Hydrogen chloride (NCERT Book Question) Solution. In this problem, hydrogen combines with chlorine to form hydrogen chloride. This has been given to us in the form of a word equation as : Hydrogen + Chlorine  o Hydrogen chloride Now, Formula of hydrogen is H2 Formula of chlorine is Cl2 And, Formula of hydrogen chloride is HCl By putting these formulae in the above word-equation, we get the following chemical equation : H2 + Cl2  o HCl Let us balance this equation now. If we look at this equation carefully, we find that there are two hydrogen atoms and two chlorine atoms on the left side but only one hydrogen atom and one chlorine atom on the right side. Now, to have two hydrogen atoms and two chlorine atoms on the right side, we have to multiply HCl by 2 and write it as 2HCl. This gives us : H2 + Cl2  o 2HCl This is a balanced equation because it contains an equal number of hydrogen atoms and chlorine atoms in the reactants and products. Hydrogen, chlorine and hydrogen chloride, are all gases, so we can write the above equation with state symbols as follows : H2 (g) + Cl2 (g)  o 2HCl (g) Sample Problem 5. Translate the following statement into chemical equation and then balance the equation : Hydrogen gas combines with nitrogen to form ammonia. (NCERT Book Question) Solution. In this reaction, hydrogen combines with nitrogen to form ammonia. This can be written as : Hydrogen + Nitrogen  o Ammonia or H2 + N2  o NH3 This equation has two H atoms on the left side but three H atoms on the right side. So, let us multiply H2 by 3 and NH3 by 2 so that each side gets 6H atoms : 3H2 + N2  o 2NH3 Now, this equation contains an equal number of hydrogen atoms and nitrogen atoms on both the sides, so this is a balanced chemical equation. CHEMICAL REACTIONS AND EQUATIONS 17 Hydrogen, nitrogen and ammonia, are all gases, so we can write the above equation with state symbols as follows : 3H2 (g) + N2 (g) o 2NH3 (g) Figure 20. This is an ammonia manufacturing plant Figure 21. This beaker has a piece of Figure 22. Sodium reacting with in which hydrogen and nitrogen are made to sodium metal (which is stored under water to form sodium hydroxide combine to form ammonia. kerosene oil). and hydrogen. The heat produced during this reaction makes the hydrogen gas burn. Sample Problem 6. Write the balanced chemical equation for the following reaction : Sodium metal reacts with water to give sodium hydroxide and hydrogen. (NCERT Book Question) Solution. Here, sodium reacts with water to form sodium hydroxide and hydrogen. This can be written as : Sodium + o Sodium hydroxide + Hydrogen Water  or Na + H2O  o NaOH + H2 This equation has two H atoms on the left side but three H atoms on the right side. So, let us multiply H2O by 2 and NaOH also by 2 so as to have an equal number of H atoms (4 each) on both the sides : Na + 2H2O  o 2NaOH + H2 Now we have only one Na atom on left side but two Na atoms on the right side. So, let us take 2Na atoms on the left side. This gives us : 2Na + 2H2O  o 2NaOH + H2 This equation contains an equal number of sodium, hydrogen and oxygen atoms on both the sides, so this is a balanced chemical equation. Sodium is a solid (s), water is a liquid (l), sodium hydroxide is an aqueous solution (aq) whereas hydrogen is a gas (g). So, we can write the above chemical equation with state symbols as follows : 2Na (s) + 2H2O (l)  o 2NaOH (aq) + H2 (g) Sample Problem 7. Write a balanced chemical equation for the following chemical reaction : Magnesium burns in oxygen to form magnesium oxide. (NCERT Book Question) Solution. Magnesium burns in oxygen (of air) to form magnesium oxide. This reaction can be written in (a) This is magnesium metal (b) This is magnesium oxide the form of a word equation as : Figure 23. When magnesium burns in oxygen (of air), then magnesium oxide is formed. 18 SCIENCE FOR TENTH CLASS : CHEMISTRY Magnesium + Oxygen  o Magnesium oxide Now, Symbol of magnesium is Mg Formula of oxygen is O2 And, Formula of magnesium oxide is MgO By putting these symbols and formulae in the above word equation, we get the following chemical equation : Mg + O2  o MgO Let us balance this equation now. We can see from the above equation that there are 2 oxygen atoms on left side but only 1 oxygen atom on the right side. So, to have 2 oxygen atoms on the right side, we write 2MgO. Thus, Mg + O2  o 2MgO Now we have 1 magnesium atom on left side but 2 magnesium atoms on the right side. To have 2 magnesium atoms on the left side, we write 2Mg. This gives us : 2Mg + O2  o 2MgO This equation contains an equal number of Mg atoms and O atoms on both the sides, so this is a balanced equation. Magnesium is a solid, oxygen is a gas and magnesium oxide is also a solid. So, we can write the above chemical equation with state symbols as follows : 2Mg (s) + O2 (g)  o 2MgO (s) Before we go further and study the types of chemical reactions, please answer the following questions : Very Short Answer Type Questions 1. Why is respiration considered an exothermic process ? 2. On what basis is a chemical equation balanced ? 3. What happens chemically when quicklime is added to water filled in a bucket ? 4. Why should magnesium ribbon be cleaned before burning in air ? 5. State whether the following statement is true or false : A chemical equation can be balanced easily by altering the formula of a reactant or product. 6. What is wrong with the following chemical equation ? Mg + O  o MgO Correct and balance it. 7. What does the symbol (aq) represent in a chemical equation ? 8. Why is photosynthesis considered an endothermic reaction ? 9. How will you indicate the following effects in a chemical equation ? (a) A solution made in water (b) Exothermic reaction (c) Endothermic reaction 10. Translate the following statements into chemical equations and then balance the equations : (a) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (b) Phosphorus burns in oxygen to give phosphorus pentoxide. (c) Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide. (d) Aluminium metal replaces iron from ferric oxide, Fe2O3, giving aluminium oxide and iron. (e) Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate. 11. Write the balanced chemical equations for the following reactions : (a) Calcium hydroxide + Carbon dioxide  o Calcium carbonate + Water (b) Aluminium + Copper chloride  o Aluminium chloride + Copper CHEMICAL REACTIONS AND EQUATIONS 19 12. Complete and balance the following equations : (a) NaOH +.............  o Na2SO4 + H2O (b) Ca(OH)2 +...........  o CaCO3 + H2O 13. Correct and balance the following equations : (i) Ca + H2O  o CaOH + H (ii) N + H  o NH3 14. Write complete balanced equations for the following reactions : (a) Calcium (solid) + Water (liquid) o Calcium hydroxide (solution) + Hydrogen (gas) (b) Sulphur dioxide (gas) + Oxygen (gas)  o Sulphur trioxide (gas) 15. Balance the following equations : (i) Na + O2  o Na2O (ii) H2O2  o H2O + O2 (iii) Mg(OH)2 + HCl  o MgCl2 + H2O (iv) Fe + O2  o Fe2O3 (v) Al(OH)3 o Al2O3 + H2O (vi) NH3 + CuO  o Cu + N2 + H2O (vii) Al2(SO4)3 + NaOH  o Al(OH)3 + Na2SO4 (viii) HNO3 + Ca(OH)2  o Ca(NO3)2 + H2O (ix) NaOH + H2SO4  o Na2SO4 + H2O (x) BaCl2 + H2SO4  o BaSO4 + HCl 16. Fill in the following blanks with suitable words : (a) Chemical equations are balanced to satisfy the law of........ (b) A solution made in water is known as an............ solution and indicated by the symbol.............. Short Answer Type Questions 17. (a) Give one example of a chemical reaction. (b) State two characteristics of the chemical reaction which takes place when dilute sulphuric acid is poured over zinc granules. (c) Give two characteristics of the chemical reaction which occurs on adding potassium iodide solution to lead nitrate solution. 18. (a) What is a chemical equation ? Explain with the help of an example. (b) Giving examples, state the difference between balanced and unbalanced chemical equations. (c) Balance the following chemical equations : (i) NH3  o N2 + H2 (ii) C + CO2  o CO 19. When hydrogen is passed over copper oxide, copper and steam are formed. Write a balanced equation for this reaction and state which of the chemicals are : (i) elements (ii) compounds (iii) reactants (iv) products (v) metals (vi) non-metals 20. (a) What are the various ways in which a chemical equation can be made more informative ? Give examples to illustrate your answer. (b) Write balanced chemical equation from the following information : An aqueous calcium hydroxide solution (lime water) reacts with carbon dioxide gas to produce a solid calcium carbonate precipitate and water. 21. (a) What is a balanced chemical equation ? Why should chemical equations be balanced ? (b) Aluminium burns in chlorine to form aluminium chloride (AlCl3). Write a balanced chemical equation for this reaction. 20 SCIENCE FOR TENTH CLASS : CHEMISTRY (c) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. Write a balanced chemical equation for this reaction. 22. (a) Explain, with example, how the physical states of the reactants and products can be shown in a chemical equation. (b) Balance the following equation and add state symbols : Zn + HCl  o ZnCl2 + H2 (c) Convey the following information in the form of a balanced chemical equation : “An aqueous solution of ferrous sulphate reacts with an aqueous solution of sodium hydroxide to form a precipitate of ferrous hydroxide and sodium sulphate remains in solution.” 23. Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer. 24. (a) Aluminium hydroxide reacts with sulphuric acid to form aluminium sulphate and water. Write a balanced equation for this reaction. (b) Balance the following chemical equation : MnO2 + HCl  o MnCl2 + Cl2 + H2O 25. Write the balanced equations for the following reactions, and add the state symbols : (a) Magnesium carbonate reacts with hydrochloric acid to produce magnesium chloride, carbon dioxide and water. (b) Sodium hydroxide reacts with sulphuric acid to produce sodium sulphate and water. 26. Carbon monoxide reacts with hydrogen under certain conditions to form methanol (CH3OH). Write a balanced chemical equation for this reaction indicating the physical states of reactants and product as well as the conditions under which this reaction takes place. 27. (a) Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Write a balanced equation for this reaction and indicate the evolution of gas. (b) Rewrite the following information in the form of a balanced chemical equation : Magnesium burns in carbon dioxide to form magnesium oxide and carbon. 28. (a) Substitute formulae for names and balance the following equation : Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water and carbon dioxide gas. (b) Write balanced chemical equation with state symbols for the following reaction : Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water. 29. Ammonia reacts with oxygen to form nitrogen and water. Write a balanced chemical equation for this reaction. Add the state symbols for all the reactants and products. 30. Write a balanced chemical equation for the process of photosynthesis giving the physical states of all the substances involved and the conditions of the reaction. 31. Translate the following statement into chemical equation and then balance it : Barium chloride solution reacts with aluminium sulphate solution to form a precipitate of barium sulphate and aluminium chloride solution. 32. When potassium nitrate is heated, it decomposes into potassium nitrite and oxygen. Write a balanced equation for this reaction and add the state symbols of the reactants and products. Long Answer Type Questions 33. (a) What is meant by a chemical reaction ? Explain with the help of an example. (b) Give one example each of a chemical reaction characterised by : (i) evolution of a gas (ii) change in colour (iii) formation of a precipitate (iv) change in temperature (v) change in state. CHEMICAL REACTIONS AND EQUATIONS 21 34. (a) State the various characteristics of chemical reactions. (b) State one characteristic each of the chemical reaction which takes place when : (i) dilute hydrochloric acid is added to sodium carbonate (ii) lemon juice is added gradually to potassium permanganate solution (iii) dilute sulphuric acid is added to barium chloride solution (iv) quicklime is treated with water (v) wax is burned in the form of a candle 35. (a) What do you understand by exothermic and endothermic reactions ? (b) Give one example of an exothermic reaction and one of an endothermic reaction. (c) Which of the following are endothermic reactions and which are exothermic reactions ? (i) Burning of natural gas (ii) Photosynthesis (iii) Electrolysis of water (iv) Respiration (v) Decomposition of calcium carbonate Multiple Choice Questions (MCQs) 36. One of the following does not happen during a chemical reaction. This is : (a) Breaking of old chemical bonds and formation of new chemical bonds (b) Formation of new substances with entirely different properties (c) Atoms of one element change into those of another element to form new products. (d) A rearrangement of atoms takes place to form new products.