Science Textbook for Class 9 PDF

SCIENCE TEXTBOOK FOR CLASS IX 2024-25 0964 – SCIENCE ISBN 81-7450-492-3 Textbook for Class IX First Edition February 2006 Phalguna 1927 ALL RIGHTS RESERVED Reprinte...

SCIENCE TEXTBOOK FOR CLASS IX 2024-25 0964 – SCIENCE ISBN 81-7450-492-3 Textbook for Class IX First Edition February 2006 Phalguna 1927 ALL RIGHTS RESERVED Reprinted q No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, November 2006, November 2007, recording or otherwise without the prior permission of the publisher. January 2009, December 2009, q This book is sold subject to the condition that it shall not, by way of trade, be lent, re- November 2010, December 2011, sold, hired out or otherwise disposed of without the publisher’s consent, in any form of binding or cover other than that in which it is published. October 2012, October 2013, q The correct price of this publication is the price printed on this page, Any revised December 2014, December 2015, price indicated by a rubber stamp or by a sticker or by any other means is incorrect February 2017, December 2017, and should be unacceptable. December 2018, October 2019, January 2021 and November 2021 OFFICES OF THE PUBLICATION DIVISION, NCERT Revised Edition NCERT Campus October 2022 Ashwina 1944 Sri Aurobindo Marg New Delhi 110 016 Phone : 011-26562708 Reprinted 108, 100 Feet Road March 2024 Chaitra 1946 Hosdakere Halli Extension Banashankari III Stage Bengaluru 560 085 Phone : 080-26725740 PD 520T SU Navjivan Trust Building P.O.Navjivan © National Council of Educational Ahmedabad 380 014 Phone : 079-27541446 Research and Training, 2006, 2022 CWC Campus Opp. Dhankal Bus Stop Panihati Kolkata 700 114 Phone : 033-25530454 CWC Complex Maligaon Guwahati 781 021 Phone : 0361-2674869 Publication Team Head, Publication : Anup Kumar Rajput Division ` 155.00 Chief Editor : Shveta Uppal Chief Production : Arun Chitkara Officer Chief Business : Amitabh Kumar Manager (In charge) Printed on 80 GSM paper with NCERT Production Assistant : Prakash Veer Singh watermark Published at the Publication Division Cover by the Secretary, National Council of Educational Research and Training, Nidhi Wadhwa Sri Aurobindo Marg, New Delhi 110 016 Layout and Illustrations and printed at SDA Print ‘N’ Pack, D-19, Digital Expressions Sector-B1, Tronica City Industrial Area, Loni, District Ghaziabad- 201 102 (U.P.) 2024-25  FOREWORD The National Curriculum Framework (NCF) 2005, recommends that children’s life at school must be linked to their life outside the school. This principle marks a departure from the legacy of bookish learning which continues to shape our system and causes a gap between the school, home and community. The syllabi and textbooks developed on the basis of NCF signify an attempt to implement this basic idea. They also attempt to discourage rote learning and the maintenance of sharp boundaries between different subject areas. We hope these measures will take us significantly further in the direction of a child- centred system of education outlined in the National Policy on Education (1986). The success of this effort depends on the steps that school principals and teachers will take to encourage children to reflect on their own learning and to pursue imaginative activities and questions. We must recognise that, given space, time and freedom, children generate new knowledge by engaging with the information passed on to them by adults. Treating the prescribed textbook as the sole basis of examination is one of the key reasons why other resources and sites of learning are ignored. Inculcating creativity and initiative is possible if we perceive and treat children as participants in learning, not as receivers of a fixed body of knowledge. These aims imply considerable change in school routines and mode of functioning. Flexibility in the daily time-table is as necessary as rigour in implementing the annual calendar so that the required number of teaching days are actually devoted to teaching. The methods used for teaching and evaluation will also determine how effective this textbook proves for making children’s life at school a happy experience, rather than a source of stress or boredom. Syllabus designers have tried to address the problem of curricular burden by restructuring and reorienting knowledge at different stages with greater consideration for child psychology and the time available for teaching. The textbook attempts to enhance this endeavour by giving higher priority and 2024-25 space to opportunities for contemplation and wondering, discussion in small groups, and activities requiring hands-on experience. The National Council of Educational Research and Training (NCER T) appreciates the hard work done by the textbook development team responsible for this book. We wish to thank the Chairman of the advisory group in science and mathematics, Professor J.V. Narlikar and the Chief Advisor for this book, Professor Rupamanjari Ghosh, School of Physical Sciences, Jawaharlal Nehru University, New Delhi, for guiding the work of this committee. Several teachers contributed to the development of this textbook; we are grateful to them and their principals for making this possible. We are indebted to the institutions and organisations which have generously permitted us to draw upon their resources, material and personnel. We are especially grateful to the members of the National Monitoring Committee, appointed by the Department of Secondary and Higher Education, Ministry of Human Resource Development under the Chairmanship of Professor Mrinal Miri and Professor G.P. Deshpande, for their valuable time and contribution. As an organisation committed to systemic reform and continuous improvement in the quality of its products, NCERT welcomes comments and suggestions which will enable us to undertake further revision and refinement. Director New Delhi National Council of Educational 20 December 2005 Research and Training (iv) 2024-25 RATIONALISATION OF CONTENT IN THE TEXTBOOK In view of the COVID-19 pandemic, it is imperative to reduce content load on students. The National Education Policy 2020, also emphasises reducing the content load and providing opportunities for experiential learning with creative mindset. In this backdrop, the NCERT has undertaken the exercise to rationalise the textbooks across all classes. Learning Outcomes already developed by the NCERT across classes have been taken into consideration in this exercise. Contents of the textbooks have been rationalised in view of the following: Overlapping with similar content included in other subject areas in the same class Similar content included in the lower or higher class in the same subject Difficulty level Content, which is easily accessible to students without much interventions from teachers and can be learned by children through self-learning or peer-learning Content, which is irrelevant in the present context This present edition, is a reformatted version after carrying out the changes given above. 2024-25 2024-25  TEXTBOOK DEVELOPMENT COMMITTEE CHAIRMAN, ADVISORY GROUP FOR TEXTBOOKS IN SCIENCE AND MATHEMATICS J.V. Narlikar, Emeritus Professor, Chairman, Advisory Committee Inter Uni- versity Centre for Astronomy & Astrophysics (IUCCA), Ganeshbhind, Pune University, Pune CHIEF ADVISOR Rupamanjari Ghosh, Professor, School of Physical Sciences, Jawaharlal Nehru University, New Delhi MEMBERS Anjni Koul, Lecturer, Department of Education in Science and Mathematics (DESM), NCERT, New Delhi Anupam Pachauri, 1317, Sector 37, Faridabad, Haryana Anuradha Gulati, TGT, CRPF Public School, Rohini, Delhi Asfa M. Yasin, Reader, Pandit Sunderlal Sharma Central Institute of Vocational Education, NCERT, Bhopal Charu Maini, PGT, DAV School, Sector 14, Gurgaon, Haryana Dinesh Kumar, Reader, DESM, NCERT, New Delhi Gagan Gupta, Reader, DESM, NCERT, New Delhi H.L. Satheesh, TGT , DM School, Regional Institute of Education, Mysore Madhuri Mahapatra, Reader, Regional Institute of Education, Bhubaneswar, Orissa Puran Chand, Joint Director, Central Institute of Educational Technology, NCERT, New Delhi S.C. Jain, Professor, DESM, NCERT, New Delhi Sujatha G.D., Assistant Mistress, V.V.S. Sardar Patel High School, Rajaji Nagar, Bangalore S.K. Dash, Reader, DESM, NCERT, New Delhi Seshu Lavania, Reader, Department of Botany, University of Lucknow, Lucknow Satyajit Rath, Scientist, National Institute of Immunology, JNU Campus, New Delhi Sukhvir Singh, Reader, DESM, Regional Institute of Education, Ajmer, Rajasthan Uma Sudhir, Eklavya, Indore MEMBER-COORDINATOR Brahm Parkash, Professor, DESM, NCERT, New Delhi 2024-25  A CKNOWLEDGEMENTS The National Council of Educational Research and Training is grateful to the members of the Textbook Development Team, whose names are given separately, for their contribution in the development of the Science textbook for Class IX. The Council also gratefully acknowledges the contribution of the participating members of the Review Workshop in the finalisation of the book: P.K. Bhattacharya, Professor, DESM, NCERT; Anita Julka, Reader, DEGSN, NCERT; Tausif Ahmad, PGT, New Era Sr. Sec. School, New Delhi; Samarketu, PGT in Physics, JNV, MESRA, Ranchi; Meenakshi Sharma, PGT in Biology, SVEM, Ankleshwar, Gujarat; Raji Kamlasanan, PGT in Biology, DTEA SNSU School, R.K. Puram, New Delhi; Meenambika Menon, TGT in Science, Cambridge School, Noida; Lalit Gupta, TGT in Science, Govt. Boys Sr. Sec. School No. 2, Uttam Nagar, New Delhi; Manoj Kumar Gupta, Lecturer in Chemistry, Mukherji Memorial Sr. Sec. School, Shahdara, Delhi; Vijay Kumar, Vice-Principal, Govt. Sarvodaya, Co. Edu. Sr. Sec. School, Anand Vihar, Delhi; Kanhaya Lal, Principal (Retd.), Deptt. of Education, GNCT of Delhi, Delhi; K.B. Gupta, Professor (Retd.), NCERT, New Delhi; Kuldeep Singh, TGT in Science, JNV, Meerut; R.A. Goel, Principal (Retd.), Delhi; Sumit Kumar Bhatnagar, Department of Education, GNCT of Delhi, Delhi. Acknowledgements are due to M. Chandra, Professor and Head, Department of Education in Science and Mathematics, NCERT, New Delhi for providing all academic and administrative support. The Council also gratefully acknowledges the support provided by the APC Office of DESM, administrative staff of DESM; Deepak Kapoor, Incharge Computer Centre, DESM; Saima, DTP Operator; Mohd. Qamar Tabrez, Copy Editor; Mathew John and Randhir Thakur, Proofreaders. The efforts of the Publication Department, NCERT are also highly appreciated. 2024-25  CONTENTS FOREWORD iii RATIONALISATION OF CONTENT IN THE TEXTBOOK V Chapter 1 MATTER IN OUR SURROUNDINGS 1 Chapter 2 IS MATTER AROUND US PURE? 14 Chapter 3 ATOMS AND MOLECULES 26 Chapter 4 STRUCTURE OF THE ATOM 38 Chapter 5 THE FUNDAMENTAL UNIT OF LIFE 49 Chapter 6 TISSUES 60 Chapter 7 MOTION 72 Chapter 8 FORCE AND LAWS OF MOTION 87 Chapter 9 GRAVITATION 100 Chapter 10 WORK AND ENERGY 113 Chapter 11 SOUND 127 Chapter 12 IMPROVEMENT IN FOOD RESOURCES 140 ANSWERS 152– 154 2024-25  THE CONSTITUTION OF INDIA PREAMBLE WE, THE PEOPLE OF INDIA, having solemnly resolved to constitute India into a 1 [SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC] and to secure to all its citizens : JUSTICE, social, economic and political; LIBERTY of thought, expression, belief, faith and worship; EQUALITY of status and of opportunity; and to promote among them all FRATERNITY assuring the dignity of the individual and the 2[unity and integrity of the Nation]; IN OUR CONSTITUENT ASSEMBLY this twenty-sixth day of November, 1949 do HEREBY ADOPT, ENACT AND GIVE TO OURSELVES THIS CONSTITUTION. 1. Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2, for "Sovereign Democratic Republic" (w.e.f. 3.1.1977) 2. Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2, for "Unity of the Nation" (w.e.f. 3.1.1977) 2024-25 C hapter 1 MATTER IN OUR SURROUNDINGS As we look at our surroundings, we see a large Activity ______________ 1.1 variety of things with different shapes, sizes and textures. Everything in this universe is Take a 100 mL beaker. Fill half the beaker with water and made up of material which scientists have mark the level of water. named “matter”. The air we breathe, the food Dissolve some salt/ sugar with the help we eat, stones, clouds, stars, plants and of a glass rod. animals, even a small drop of water or a Observe any change in water level. particle of sand — every thing is matter. We What do you think has happened to can also see as we look around that all the the salt? things mentioned above occupy space and Where does it disappear? have mass. In other words, they have both Does the level of water change? mass* and volume**. In order to answer these questions we Since early times, human beings have need to use the idea that matter is made up been trying to understand their surroundings. of particles. What was there in the spoon, salt Early Indian philosophers classified matter in or sugar, has now spread throughout water. the form of five basic elements — the “Panch This is illustrated in Fig. 1.1. Tatva” — air, earth, fire, sky and water. According to them everything, living or non- living, was made up of these five basic elements. Ancient Greek philosophers had arrived at a similar classification of matter. Modern day scientists have evolved two types of classification of matter based on their physical properties and chemical nature. In this chapter we shall learn about matter based on its physical properties. Chemical aspects of matter will be taken up in subsequent chapters. Fig. 1.1: When we dissolve salt in water, the particles of salt get into the spaces between particles 1.1 Physical Nature of Matter of water. 1.1.1 MATTER IS MADE UP OF PARTICLES 1.1.2 HOW SMALL ARE THESE PARTICLES For a long time, two schools of thought prevailed regarding the nature of matter. One school OF MATTER? believed matter to be continuous like a block of wood, whereas, the other thought that matter Activity ______________ 1.2 was made up of particles like sand. Let us Take 2–3 crystals of potassium perform an activity to decide about the nature permanganate and dissolve them in of matter — is it continuous or particulate? 100 mL of water. * The SI unit of mass is kilogram (kg). ** The SI unit of volume is cubic metre (m3). The common unit of measuring volume is litre (L) such that 1L = 1 dm3, 1L = 1000 mL, 1 mL = 1 cm3. 2024-25  Take out approximately 10 mL of this 1.2.2 P ARTICLES OF MATTER ARE solution and put it into 90 mL of clear water. CONTINUOUSLY MOVING Take out 10 mL of this solution and put it into another 90 mL of clear water. Keep diluting the solution like this 5 to Activity 1.3 8 times. Is the water still coloured ? Put an unlit incense stick in a corner of your class. How close do you have to go near it so as to get its smell? Now light the incense stick. What happens? Do you get the smell sitting at a distance? Record your observations. Activity 1.4 Take two glasses/beakers filled with Fig. 1.2: Estimating how small are the particles of water. matter. With every dilution, though the colour Put a drop of blue or red ink slowly becomes light, it is still visible. and carefully along the sides of the first beaker and honey in the same way in This experiment shows that just a few the second beaker. crystals of potassium permanganate can Leave them undisturbed in your house colour a large volume of water (about or in a corner of the class. 1000 L). So we conclude that there must be Record your observations. millions of tiny particles in just one crystal What do you observe immediately after of potassium permanganate, which keep adding the ink drop? on dividing themselves into smaller and What do you observe immediately after smaller particles. adding a drop of honey? The same activity can be done using 2 mL How many hours or days does it take of Dettol instead of potassium permanganate. for the colour of ink to spread evenly The smell can be detected even on throughout the water? repeated dilution. The particles of matter are very small – Activity 1.5 they are small beyond our imagination!!!! Drop a crystal of copper sulphate or 1.2 Characteristics of Particles of potassium permanganate into a glass of hot water and another containing Matter cold water. Do not stir the solution. Allow the crystals to settle at the 1.2.1 PARTICLES OF MATTER HAVE SPACE bottom. BETWEEN THEM What do you observe just above the solid crystal in the glass? In activities 1.1 and 1.2 we saw that particles What happens as time passes? of sugar, salt, Dettol, or potassium What does this suggest about the permanganate got evenly distributed in water. particles of solid and liquid? Similarly, when we make tea, coffee or Does the rate of mixing change with lemonade (nimbu paani ), particles of one type temperature? Why and how? of matter get into the spaces between particles From the above three activities (1.3, 1.4 and of the other. This shows that there is enough 1.5), we can conclude the following: space between particles of matter. 2 SCIENCE 2024-25  Particles of matter are continuously If we consider each student as a moving, that is, they possess what we call particle of matter, then in which group the kinetic energy. As the temperature rises, the particles held each other with the maximum force? particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases. Activity ______________ 1.7 In the above three activities we observe Take an iron nail, a piece of chalk and that particles of matter intermix on their own a rubber band. with each other. They do so by getting into Try breaking them by hammering, the spaces between the particles. This cutting or stretching. intermixing of particles of two different types In which of the above three of matter on their own is called diffusion. We substances do you think the particles also observe that on heating, diffusion are held together with greater force? becomes faster. Why does this happen? Activity ______________ 1.8 1.2.3 PARTICLES OF MATTER ATTRACT Take some water in a container, try EACH OTHER cutting the surface of water with your fingers. Activity ______________ 1.6 Were you able to cut the surface of water? Play this game in the field— make four What could be the reason behind the groups and form human chains as surface of water remaining together? suggested: The first group should hold each The above three activities (1.6, 1.7 and 1.8) other from the back and lock arms suggest that particles of matter have force like Idu-Mishmi dancers (Fig. 1.3). acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another. Q uestions 1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume. 2. Give reasons for the following Fig. 1.3 observation: The second group should hold hands The smell of hot sizzling food to form a human chain. reaches you several metres The third group should form a chain away, but to get the smell from by touching each other with only their cold food you have to go close. finger tips. 3. A diver is able to cut through Now, the fourth group of students water in a swimming pool. Which should run around and try to break the property of matter does this three human chains one by one into observation show? as many small groups as possible. Which group was the easiest to break? 4. What are the characteristics of Why? the particles of matter? M AT T E R IN O U R S U R R O U N D I N GS 3 2024-25 1.3 States of Matter the force is removed. If excessive force is applied, it breaks. Observe different types of matter around you. The shape of each individual sugar or What are its different states? We can see that salt crystal remains fixed, whether we matter around us exists in three different take it in our hand, put it in a plate or in states– solid, liquid and gas. These states of a jar. matter arise due to the variation in the A sponge has minute holes, in which characteristics of the particles of matter. air is trapped, when we press it, the air Now, let us study about the properties of is expelled out and we are able to these three states of matter in detail. compress it. 1.3.1 THE SOLID STATE 1.3.2 THE LIQUID STATE Activity _____________ 1.9 Activity _____________ 1.10 Collect the following articles — a pen, Collect the following: a book, a needle and a piece of wooden (a) water, cooking oil, milk, juice, a stick. cold drink. Sketch the shape of the above articles (b) containers of different shapes. Put in your notebook by moving a pencil a 50 mL mark on these containers around them. using a measuring cylinder from Do all these have a definite shape, the laboratory. distinct boundaries and a fixed volume? What will happen if these liquids are What happens if they are hammered, spilt on the floor? pulled or dropped? Measure 50 mL of any one liquid and Are these capable of diffusing into each transfer it into different containers one other? by one. Does the volume remain the same? Try compressing them by applying Does the shape of the liquid remain the force. Are you able to compress them? same ? All the above are examples of solids. We When you pour the liquid from one can observe that all these have a definite container into another, does it flow shape, distinct boundaries and fixed volumes, easily? that is, have negligible compressibility. Solids We observe that liquids have no fixed have a tendency to maintain their shape when shape but have a fixed volume. They take up subjected to outside force. Solids may break the shape of the container in which they are under force but it is difficult to change their kept. Liquids flow and change shape, so they shape, so they are rigid. are not rigid but can be called fluid. Refer to activities 1.4 and 1.5 where we Consider the following: saw that solids and liquids can diffuse into (a) What about a rubber band, can it liquids. The gases from the atmosphere change its shape on stretching? Is it diffuse and dissolve in water. These gases, a solid? especially oxygen and carbon dioxide, are (b) What about sugar and salt? When essential for the survival of aquatic animals kept in different jars these take the and plants. shape of the jar. Are they solid? All living creatures need to breathe for (c) What about a sponge? It is a solid survival. The aquatic animals can breathe yet we are able to compress it. Why? under water due to the presence of dissolved All the above are solids as: oxygen in water. Thus, we may conclude that A rubber band changes shape under solids, liquids and gases can diffuse into force and regains the same shape when liquids. The rate of diffusion of liquids is 4 SCIENCE 2024-25 higher than that of solids. This is due to the We have observed that gases are highly fact that in the liquid state, particles move compressible as compared to solids and freely and have greater space between each liquids. The liquefied petroleum gas (LPG) other as compared to particles in the solid cylinder that we get in our home for cooking state. or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed 1.3.3 THE GASEOUS STATE natural gas (CNG) is used as fuel these days in vehicles. Due to its high compressibility, Have you ever observed a balloon seller filling large volumes of a gas can be compressed a large number of balloons from a single into a small cylinder and transported easily. cylinder of gas? Enquire from him how many We come to know of what is being cooked balloons is he able to fill from one cylinder. in the kitchen without even entering there, Ask him which gas does he have in the cylinder. by the smell that reaches our nostrils. How does this smell reach us? The particles of the Activity _____________ 1.11 aroma of food mix with the particles of air spread from the kitchen, reach us and even Take three 100 mL syringes and close farther away. The smell of hot cooked food their nozzles by rubber corks, as shown in Fig.1.4. reaches us in seconds; compare this with the Remove the pistons from all the rate of diffusion of solids and liquids. Due to syringes. high speed of particles and large space Leaving one syringe untouched, fill between them, gases show the property of water in the second and pieces of chalk diffusing very fast into other gases. in the third. In the gaseous state, the particles move Insert the pistons back into the about randomly at high speed. Due to this syringes. You may apply some vaseline random movement, the particles hit each on the pistons before inserting them other and also the walls of the container. The into the syringes for their smooth pressure exerted by the gas is because of this movement. force exerted by gas particles per unit area Now, try to compress the content by pushing the piston in each syringe. on the walls of the container. Fig. 1.4 What do you observe? In which case Fig.1.5: a, b and c show the magnified schematic was the piston easily pushed in? pictures of the three states of matter. The What do you infer from your motion of the particles can be seen and observations? compared in the three states of matter. M AT T E R IN O U R S U R R O U N D I N GS 5 2024-25  uestions 1.4.1 EFFECT OF CHANGE OF TEMPERATURE Q 1. The mass per unit volume of a substance is called density. (density = mass/volume). Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron. 2. (a) Tabulate the differences in the characterisitcs of states Activity _____________ 1.12 Take about 150 g of ice in a beaker and suspend a laboratory thermometer so that its bulb is in contact with the ice, as in Fig. 1.6. of matter. (b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density. 3. Give reasons (a) A gas fills completely the vessel in which it is kept. (b) A gas exerts pressure on the walls of the container. (c) A wooden table should be called a solid. (d) We can easily move our hand (a) in air but to do the same through a solid block of wood we need a karate expert. 4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why. 1.4 Can Matter Change its State? We all know from our observation that water can exist in three states of matter– solid, as ice, liquid, as the familiar water, and gas, as water vapour. What happens inside the matter during this change of state? What happens to the (b) particles of matter during the change of states? How does this change of state take place? We need answers to these questions, Fig. 1.6: (a) Conversion of ice to water, (b) conversion isn’t it? of water to water vapour 6 SCIENCE 2024-25  Start heating the beaker on a low flame. state by overcoming the forces of attraction Note the temperature when the ice between the particles. As this heat energy is starts melting. absorbed by ice without showing any rise in Note the temperature when all the ice temperature, it is considered that it gets has converted into water. hidden into the contents of the beaker and is Record your observations for this known as the latent heat. The word latent conversion of solid to liquid state. means hidden. The amount of heat energy Now, put a glass rod in the beaker and that is required to change 1 kg of a solid into heat while stirring till the water starts liquid at atmospheric pressure at its melting boiling. point is known as the latent heat of fusion. Keep a careful eye on the thermometer So, particles in water at 0oC (273 K) have more reading till most of the water has vaporised. energy as compared to particles in ice at the Record your observations for the same temperature. conversion of water in the liquid state When we supply heat energy to water, to the gaseous state. particles start moving even faster. At a certain temperature, a point is reached when the On increasing the temperature of solids, particles have enough energy to break free the kinetic energy of the particles increases. from the forces of attraction of each other. At Due to the increase in kinetic energy, the this temperature the liquid starts changing particles start vibrating with greater speed. into gas. The temperature at which a liquid The energy supplied by heat overcomes the starts boiling at the atmospheric pressure is forces of attraction between the particles. The known as its boiling point. Boiling is a bulk particles leave their fixed positions and start phenomenon. Particles from the bulk of the moving more freely. A stage is reached when liquid gain enough energy to change into the the solid melts and is converted to a liquid. vapour state. The minimum temperature at which a solid For water this temperature is 373 K melts to become a liquid at the atmospheric (100oC = 273 + 100 = 373 K). pressure is called its melting point. Can you define the latent heat of vaporisation? Do it in the same way as we The melting point of a solid is an indication have defined the latent heat of fusion. of the strength of the force of attraction Particles in steam, that is, water vapour at between its particles. 373 K (1000 C) have more energy than water at the same temperature. This is because The melting point of ice is 273.15 K*. The particles in steam have absorbed extra energy process of melting, that is, change of solid in the form of latent heat of vaporisation. state into liquid state is also known as fusion. When a solid melts, its temperature remains the same, so where does the heat energy go? You must have observed, during the experiment of melting, that the temperature So, we infer that the state of matter can of the system does not change after the be changed into another state by changing melting point is reached, till all the ice melts. the temperature. This happens even though we continue to We have learnt that substances around heat the beaker, that is, we continue to supply us change state from solid to liquid and from heat. This heat gets used up in changing the liquid to gas on application of heat. But there *Note: Kelvin is the SI unit of temperature, 0o C =273.15 K. For convenience, we take 0o C = 273 K after rounding off the decimal. To change a temperature on the Kelvin scale to the Celsius scale you have to subtract 273 from the given temperature, and to convert a temperature on the Celsius scale to the Kelvin scale you have to add 273 to the given temperature. M AT T E R IN O U R S U R R O U N D I N GS 7 2024-25 are some that change directly from solid state enclosed in a cylinder? Will the particles come to gaseous state and vice versa without closer? Do you think that increasing or changing into the liquid state. decreasing the pressure can change the state of matter? Activity _____________ 1.13 Take some camphor. Crush it and put it in a china dish. Put an inverted funnel over the china dish. Put a cotton plug on the stem of the funnel, as shown in Fig. 1.7. Fig. 1.8: By applying pressure, particles of matter can be brought close together Applying pressure and reducing temperature can liquefy gases. Have you heard of solid carbon dioxide (CO2)? It is stored under high pressure. Solid CO2 gets converted directly into gaseous state on decrease of pressure to 1 atmosphere* without coming into liquid state. This is the Fig. 1.7: Sublimation of camphor reason that solid carbon dioxide is also known as dry ice. Now, heat slowly and observe. Thus, we can say that pressure and What do you infer from the above temperature deter mine the state of a activity? substance, whether it will be solid, liquid A change of state directly from solid to gas or gas. without changing into liquid state is called sublimation and the direct change of gas to solid without changing into liquid is called deposition. 1.4.2 EFFECT OF CHANGE OF PRESSURE We have already learnt that the difference in various states of matter is due to the Deposition difference in the distances between the constituent particles. What will happen when we start putting pressure and compress a gas Fig. 1.9: Interconversion of the three states of matter * atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal (Pa): 1 atmosphere = 1.01 × 105 Pa. The pressure of air in atmosphere is called atmospheric pressure. The atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure. 8 SCIENCE 2024-25 Q dish and keep it inside a cupboard or uestions on a shelf in your class. 1. Convert the following temperature Record the room temperature. to celsius scale: Record the time or days taken for the a. 300 K b. 573 K evaporation process in the above cases. 2. What is the physical state of Repeat the above three steps of activity water at: on a rainy day and record your a. 250oC b. 100oC ? observations. 3. For any substance, why does the What do you infer about the effect of temperature remain constant temperature, surface area and wind during the change of state? velocity (speed) on evaporation? 4. Suggest a method to liquefy You must have observed that the rate of atmospheric gases. evaporation increases with– an increase of surface area: 1.5 Evaporation We know that evaporation is a surface Do we always need to heat or change pressure phenomenon. If the surface area is for changing the state of matter? Can you increased, the rate of evaporation quote some examples from everyday life where increases. For example, while putting change of state from liquid to vapour takes clothes for drying up we spread them out. place without the liquid reaching the boiling an increase of temperature: point? Water, when left uncovered, slowly With the increase of temperature, more changes into vapour. Wet clothes dry up. What number of particles get enough kinetic happens to water in the above two examples? energy to go into the vapour state. We know that particles of matter are a decrease in humidity: always moving and are never at rest. At a given Humidity is the amount of water vapour temperature in any gas, liquid or solid, there present in air. The air around us cannot are particles with different amounts of kinetic hold more than a definite amount of energy. In the case of liquids, a small fraction of particles at the surface, having higher water vapour at a given temperature. If kinetic energy, is able to break away from the the amount of water in air is already high, forces of attraction of other particles and gets the rate of evaporation decreases. converted into vapour. This phenomenon of an increase in wind speed: change of liquid into vapours at any It is a common observation that clothes temperature below its boiling point is called dry faster on a windy day. With the evaporation. increase in wind speed, the particles of water vapour move away with the wind, 1.5.1 FACTORS AFFECTING decreasing the amount of water vapour EVAPORATION in the surrounding. Let us understand this with an activity. 1.5.2 HOW DOES EVAPORATION CAUSE COOLING? Activity _____________ 1.14 In an open vessel, the liquid keeps on Take 5 mL of water in a test tube and evaporating. The particles of liquid absorb keep it near a window or under a fan. energy from the surrounding to regain the Take 5 mL of water in an open china energy lost during evaporation. This dish and keep it near a window or absorption of energy from the surroundings under a fan. make the surroundings cold. Take 5 mL of water in an open china M AT T E R IN O U R S U R R O U N D I N GS 9 2024-25  What happens when you pour some Why do we see water droplets on the outer acetone (nail polish remover) on your palm? surface of a glass containing ice-cold The particles gain energy from your palm or water? surroundings and evaporate causing the palm Let us take some ice-cold water in a to feel cool. tumbler. Soon we will see water droplets on After a hot sunny day, people sprinkle the outer surface of the tumbler. The water water on the roof or open ground because vapour present in air, on coming in contact the large latent heat of vaporisation of water with the cold glass of water, loses energy and helps to cool the hot surface. gets converted to liquid state, which we see Can you cite some more examples from as water droplets. daily life where we can feel the effect of cooling due to evaporation? uestions Q Why should we wear cotton clothes in summer? 1. Why does a desert cooler cool During summer, we perspire more better on a hot dry day? because of the mechanism of our body which 2. How does the water kept in an keeps us cool. We know that during earthen pot (matka) become cool evaporation, the particles at the surface of during summer? the liquid gain energy from the surroundings 3. Why does our palm feel cold or body surface and change into vapour. The when we put some acetone or heat energy equal to the latent heat of petrol or perfume on it? vaporisation is absorbed from the body 4. Why are we able to sip hot tea or leaving the body cool. Cotton, being a good milk faster from a saucer rather absorber of water helps in absorbing the than a cup? sweat and exposing it to the atmosphere for 5. What type of clothes should we easy evaporation. wear in summer? What you have learnt Matter is made up of small particles. The matter around us exists in three states—solid, liquid and gas. The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases. The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases. 10 SCIENCE 2024-25  The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly. The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure. Sublimation is the change of solid state directly to gaseous state without going through liquid state. Deposition is the change of gaseous state directly to solid state without going through liquid state. Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state. Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state. The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed. Evaporation causes cooling. Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point. Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point. Some measurable quantities and their units to remember: Quantity Unit Symbol Temperature kelvin K Length metre m Mass kilogram kg Weight newton N Volume cubic metre m3 Density kilogram per cubic metre kg m–3 Pressure pascal Pa M AT T E R IN O U R S U R R O U N D I N GS 11 2024-25  Exercises 1. Convert the following temperatures to the celsius scale. (a) 293 K (b) 470 K 2. Convert the following temperatures to the kelvin scale. (a) 25o°C (b) 373o°C 3. Give reason for the following observations. (a) Naphthalene balls disappear with time without leaving any solid. (b) We can get the smell of perfume sitting several metres away. 4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen. 5. What is the physical state of water at— (a) 25o°C (b) 0o°C (c) 100o°C ? 6. Give two reasons to justify— (a) water at room temperature is a liquid. (b) an iron almirah is a solid at room temperature. 7. Why is ice at 273 K more effective in cooling than water at the same temperature? 8. What produces more severe burns, boiling water or steam? 9. Name A,B,C,D,E and F in the following diagram showing change in its state 12 SCIENCE 2024-25  Group Activity Prepare a model to demonstrate movement of particles in solids, liquids and gases. For making this model you will need A transparent jar A big rubber balloon or piece of stretchable rubber sheet A string Few chickpeas or black gram or dry green peas. How to make? Put the seeds in the jar. Sew the string to the centre of the rubber sheet and put some tape to keep it tied securely. Stretch and tie the rubber sheet on the mouth of the jar. Your model is ready. Now run your fingers up and down the string by first tugging at it slowly and then rapidly. Fig. 1.10: A model for converting of solid to liquid and liquid to gas. M AT T E R IN O U R S U R R O U N D I N GS 13 2024-25 C hapter 2 IS MATTER AROUND US PURE? How do we judge whether milk, ghee, butter, a pure substance and cannot be separated by salt, spices, mineral water or juice that we buy physical process into its chemical constituents. from the market are pure? Similarly, sugar is a substance which contains only one kind of pure matter and its composition is the same throughout. Soft drink and soil are not single pure substances. Whatever the source of a pure substance may be, it will always have the same characteristic properties. Therefore, we can say that a mixture contains more than one pure substance. 2.1.1 TYPES OF MIXTURES Fig. 2.1: Some consumable items Depending upon the nature of the components Have you ever noticed the word ‘pure’ that form a mixture, we can have different written on the packs of these consumables? types of mixtures. For a common person pure means having no adulteration. But, for a scientist all these things Activity ______________ 2.1 are actually mixtures of different substances Let us divide the class into groups A, and hence not pure. For example, milk is B, C and D. actually a mixture of water, fat, proteins, etc. Group A takes a beaker containing When a scientist says that something is pure, 50 mL of water and one spatula full of copper sulphate powder. Group B it means that all the constituent particles of takes 50 mL of water and two spatula that substance are the same in their chemical full of copper sulphate powder in a nature. A pure substance consists of a single beaker. type of particle. In other words, a substance is Groups C and D can take different a pure single form of matter. amounts of copper sulphate and As we look around, we can see that most potassium permanganate or common salt (sodium chloride) and mix the of the matter around us exists as mixtures of given components to form a mixture. two or more pure components, for example, Report the observations on the sea water, minerals, soil, etc., are all mixtures. uniformity in colour and texture. Groups A and B have obtained a 2.1 What is a Mixture? mixture which has a uniform Mixtures are constituted by more than one composition throughout. Such mixtures are called homogeneous kind of pure form of matter. We know that mixtures or solutions. Some other dissolved sodium chloride can be separated examples of such mixtures are: (i) salt from water by the physical process of dissolved in water and (ii) sugar evaporation. However, sodium chloride is itself dissolved in water. Compare the 2024-25  colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution but the intensity of colour of the solutions is different. This shows that a homogeneous mixture can have a variable composition. Groups C and D have obtained mixtures, which contain physically distinct parts and have non-uniform compositions. Such mixtures are called heterogeneous mixtures. Mixtures of Fig. 2.2: Filtration sodium chloride and iron filings, salt Now, we shall learn about solutions, and sulphur, and oil and water are suspensions and colloidal solutions in the examples of heterogeneous mixtures. following sections. Activity ______________ 2.2 Q uestions Let us again divide the class into four 1. What is meant by a substance? groups— A, B, C and D. 2. List the points of differences Distribute the following samples to between homogeneous and each group: heterogeneous mixtures. − Few crystals of copper sulphate to group A. − One spatula full of copper sulphate to group B. 2.2 What is a Solution? − Chalk powder or wheat flour to group C. A solution is a homogeneous mixture of two − Few drops of milk or ink to or more substances. You come across various group D. types of solutions in your daily life. Lemonade, Each group should add the given soda water, etc., are all examples of solutions. sample in water and stir properly Usually we think of a solution as a liquid that using a glass rod. Are the particles contains either a solid, liquid or a gas dissolved in the mixture visible? in it. But, we can also have solid solutions Direct a beam of light from a torch (alloys) and gaseous solutions (air). In a through the beaker containing the solution there is homogeneity at the particle mixture and observe from the front. Was level. For example, lemonade tastes the same the path of the beam of light visible? throughout. This shows that particles of sugar Leave the mixtures undisturbed for or salt are evenly distributed in a few minutes (and set up the the solution. filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some Alloys: Alloys are mixtures of two or more metals or a metal and a non-metal More to know time? Filter the mixture. Is there any and cannot be separated into their residue on the filter paper? components by physical methods. But still, an alloy is considered as a mixture Discuss the results and form an because it shows the properties of its opinion. constituents and can have variable Groups A and B have got a solution. composition. For example, brass is a Group C has got a suspension. mixture of approximately 30% zinc and Group D has got a colloidal solution. 70% copper. IS MATTER AROUND US PURE? 15 2024-25  A solution has a solvent and a solute as its proportion of the solute and solvent can be components. The component of the solution varied. Depending upon the amount of solute that dissolves the other component in it present in a solution, it can be called dilute, (usually the component present in larger concentrated or saturated solution. Dilute amount) is called the solvent. The component and concentrated are comparative terms. In of the solution that is dissolved in the solvent activity 2.2, the solution obtained by group (usually present in lesser quantity) is called A is dilute as compared to that obtained by the solute. group B. Examples: Activity ______________ 2.3 (i) A solution of sugar in water is a solid Take approximately 50 mL of water in liquid solution. In this solution, each in two separate beakers. sugar is the solute and water is Add salt in one beaker and sugar or the solvent. barium chloride in the second beaker with continuous stirring. (ii) A solution of iodine in alcohol known When no more solute can be dissolved, as ‘tincture of iodine’, has iodine (solid) heat the contents of the beaker to raise as the solute and alcohol (liquid) as the the temperature by about 5oC.° solvent. Start adding the solute again. (iii) Aerated drinks like soda water, etc., are gas in liquid solutions. These contain Is the amount of salt and sugar or barium carbon dioxide (gas) as solute and water chloride, that can be dissolved in water at a (liquid) as solvent. given temperature, the same? (iv) Air is a mixture of gas in gas. Air is a At any particular temperature, a solution homogeneous mixture of a number of that has dissolved as much solute as it is gases. Its two main constituents are: capable of dissolving, is said to be a saturated oxygen (21%) and nitrogen (78%). The solution. In other words, when no more solute other gases are present in very can be dissolved in a solution at a given small quantities. temperature, it is called a saturated solution. The amount of the solute present in the Properties of a Solution saturated solution at this temperature is called its solubility. A solution is a homogeneous mixture. If the amount of solute contained in a The particles of a solution are smaller solution is less than the saturation level, it is than 1 nm (10-9 metre) in diameter. So, called an unsaturated solution. they cannot be seen by naked eyes. What would happen if you were to take a Because of very small particle size, they saturated solution at a certain temperature do not scatter a beam of light passing and cool it slowly. through the solution. So, the path of We can infer from the above activity that light is not visible in a solution. different substances in a given solvent have The solute particles cannot be separated different solubilities at the same temperature. from the mixture by the process of The concentration of a solution is the amount filtration. The solute particles do not (mass or volume) of solute present in a given settle down when left undisturbed, that amount (mass or volume) of solution. is, a solution is stable. There are various ways of expressing the concentration of a solution, but here we will 2.2.1 CONCENTRATION OF A SOLUTION learn only three methods. In activity 2.2, we observed that groups A and (i) Mass by mass percentage of a solution B obtained different shades of solutions. So, Mass of solute = ×100 we understand that in a solution the relative Mass of solution 16 SCIENCE 2024-25  (ii) Mass by volume percentage of a solution The particles of a suspension scatter a beam of light passing through it and Mass of solute = ×100 make its path visible. Volume of solution The solute particles settle down when a (iii) Volume by volume percentage of a suspension is left undisturbed, that is, solution a suspension is unstable. They can be separated from the mixture by the Volume of solute = ×100 process of filtration. When the particles Volume of solution settle down, the suspension breaks and it does not scatter light any more. Example 2.1 A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by 2.2.3 WHAT IS A COLLOIDAL SOLUTION? mass percentage of the solution. The mixture obtained by group D in activity Solution: 2.2 is called a colloid or a colloidal solution. The particles of a colloid are uniformly spread Mass of solute (salt) = 40 g throughout the solution. Due to the relatively Mass of solvent (water) = 320 g We know, smaller size of particles, as compared to that of Mass of solution = Mass of solute + a suspension, the mixture appears to be Mass of solvent homogeneous. But actually, a colloidal solution = 40 g + 320 g is a heterogeneous mixture, for example, milk. = 360 g Because of the small size of colloidal particles, we cannot see them with naked eyes. Mass percentage of solution But, these particles can easily scatter a beam Mass of solute of visible light as observed in activity 2.2. This = ×100 scattering of a beam of light is called the Mass of solution Tyndall effect after the name of the scientist 40 who discovered this effect. = ×100 =11.1% Tyndall effect can also be observed when a 360 fine beam of light enters a room through a small hole. This happens due to the scattering of light 2.2.2 WHAT IS A SUSPENSION? by the particles of dust and smoke in the air. Non-homogeneous systems, like those obtained by group C in activity 2.2, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye. Properties of a Suspension (a) (b) Suspension is a heterogeneous mixture. Fig. 2.3: (a) Solution of copper sulphate does not The particles of a suspension can be seen show Tyndall effect, (b) mixture of water by the naked eye. and milk shows Tyndall effect. IS MATTER AROUND US PURE? 17 2024-25  Tyndall effect can be observed when Colloids are big enough to scatter a sunlight passes through the canopy of a dense beam of light passing through it and forest. In the forest, mist contains tiny droplets make its path visible. of water, which act as particles of colloid They do not settle down when left dispersed in air. undisturbed, that is, a colloid is quite stable. They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles. The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known Fig. 2.4: The Tyndall effect as the dispersing medium. Colloids are classified according to the state (solid, liquid Properties of a Colloid or gas) of the dispersing medium and the A colloid is a heterogeneous mixture. dispersed phase. A few common examples are The size of particles of a colloid is too given in Table 2.1. From this table you can small to be individually seen with see that they are very common everyday life. naked eyes. Q uestions 1. Differentiate between homogen eous and heterogeneous mixtures with examples. 2. How are sol, solution and suspension different from each other? 3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature. Table 2.1: Common examples of colloids Dispersed Dispersing Type Example phase Medium Liquid Gas Aerosol Fog, clouds, mist Solid Gas Aerosol Smoke, automobile exhaust Gas Liquid Foam Shaving cream Liquid Liquid Emulsion Milk, face cream Solid Liquid Sol Milk of magnesia, mud Gas Solid Foam Foam, rubber, sponge, pumice Liquid Solid Gel Jelly, cheese, butter Solid Solid Solid Sol Coloured gemstone, milky glass 18 S CIENCE 2024-25 2.3 Physical and Chemical burning of paper and wood. 2. T ry segregating the things Changes around you as pure substances or mixtures. In the previous chapter, we have learnt about a few physical properties of matter. The properties that can be observed and specified 2.4 What are the Types of Pure like colour, hardness, rigidity, fluidity, Substances? density, melting point, boiling point etc. are the physical properites. On the basis of their chemical composition, The interconversion of states is a physical substances can be classified either as elements change because these changes occur without or compounds. a change in composition and no change in the chemical nature of the substance. Although 2.4.1 ELEMENTS ice, water and water vapour all look different and display different physical properties, they Robert Boyle was the first scientist to use the are chemically the same. term element in 1661. Antoine Laurent Both water and cooking oil are liquid but Lavoisier (1743–94), a French chemist, was the their chemical characteristics are different. first to establish an experimentally useful They differ in odour and inflammability. We definition of an element. He defined an element know that oil burns in air whereas water as a basic form of matter that cannot be broken extinguishes fire. It is this chemical property down into simpler substances by chemical of oil that makes it different from water. reactions. Burning is a chemical change. During this Elements can be normally divided into process one substance reacts with another to metals, non-metals and metalloids. undergo a change in chemical composition. Metals usually show some or all of the Chemical change brings change in the following properties: chemical properties of matter and we get new They have a lustre (shine). substances. A chemical change is also called They have silvery-grey or golden-yellow a chemical reaction. colour. During burning of a candle, both physical They conduct heat and electricity. and chemical changes take place. Can you They are ductile (can be drawn into distinguish these? wires). They are malleable (can be hammered Q into thin sheets). uestions They are sonorous (make a ringing sound when hit). 1. Classify the following as chemical or physical changes: Examples of metals are gold, silver, cutting of trees, copper, iron, sodium, potassium etc. Mercury melting of butter in a pan, is the only metal that is liquid at room rusting of almirah, temperature. boiling of water to form steam, Non-metals usually show some or all of the passing of electric current, following properties: through water and the water They display a variety of colours. breaking down into hydrogen They are poor conductors of heat and and oxygen gases, electricity. dissolving common salt in They are not lustrous, sonorous water, or malleable. making a fruit salad with raw Examples of non-metals are hydrogen, fruits, and oxygen, iodine, carbon (coal, coke), bromine, IS MATTER AROUND US PURE? 19 2024-25 chlorine etc. Some elements have intermediate Compare the texture and colour of the properties between those of metals and non- material obtained by the groups. metals, they are called metalloids; examples Add carbon disulphide to one part of are boron, silicon, germanium, etc. the material obtained. Stir well and filter. Add dilute sulphuric acid or dilute The number of elements hydrochloric acid to the other part of known at present are more the material obtained.(Note: teacher than 100. Ninety-two elements supervision is necessary for are naturally occurring and the rest are man-made. this activity). More to know Perform all the above steps with both Majority of the elements are the elements (iron and sulphur) solid. separately. Eleven elements are in gaseous state at room temperature. Two elements are liquid at Now answer room temperature—mercury and bromine. Did the material obtained by the two Elements, gallium and cesium groups look the same? become liquid at a temperature Which group has obtained a material slightly above room with magnetic properties? temperature (303 K). Can we separate the components of the material obtained? 2.4.2 COMPOUNDS On adding dilute sulphuric acid or A compound is a substance composed of two dilute hydrochloric acid, did both the or more elements, chemically combined with groups obtain a gas? Did the gas in one another in a fixed proportion. both the cases smell the same What do we get when two or more elements or different? are combined? The gas obtained by Group I is hydrogen, Activity ______________ 2.4 it is colourless, odourless and combustible– it is not advised to do the combustion test for Divide the class into two groups. Give hydrogen in the class. The gas obtained by 5 g of iron filings and 3 g of sulphur Group II is hydrogen sulphide. It is a colourless powder in a china dish to both the groups. gas with the smell of rotten eggs. You must have observed that the products Group I Mix and crush iron filings and obtained by both the groups show different sulphur powder. properties, though the starting materials were the same. Group I has carried out the activity Group II Mix and crush iron filings and involving a physical change whereas in case sulphur powder. Heat this mixture of Group II, a chemical change (a chemical strongly till red hot. Remove from reaction) has taken place. flame and let the mixture cool. The material obtained by group I is a Groups I and II mixture of the two substances. The Check for magnetism in the material substances given are the elements—iron obtained. Bring a magnet near the and sulphur. material and check if the material is The properties of the mixture are the attracted towards the magnet. same as that of its constituents. 20 SCIENCE 2024-25  Table 2.2: Mixtures and Compounds Mixtures Compounds 1. Elements or compounds just mix 1. Elements react to form new compounds. together to form a mixture and no new compound is formed. 2. A mixture has a variable composition. 2. The composition of each new substance is always fixed. 3, A mixture shows the properties of the 3. The new substance has totally different constituent substances. properties. 4. The constituents can be seperated 4. The constituents can be separated only fairly easily by physical methods. by chemical or electrochemical reactions. The material obtained by group II is a The composition of a compound is the compound. same throughout. We can also observe On heating the two elements strongly we that the texture and the colour of the get a compound, which has totally compound are the same throughout. different properties compared to the Thus, we can summarise the physical combining elements. and chemical nature of matter in the following graphical organiser: What you have learnt A mixture contains more than one substance (element and/ or compound) mixed in any proportion. IS MATTER AROUND US PURE? 21 2024-25  Mixtures can be separated into pure substances using appropriate separation techniques. A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute. The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution. Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture. Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. Pure substances can be elements or compounds. An element is a form of matter that cannot be broken down by chemical reactions into simpler substances. A compound is a substance composed of two or more different types of elements, chemically combined in a fixed proportion. Properties of a compound are different from its constituent elements, whereas a mixture shows the properties of its constituting elements or compounds. Exercises 1. Which separation techniques will you apply for the separation of the following? (a) Sodium chloride from its solution in water (b) Amm