Welcome dear students! Today we are going to learn about Matter in Our Surroundings from Class 9 Science. As we look at our surroundings, we see a large variety of things with different shapes, sizes and textures. Everything in this universe is made up of material which scientists have named matter. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand, every thing is matter. We can also see as we look around that all the things mentioned above occupy space and have mass. In other words, they have both mass and volume. The SI unit of mass is kilogram. The SI unit of volume is cubic metre. The common unit of measuring volume is litre such that 1 L equals 1 dm^3, 1 L equals 1000 mL, and 1 mL equals 1 cm^3. Since early times, human beings have been trying to understand their surroundings. Early Indian philosophers classified matter in the form of five basic elements, the Panch Tatva, which are 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.
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Let us begin with Activity 1.1. Take a 100 mL beaker. Fill half the beaker with water and mark the level of water. Dissolve some salt or sugar with the help of a glass rod. Observe any change in water level. Ask yourself, what do you think has happened to the salt? Where does it disappear? Does the level of water change? In order to answer these questions we need to use the idea that matter is made up of particles. What was there in the spoon, salt or sugar, has now spread throughout water. This is illustrated in Figure 1.1. In this diagram, we can see that when we dissolve salt in water, the particles of salt get into the spaces between particles of water. Now, section 1.1.1 tells us that matter is made up of particles. For a long time, two schools of thought prevailed regarding the nature of matter. One school believed matter to be continuous like a block of wood, whereas, the other thought that matter was made up of particles like sand. Let us perform an activity to decide about the nature of matter, is it continuous or particulate? Moving to section 1.1.2, how small are these particles of matter? Activity 1.2 instructs us to take 2 to 3 crystals of potassium permanganate and dissolve them in 100 mL of water. Take out approximately 10 mL of this solution and put it into 90 mL of clear water. 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 8 times. Is the water still coloured? Figure 1.2 illustrates estimating how small the particles of matter are. With every dilution, though the colour becomes light, it is still visible. This experiment shows that just a few crystals of potassium permanganate can colour a large volume of water, about 1000 L. So we conclude that there must be millions of tiny particles in just one crystal of potassium permanganate, which keep on dividing themselves into smaller and smaller particles. The same activity can be done using 2 mL of Dettol instead of potassium permanganate. The smell can be detected even on repeated dilution. The particles of matter are very small, they are small beyond our imagination.
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Now we move to section 1.2, characteristics of particles of matter. First, particles of matter have space between them. In activities 1.1 and 1.2 we saw that particles of sugar, salt, Dettol, or potassium permanganate got evenly distributed in water. Similarly, when we make tea, coffee or lemonade, particles of one type of matter get into the spaces between particles of the other. This shows that there is enough space between particles of matter. Next, particles of matter are continuously moving. Activity 1.3 says to 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 instructs to take two glasses or beakers filled with water. Put a drop of blue or red ink slowly and carefully along the sides of the first beaker and honey in the same way in the second beaker. Leave them undisturbed in your house or in a corner of the class. Record your observations. What do you observe immediately after adding the ink drop? What do you observe immediately after adding a drop of honey? How many hours or days does it take for the colour of ink to spread evenly throughout the water? Activity 1.5 says to drop a crystal of copper sulphate or potassium permanganate into a glass of hot water and another containing cold water. Do not stir the solution. Allow the crystals to settle at the bottom. What do you observe just above the solid crystal in the glass? What happens as time passes? What does this suggest about the particles of solid and liquid? Does the rate of mixing change with temperature? Why and how? From the above three activities, we can conclude that particles of matter are continuously moving, that is, they possess what we call the kinetic energy. As the temperature rises, particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases. In the above three activities we observe that particles of matter intermix on their own with each other. They do so by getting into the spaces between the particles. This intermixing of particles of two different types of matter on their own is called diffusion. We also observe that on heating, diffusion becomes faster.
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Let us pause to answer the questions following this section. Question one asks which of the following are matter: chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume. The answer is chair, air, almonds, lemon water, and smell of perfume are matter because they occupy space and have mass. Love, smell, hate, thought, and cold are not matter as they do not have mass or occupy space. Question two asks for reasons why the smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close. The answer is that particles of hot food have higher kinetic energy, so they diffuse faster and travel farther. Cold food particles have lower kinetic energy, so diffusion is slower. Question three states a diver is able to cut through water in a swimming pool. Which property of matter does this observation show? It shows that there is space between the particles of water, and the force of attraction between them is not extremely strong, allowing the diver to move through. Question four asks what are the characteristics of the particles of matter? The characteristics are: they have space between them, they are continuously moving, and they attract each other. Now, particles of matter attract each other. Activity 1.6 is a game to play in the field. Make four groups and form human chains as suggested. The first group should hold each other from the back and lock arms like Idu Mishmi dancers, as shown in Figure 1.3. The second group should hold hands to form a human chain. The third group should form a chain by touching each other with only their finger tips. Now, the fourth group of students should run around and try to break the three human chains one by one into as many small groups as possible. Which group was the easiest to break? Why? If we consider each student as a particle of matter, then in which group the particles held each other with the maximum force? The third group with fingertips is easiest to break because the force of attraction is weakest. The first group has the maximum force of attraction. Activity 1.7 says to take an iron nail, a piece of chalk and a rubber band. Try breaking them by hammering, cutting or stretching. In which of the above three substances do you think the particles are held together with greater force? The particles in the iron nail are held together with the greatest force. Activity 1.8 says to take some water in a container, try cutting the surface of water with your fingers. Were you able to cut the surface of water? What could be the reason behind the surface of water remaining together? You cannot cut it because the force of attraction between water particles keeps them together. The above three activities suggest that particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.
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Let us move to section 1.3, states of matter. Observe different types of matter around you. What are its different states? We can see that matter around us exists in three different states, solid, liquid and gas. These states of matter arise due to the variation in the characteristics of the particles of matter. Now, let us study about the properties of these three states of matter in detail. First, the solid state. Activity 1.9 instructs to collect a pen, a book, a needle and a piece of wooden stick. Sketch the shape of the above articles in your notebook by moving a pencil around them. Do all these have a definite shape, distinct boundaries and a fixed volume? What happens if they are hammered, pulled or dropped? Are these capable of diffusing into each other? Try compressing them by applying force. Are you able to compress them? All the above are examples of solids. We can observe that all these have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility. Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid. Consider the following. What about a rubber band, can it change its shape on stretching? Is it a solid? What about sugar and salt? When kept in different jars these take the shape of the jar. Are they solid? What about a sponge? It is a solid yet we are able to compress it. Why? All the above are solids as a rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks. The shape of each individual sugar or salt crystal remains fixed, whether we take it in our hand, put it in a plate or in a jar. A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we are able to compress it.
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Next, the liquid state. Activity 1.10 instructs to collect water, cooking oil, milk, juice, a cold drink, and containers of different shapes. Put a 50 mL mark on these containers using a measuring cylinder from the laboratory. What will happen if these liquids are spilt on the floor? Measure 50 mL of any one liquid and transfer it into different containers one by one. Does the volume remain the same? Does the shape of the liquid remain the same? When you pour the liquid from one container into another, does it flow easily? We observe that liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid. Refer to activities 1.4 and 1.5 where we saw that solids and liquids can diffuse into liquids. The gases from the atmosphere diffuse and dissolve in water. These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants. All living creatures need to breathe for survival. The aquatic animals can breathe under water due to the presence of dissolved oxygen in water. Thus, we may conclude that solids, liquids and gases can diffuse into liquids. The rate of diffusion of liquids is higher than that of solids. This is due to the fact that in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state. Now, the gaseous state. Have you ever observed a balloon seller filling a large number of balloons from a single cylinder of gas? Enquire from him how many balloons is he able to fill from one cylinder. Ask him which gas does he have in the cylinder. Activity 1.11 says to take three 100 mL syringes and close their nozzles by rubber corks, as shown in Figure 1.4. Remove the pistons from all the syringes. Leaving one syringe untouched, fill water in the second and pieces of chalk in the third. Insert the pistons back into the syringes. You may apply some vaseline on the pistons before inserting them into the syringes for their smooth movement. Now, try to compress the content by pushing the piston in each syringe. What do you observe? In which case was the piston easily pushed in? What do you infer from your observations? We have observed that gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily. We come to know of what is being cooked in the kitchen without even entering there, by the smell that reaches our nostrils. How does this smell reach us? The particles of the aroma of food mix with the particles of air spread from the kitchen, reach us and even farther away. The smell of hot cooked food reaches us in seconds; compare this with the rate of diffusion of solids and liquids. Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases. In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container. Figure 1.5 shows magnified schematic pictures of the three states of matter. The motion of the particles can be seen and compared in the three states of matter.
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Let us answer the questions following this section. Question one defines density as mass per unit volume. Arrange air, exhaust from chimneys, honey, water, chalk, cotton and iron in order of increasing density. The order is air, exhaust from chimneys, cotton, water, honey, chalk, iron. Question two part a asks to tabulate differences in characteristics of states of matter. Solids have definite shape and volume, negligible compressibility, high density, and particles vibrate in fixed positions. Liquids have indefinite shape but definite volume, low compressibility, intermediate density, and particles slide past each other. Gases have indefinite shape and volume, high compressibility, low density, and particles move randomly at high speed. Part b asks to comment on rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density. Rigidity is the tendency to maintain shape under force, highest in solids. Compressibility is the ability to reduce volume under pressure, highest in gases. Fluidity is the ability to flow, present in liquids and gases. Filling a gas container utilizes high compressibility to store large volumes. Shape is definite in solids, takes container shape in liquids and gases. Kinetic energy is lowest in solids, intermediate in liquids, highest in gases. Density is highest in solids, intermediate in liquids, lowest in gases. Question three part a asks why a gas fills completely the vessel. Because gas particles have high kinetic energy and move randomly, filling all available space. Part b asks why a gas exerts pressure. Because gas particles collide with container walls continuously. Part c asks why a wooden table is a solid. Because it has definite shape, volume, and high force of attraction between particles. Part d asks why we can move hand in air but not wood. Because air particles have large spaces and weak attraction, while wood particles are tightly packed with strong attraction. Question four asks why ice floats on water. Ice has a cage-like structure with empty spaces, making its density lower than liquid water, so it floats. Moving to section 1.4, can matter change its state? We all know water exists as solid ice, liquid water, and gas water vapour. What happens inside the matter during this change? What happens to the particles? How does it take place?
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Effect of change of temperature. Activity 1.12 instructs to 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 Figure 1.6. Figure 1.6 shows two setups. Setup a shows conversion of ice to water with an iron stand, thermometer, glass stirrer, beaker with ice, and a burner. Setup b shows conversion of water to water vapour with the same apparatus but with water in the beaker. Start heating the beaker on a low flame. Note the temperature when the ice starts melting. Note the temperature when all the ice has converted into water. Record your observations for this conversion of solid to liquid state. Now, put a glass rod in the beaker and heat while stirring till the water starts boiling. Keep a careful eye on the thermometer reading till most of the water has vaporised. Record your observations for the conversion of water in the liquid state to the gaseous state. On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of a solid is an indication of the strength of the force of attraction between its particles. The melting point of ice is 273.15 K. The process of melting, that is, change of solid 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 of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat. The word latent means hidden. The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. So, particles in water at 0°C or 273 K have more energy as compared to particles in ice at the same temperature. When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Boiling is a bulk phenomenon. Particles from the bulk of the liquid gain enough energy to change into the vapour state. For water this temperature is 373 K, which is 100°C + 273 = 373 K. Can you define the latent heat of vaporisation? Do it in the same way as we have defined the latent heat of fusion. Particles in steam, that is, water vapour at 373 K or 100°C have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporisation. So, we infer that the state of matter can be changed into another state by changing the temperature.
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Note that kelvin is the SI unit of temperature, 0°C equals 273.15 K. For convenience, we take 0°C equals 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. Let us answer the questions. Question one asks to convert 300 K and 573 K to celsius. 300 minus 273 equals 27°C. 573 minus 273 equals 300°C. Question two asks physical state of water at 250°C and 100°C. At 250°C, water is gas. At 100°C, water is changing from liquid to gas, so it exists as both liquid and gas. Question three asks why temperature remains constant during change of state. Because the supplied heat is used as latent heat to overcome intermolecular forces, not to increase kinetic energy. Question four asks to suggest a method to liquefy atmospheric gases. By applying high pressure and reducing temperature. Now, effect of change of pressure. We know difference in states is due to distance between particles. What happens when we compress a gas? Will particles come closer? Does pressure change state? Figure 1.8 shows that 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 atm without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. Thus, we can say that pressure and temperature determine the state of a substance. Figure 1.9 shows interconversion of the three states of matter. Solid to liquid is fusion. Liquid to solid is solidification. Liquid to gas is vaporisation. Gas to liquid is condensation. Solid to gas is sublimation. Gas to solid is deposition. Activity 1.13 says to 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 Figure 1.7. Figure 1.7 shows a china dish with camphor being heated by a burner, covered by an inverted funnel with a cotton plug on the stem. Camphor vapours rise and solidify on the cooler funnel walls. Now, heat slowly and observe. What do you infer? A change of state directly from solid to gas without changing into liquid state is called sublimation and the direct change of gas to solid without changing into liquid is called deposition. Note that atmosphere is a unit of measuring pressure exerted by a gas. The unit of pressure is pascal. 1 atm equals 1.01 × 10^5 Pa. The pressure of air in atmosphere is called atmospheric pressure. The atmospheric pressure at sea level is 1 atm, and is taken as the normal atmospheric pressure.
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Now, section 1.5, evaporation. Do we always need to heat or change pressure? Can you quote examples where liquid changes to vapour without boiling? Water left uncovered changes to vapour. Wet clothes dry. What happens? Particles are always moving. At a given temperature, some particles have higher kinetic energy. In liquids, a small fraction at the surface, having higher kinetic energy, breaks away from forces of attraction and gets converted into vapour. This phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation. Factors affecting evaporation. Let us understand with Activity 1.14. Take 5 mL of water in a test tube and keep it near a window or under a fan. Take 5 mL of water in an open china dish and keep it near a window or under a fan. Take 5 mL of water in an open china dish and keep it inside a cupboard or on a shelf in your class. Record the room temperature. Record the time or days taken for the evaporation process in the above cases. Repeat the above three steps on a rainy day and record your observations. What do you infer about the effect of temperature, surface area and wind velocity on evaporation? You must have observed that the rate of evaporation increases with an increase of surface area. Evaporation is a surface phenomenon. If surface area increases, rate increases. For example, spreading clothes to dry. It increases with an increase of temperature. More particles get enough kinetic energy to go into vapour state. It increases with a decrease in humidity. Humidity is the amount of water vapour present in air. Air cannot hold more than a definite amount at a given temperature. If air already has high water content, evaporation decreases. It increases with an increase in wind speed. Clothes dry faster on a windy day. Water vapour particles move away with wind, decreasing surrounding water vapour. How does evaporation cause cooling? In an open vessel, liquid keeps evaporating. Particles absorb energy from surroundings to regain energy lost during evaporation. This absorption makes surroundings cold. What happens when you pour acetone on your palm? Particles gain energy from palm and evaporate, making palm feel cool. After a hot sunny day, people sprinkle water on roofs because the large latent heat of vaporisation of water helps cool the surface. Why should we wear cotton clothes in summer? We perspire more. During evaporation, particles at liquid surface gain energy from body and change to vapour. Heat equal to latent heat of vaporisation is absorbed from body, leaving it cool. Cotton absorbs sweat and exposes it to atmosphere for easy evaporation. Why do we see water droplets on outer surface of a glass with ice-cold water? Water vapour in air contacts cold glass, loses energy, gets converted to liquid state, seen as droplets.
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Let us answer the questions following this section. Question one asks why a desert cooler cools better on a hot dry day. Because dry air has low humidity, allowing faster evaporation, which causes more cooling. Question two asks how water in an earthen pot becomes cool. The pot has tiny pores. Water seeps out and evaporates, absorbing latent heat from the pot and remaining water, cooling it. Question three asks why palm feels cold with acetone or petrol or perfume. Because they evaporate quickly, absorbing latent heat of vaporisation from the palm. Question four asks why we sip hot tea faster from a saucer. Saucer has larger surface area than a cup, increasing evaporation rate and cooling the liquid faster. Question five asks what clothes to wear in summer. Cotton clothes, because they absorb sweat and allow easy evaporation, keeping the body cool. Let us review the key points. 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. The arrangement of particles is most ordered in solids, in liquids layers can slip and slide, while for gases there is no order, particles move randomly. The states of matter are inter-convertible. State can be changed by changing temperature or pressure. Sublimation is change of solid directly to gas without liquid. Deposition is change of gas directly to solid without liquid. Boiling is a bulk phenomenon. Particles from the bulk change to vapour. Evaporation is a surface phenomenon. Particles from surface gain energy to change to vapour. Rate of evaporation depends on surface area, temperature, humidity and wind speed. Evaporation causes cooling. Latent heat of vaporisation is heat required to change 1 kg of liquid to gas at atmospheric pressure at boiling point. Latent heat of fusion is heat required to change 1 kg of solid to liquid at melting point. Some measurable quantities and their units to remember: Temperature is measured in kelvin, symbol K. Length in metre, symbol m. Mass in kilogram, symbol kg. Weight in newton, symbol N. Volume in cubic metre, symbol m^3. Density in kilogram per cubic metre, symbol kg m^-3. Pressure in pascal, symbol Pa.
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Now let us solve the exercises completely. Question one: Convert to celsius. (a) 293 K. Subtract 273. Answer is 20°C. (b) 470 K. Subtract 273. Answer is 197°C. Question two: Convert to kelvin. (a) 25°C. Add 273. Answer is 298 K. (b) 373°C. Add 273. Answer is 646 K. Question three: Give reasons. (a) Naphthalene balls disappear without leaving solid. Because naphthalene undergoes sublimation, changing directly from solid to gas. (b) We get smell of perfume several metres away. Because perfume particles diffuse rapidly through air due to high kinetic energy and large spaces between gas particles. Question four: Arrange water, sugar, oxygen in increasing order of forces of attraction. Oxygen is gas (weakest), water is liquid (intermediate), sugar is solid (strongest). So order is oxygen, water, sugar. Question five: Physical state of water at (a) 25°C is liquid. (b) 0°C is solid or liquid at melting point. (c) 100°C is gas or liquid at boiling point. Question six: Give two reasons. (a) Water at room temperature is a liquid. Because it has no fixed shape but fixed volume, and it flows. (b) Iron almirah is solid at room temperature. Because it has definite shape and volume, and is rigid with high force of attraction. Question seven: Why is ice at 273 K more effective in cooling than water at same temperature? Because ice absorbs latent heat of fusion to melt into water, taking extra heat from surroundings, causing more cooling. Question eight: What produces more severe burns, boiling water or steam? Steam produces more severe burns because it contains extra latent heat of vaporisation, releasing more energy on contact with skin. Question nine: Name A, B, C, D, E, F in the diagram. The diagram shows state changes. A is fusion. B is vaporisation. C is condensation. D is solidification. E is sublimation. F is deposition. Finally, prepare a model to demonstrate movement of particles. You need a transparent jar, a big rubber balloon or stretchable rubber sheet, a string, and few chickpeas or black gram. Put seeds in jar. Sew string to centre of rubber sheet and tape securely. Stretch and tie rubber sheet on jar mouth. Pull string slowly to show solid vibration, moderately for liquid sliding, and rapidly for gas random motion. This demonstrates particle movement in three states.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]