Matter In Our Surroundings

Namaste students, welcome to your science class. Today we are going to study a very interesting chapter from your NCERT textbook — Chapter 1: Matter in Our Surroundings. This is the foundation of chemistry, and I must tell you, what we learn in this chapter will help you understand many phenomena that you see around you every single day. So pay attention, and we will go step by step, concept by concept.

Let us begin by looking around our classroom. What do we see? We see desks, chairs, a blackboard, our books, maybe a water bottle, and of course, we ourselves are sitting here. Now look outside the window — we can see trees, perhaps some clouds in the sky, and if there is a road nearby, we might see vehicles moving. All of these things, everything around us, is made up of something. That something is what scientists have named "matter".

Now students, let me ask you a question. What is matter? Matter is anything that occupies space and has mass. The air we breathe — that is matter. The food we eat — that is matter. The water we drink — that is matter. Stones, clouds, stars, plants, animals, even a tiny drop of water or a particle of sand — everything is matter. So basically, the entire universe is made up of matter. Isn't that wonderful?

Now, since early times, human beings have been trying to understand the nature of matter. In ancient India, our philosophers classified matter into five basic elements. They called it "Panch Tatva" — that is, five elements. Can anyone tell me what those five elements are? Yes, that's right — air, earth, fire, sky, and water. According to ancient Indian philosophy, everything in this world, whether living or non-living, was made up of these five basic elements. Interestingly, ancient Greek philosophers also arrived at a similar classification of matter. So students, you see, the quest to understand matter is not new — it has been going on for thousands of years.

Modern scientists have classified matter in different ways. In this chapter, we will learn about matter based on its physical properties. We will look at how matter behaves, what are its different states, and how we can change one state to another. The chemical aspects of matter will be taken up in later chapters, so for now, let us focus on the physical nature of matter.

Let us start with a very fundamental question: Is matter continuous, or is it made up of tiny particles? This is a question that scientists debated for a long time. One school of thought believed that matter is continuous, like a block of wood — you can keep dividing it and it will still be wood. But another school believed that matter is made up of particles, like sand — if you keep dividing sand, you eventually get individual grains. So which one is correct? Let us find out through some activities.

For this, we will perform Activity 1.1. Imagine we have a 100 millilitre beaker. We fill it half with water and mark the water level. Now, we take some salt or sugar and dissolve it in this water using a glass rod. What do we observe? The water level changes. But where did the salt go? It didn't disappear — it is still there, but now it has spread throughout the water. The salt particles have gone into the spaces between the particles of water. This shows us that matter is indeed made up of tiny particles. What was in the spoon — salt or sugar — has now spread throughout the water. The particles of salt have gotten between the particles of water.

Now, let us do another activity to understand just how small these particles are. This is Activity 1.2 and 1.3 combined. We take two to three crystals of potassium permanganate — that is a purple colored compound — and dissolve them in 100 millilitres of water. The water turns purple. Now, we take 10 millilitres of this solution and put it into 90 millilitres of clear water. The color becomes lighter, but it is still purple. We take 10 millilitres of this diluted solution and put it into another 90 millilitres of clear water. We keep doing this five to eight times. What do we observe? Even after so many dilutions, the water is still colored! This is amazing, isn't it? Just a few crystals of potassium permanganate can color a large volume of water — we are talking about about 1000 litres of water! This tells us that there must be millions of tiny particles in just one crystal of potassium permanganate. And these particles keep dividing into smaller and smaller particles. The same activity can be done using two millilitres of Dettol instead of potassium permanganate. Even after repeated dilution, you can still smell the Dettol. So students, we can conclude that the particles of matter are very, very small — small beyond our imagination!

Now, let us move to another important property of particles of matter. Are they stationary, or do they move? Let us perform some activities to find out.

Activity 1.4: Put an unlit incense stick in a corner of your class. How close do you have to go to get its smell? You have to go quite close, right? Now light the incense stick. What happens? The smell reaches you even when you are sitting far away. This is because the particles of the incense smoke mix with the air particles and spread around. So particles of matter are moving.

Activity 1.5: Take two glasses filled with water. Put a drop of blue or red ink slowly along the sides of the first glass, and honey in the same way in the second glass. Leave them undisturbed. What do you observe immediately after adding the ink drop? The ink starts spreading slowly. What about honey? Honey is denser, so it spreads even more slowly. How many hours or days does it take for the color of ink to spread evenly throughout the water? It might take several hours, maybe even a day or two. This spreading of particles is called diffusion — we will talk about this in detail soon.

Activity 1.6: This is a very interesting activity. 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? You will see that the color starts spreading from the bottom upwards. What happens as time passes? In the hot water, the color spreads faster. In the cold water, it spreads slower. What does this suggest about the particles of solid and liquid? It suggests that particles of matter are continuously moving. And does the rate of mixing change with temperature? Yes, it does! In hot water, the diffusion is faster.

So students, from these three activities, we can conclude that particles of matter are continuously moving. They possess what we call kinetic energy. And as the temperature rises, particles move faster. So we can say that with increase in temperature, the kinetic energy of the particles also increases.

Now, there is another very important observation from these activities. We see 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. Why does this happen? Because when we heat a substance, the particles gain more kinetic energy and move faster, so they can mix with each other more quickly.

Now students, let us think about another property. What holds these particles together? Why don't they just fly apart? There must be some force of attraction between them. Let us verify this through some activities.

Activity 1.7: Take an iron nail, a piece of chalk, and a rubber band. Try breaking them by hammering, cutting, or stretching. In which of these three substances do you think the particles are held together with greater force? The iron nail is very hard to break — its particles are held together with a very strong force. The chalk is also quite strong, but not as strong as iron. The rubber band can be stretched — but it regains its shape because the particles are attracted to each other.

Activity 1.8: Take some water in a container. Try cutting the surface of water with your fingers. Were you able to cut the surface of water? No, you cannot really cut the surface of water. The water molecules at the surface are held together by an attractive force, which is why the surface behaves like a stretched membrane. This is called surface tension.

Now, let us play a game to understand this better. Make four groups of students. The first group should hold each other from the back and lock arms tightly — like the Idu-Mishmi dancers you might have seen in pictures. 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 fingertips. Now, the fourth group of students should run around and try to break these three human chains into as many small groups as possible. Which group was the easiest to break? Obviously, the third group — the one with just fingertips touching — because the force of attraction between them is very weak. The first group — the one with locked arms — is the hardest to break because the force of attraction between them is very strong.

If we consider each student as a particle of matter, then in which group are the particles holding each other with maximum force? It is the first group. So students, the activities we performed tell us 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. In solids, the force of attraction is very strong. In liquids, it is intermediate. And in gases, it is very weak.

Now, let us answer some questions based on what we have learned so far.

Question 1: Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, lemon water, smell of perfume.

Let us think about each one. Chair — yes, it occupies space and has mass, so it is matter. Air — yes, air is a mixture of gases, and it definitely occupies space and has mass. Love — no, love is an emotion, it does not occupy space and has no mass. Smell — now this is interesting. Smell itself is not matter, but the particles that cause the smell are matter. The smell of something is caused by tiny particles of that substance that enter our nose. So the smell itself is not matter, but the particles causing it are. Hate — no, hate is an emotion, not matter. Almonds — yes, almonds are matter. Thought — no, thought is a mental process, not matter. Cold — now, cold is a condition, not a substance, so it is not matter in the traditional sense. But we often say "I have a cold" meaning we have caught a cold virus, and viruses are matter. However, in common language, cold is not considered matter. Lemon water — yes, it is a liquid, so it is matter. Smell of perfume — similar to smell, the perfume particles are matter, but the smell itself is not.

So the matter items are: chair, air, almonds, lemon water, and the particles that cause smell (whether it's the smell of perfume or anything else).

Question 2: Give reasons for the following observation: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.

This is because when food is hot, the particles of the food have more kinetic energy. They move faster and spread into the air more quickly. So the aroma particles from hot food diffuse into the air faster and reach us even when we are far away. In cold food, the particles have less kinetic energy, so they move slowly and do not spread much. Therefore, we have to go close to the cold food to smell it.

Question 3: A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

This shows that the particles of water are not very strongly attracted to each other. The force of attraction between water particles is relatively weak, which is why a diver can move through water. If water particles were strongly attracted like in solids, it would be impossible to move through it.

Question 4: What are the characteristics of the particles of matter?

The characteristics of particles of matter are: first, matter is made up of tiny particles. Second, these particles are continuously moving — they possess kinetic energy. Third, there is force of attraction between particles. And fourth, the size of these particles is very small — too small to be seen with our eyes.

Now students, let us move on to a very important section — the States of Matter. We all know that matter exists in three different states: solid, liquid, and gas. Let us study each of these in detail.

First, let us talk about the Solid State.

Activity 1.9: Collect some articles — a pen, a book, a needle, and a piece of wooden stick. Sketch the shape of these articles in your notebook by moving a pencil around them. Do all these have a definite shape, distinct boundaries, and a fixed volume? Yes, they do. What happens if they are hammered, pulled, or dropped? They might break or deform, but they generally maintain their shape. Are they capable of diffusing into each other? No, solids do not diffuse into each other easily. Try compressing them by applying force. Are you able to compress them? Not really — solids are very slightly compressible.

All these are examples of solids. We can observe that all solids have a definite shape, distinct boundaries, and fixed volume. Solids have negligible compressibility. They 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 we say solids are rigid.

Now, there are some special cases we need to consider. What about a rubber band? Can it change its shape on stretching? Yes, it can. Is it a solid? Yes, a rubber band is a solid because it regains its shape when the force is removed. If excessive force is applied, it breaks. So rubber band is an example of a solid that can be stretched.

What about sugar and salt? When kept in different jars, they take the shape of the jar. Are they solid? Yes, they are solids. 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. They just fill the container because we are talking about many tiny crystals together.

What about a sponge? It is a solid, yet we are able to compress it. Why? 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. But the sponge material itself is solid.

So students, we can say that solids have a definite shape, definite volume, and are rigid. The particles in a solid are very close together, and they vibrate in their positions but do not move around.

Now, let us study the Liquid State.

Activity 1.10: Collect some liquids — water, cooking oil, milk, juice, and a cold drink. Also collect containers of different shapes. Put a 50 millilitre mark on these containers using a measuring cylinder. What will happen if these liquids are spilled on the floor? They will flow and take the shape of the floor. Measure 50 millilitres of any one liquid and transfer it into different containers one by one. Does the volume remain the same? Yes, the volume remains the same. Does the shape of the liquid remain the same? No, the liquid takes the shape of the container. When you pour the liquid from one container into another, does it flow easily? Yes, it does.

So 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.

Now, from our earlier activities, 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 underwater 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 because in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state.

Now, let us study the Gaseous State.

Have you ever seen a balloon seller filling balloons from a single cylinder of gas? Ask him how many balloons he can fill from one cylinder. The gas in the cylinder is compressed, so a large number of balloons can be filled from one cylinder.

Activity 1.11: Take three 100 millilitre syringes and close their nozzles by rubber corks. Remove the pistons from all the syringes. Leave one syringe empty, fill water in the second, and fill pieces of chalk in the third. Insert the pistons back into the syringes. You may apply some vaseline on the pistons before inserting them for 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? The piston is easily pushed in the empty syringe — that is, in the case of air or gas. In the syringe with water, it is difficult to push. And in the syringe with chalk pieces, it is very difficult to push. What do we infer from this? Gases are highly compressible as compared to solids and liquids.

The liquefied petroleum gas cylinder that we use at home for cooking, or the oxygen supplied to hospitals in cylinders, is compressed gas. Compressed natural gas is used as fuel in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.

Now, how does smell reach us from the kitchen? We come to know what is being cooked in the kitchen without even entering, just by the smell that reaches our nostrils. How does this happen? The particles of the aroma of food mix with the particles of air spread from the kitchen, and reach us even farther away. The smell of hot cooked food reaches us in seconds. Compare this with the rate of diffusion of solids and liquids — it is much slower. 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 a gas is because of this force exerted by gas particles per unit area on the walls of the container.

Now, let us answer some questions about states of matter.

Let us think about each substance. Air is the least dense — it is a gas. Exhaust from chimneys contains smoke and particles, so it is denser than air but still a gas. Water is a liquid, so it is denser than gases. Honey is a thick liquid, so it is denser than water. Chalk is a solid, so it is denser than liquids. Iron is a metal, and it is very dense. Cotton is a solid fiber with true density greater than water, though it appears light due to air trapped between fibers. So in order of increasing true density: air, exhaust from chimneys, water, honey, chalk, cotton, iron (most dense). However, if we consider bulk density (including trapped air), cotton would appear less dense than water.

Question 2: (a) Tabulate the differences in the characteristics of states of matter. (b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy, and density.

Let us discuss each characteristic:

Rigidity: Solids are rigid — they maintain their shape. Liquids are not rigid — they flow and take the shape of the container. Gases are not rigid at all — they fill the entire container.

Compressibility: Solids are almost incompressible. Liquids are also very slightly compressible. Gases are highly compressible.

Fluidity: Solids do not flow. Liquids and gases flow — they are fluids.

Filling a gas container: Gases fill the container completely — they take the shape and volume of the entire container. Liquids fill only the volume but take the shape of the container. Solids maintain their own shape and volume.

Shape: Solids have a definite shape. Liquids do not have a definite shape — they take the shape of the container. Gases do not have a definite shape.

Kinetic energy: Particles in solids have the least kinetic energy — they only vibrate in their positions. Particles in liquids have more kinetic energy — they can move around. Particles in gases have the maximum kinetic energy — they move very fast.

Density: Solids have the highest density. Liquids have lower density than solids (generally). Gases have the lowest density.

Question 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 in air but to do the same through a solid block of wood we need a karate expert.

(a) A gas fills completely the vessel in which it is kept because the particles of a gas move randomly at high speed and have very weak forces of attraction between them. They keep moving in all directions and fill the entire available space.

(b) A gas exerts pressure on the walls of the container because the gas particles are moving randomly at high speed. They collide with each other and with the walls of the container. The force exerted by these collisions per unit area is what we call pressure.

(c) A wooden table should be called a solid because it has a definite shape, definite volume, and is rigid. The particles in the wood are held together by strong forces of attraction.

(d) We can easily move our hand in air because the particles in air are far apart and have weak forces of attraction between them. They easily move out of the way. But to move through a solid block of wood, we need to overcome the strong forces of attraction between the wood particles. That is why we need a karate expert to break a wooden block — the expert applies a force quickly and with precision to overcome the strong intermolecular forces.

Question 4: Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

This is a very interesting question. Ice floats on water because ice is less dense than water. But why is ice less dense? Most substances are denser in their solid state than in their liquid state. But water is an exception. When water freezes, the water molecules form a crystal structure that has more space between the molecules compared to liquid water. This makes ice less dense than water. So when we put ice in water, the ice, being less dense, floats on top. This is very important for aquatic life — if ice sank, then in very cold climates, entire lakes would freeze solid, and fish and other aquatic organisms would not survive.

Now students, let us move on to another very important topic: Can Matter Change its State?

We all know that water can exist in three states — solid as ice, liquid as water, and gas as water vapour. What happens inside the matter during this change of state? What happens to the particles of matter? How does this change take place? Let us find out.

First, let us see the effect of change of temperature on the state of matter.

Activity 1.12: Take about 150 grams of ice in a beaker and suspend a laboratory thermometer so that its bulb is in contact with the ice. 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.

What do we observe? When we start heating ice, the temperature of ice rises. At 0 degrees Celsius or 273 Kelvin, the ice starts melting. During the melting process, the temperature remains constant at 0 degrees Celsius even though we are heating. Once all the ice has melted, the temperature of the water starts rising. At 100 degrees Celsius or 373 Kelvin, the water starts boiling. During the boiling process, the temperature remains constant at 100 degrees Celsius even though we are heating. Once all the water has converted to steam, the temperature of the steam can rise above 100 degrees Celsius.

Now, let us understand what is happening at the particle level. 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 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. Higher the melting point, stronger the force of attraction.

The melting point of ice is 273.15 Kelvin. The process of melting, that is, change of solid state into liquid state is also known as fusion.

Now, here is an important question: When a solid melts, its temperature remains the same, so where does the heat energy go? You must have observed that during the experiment of melting, 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. 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 latent heat. The word latent means hidden. The amount of heat energy that is required to change 1 kilogram 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 degrees Celsius have more energy as compared to particles in ice at the same temperature. This is because they have absorbed latent heat of fusion.

Similarly, 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 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 Kelvin, which is 100 degrees Celsius.

Can you define the latent heat of vaporisation? Do it in the same way as we defined latent heat of fusion. The latent heat of vaporisation is the amount of heat energy required to change 1 kilogram of a liquid into gas at atmospheric pressure at its boiling point. Particles in steam, that is, water vapour at 373 Kelvin, 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 students, we can say that the state of matter can be changed into another state by changing the temperature. Solid state, when heated, becomes liquid state. Liquid state, when heated, becomes gaseous state. And if we cool a gas, it becomes liquid, and if we cool a liquid further, it becomes solid.

Now, there are some substances that change directly from solid to gas without becoming liquid. Have you seen naphthalene balls? They slowly disappear without leaving any solid. This is because they undergo sublimation.

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. Now heat slowly and observe. What do you infer? The camphor changes directly from solid to gas without becoming liquid. This is called sublimation. Similarly, the direct change of gas to solid without changing into liquid is called deposition. You might have seen frost forming on cold surfaces — that is deposition.

Now, let us see the effect of change of pressure on the state of matter.

We have already learnt that the difference in various states of matter is due to the difference in the distances between the constituent particles. What will happen when we start putting pressure and compress a gas enclosed in a cylinder? Will the particles come closer? Do you think that increasing or decreasing the pressure can change the state of matter?

Applying pressure and reducing temperature can liquefy gases. Have you heard of solid carbon dioxide? It is stored under high pressure. Solid carbon dioxide gets converted directly into gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. It sublimates directly from solid to gas.

Thus, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid, or gas.

Now, let us answer some questions about change of state.

Question 1: Convert the following temperatures to celsius scale: (a) 300 K (b) 573 K

To convert Kelvin to Celsius, we subtract 273. So for 300 K, the temperature in Celsius is 300 minus 273, which equals 27 degrees Celsius. For 573 K, it is 573 minus 273, which equals 300 degrees Celsius.

Question 2: What is the physical state of water at: (a) 25°C (b) 100°C?

At 25 degrees Celsius, which is room temperature, water is in liquid state. At 100 degrees Celsius, water is at its boiling point, so it is in the process of boiling. At exactly 100 degrees Celsius, water can exist in both liquid and gaseous states — it is at its boiling point. But if we say water at 100 degrees Celsius, it is typically considered to be in liquid state (unless it is boiling vigorously, in which case it is converting to steam).

Question 3: For any substance, why does the temperature remain constant during the change of state?

This is because the heat energy supplied during the change of state is used to overcome the forces of attraction between the particles. This heat is called latent heat, and it does not cause a rise in temperature. The temperature remains constant until the change of state is complete.

Question 4: Suggest a method to liquefy atmospheric gases.

Atmospheric gases can be liquefied by applying high pressure and lowering the temperature. By applying high pressure, the gas molecules are brought closer together. By lowering the temperature, the kinetic energy of the molecules decreases, making it easier to liquefy the gas.

Now students, let us study one more important topic: Evaporation.

Do we always need to heat or change pressure for changing the state of matter? Can you quote some examples from everyday life where change of state from liquid to vapour takes place without the liquid reaching the boiling point? Water, when left uncovered, slowly changes into vapour. Wet clothes dry up. What happens to water in these examples?

We know that particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid, or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation.

Now, let us understand the factors affecting evaporation.

Activity 1.14: Take 5 millilitres of water in a test tube and keep it near a window or under a fan. Take 5 millilitres of water in an open china dish and keep it near a window or under a fan. Take 5 millilitres 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 we infer about the effect of temperature, surface area, and wind velocity on evaporation?

From this activity, we can see that the rate of evaporation increases with:

An increase of surface area: Evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying, we spread them out so that more surface area is exposed.

An increase of temperature: With the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state.

A decrease in humidity: Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases. That is why clothes take longer to dry on a humid day.

An increase in wind speed: It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding. This allows more evaporation to take place.

Now, how does evaporation cause cooling? In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cold.

What happens when you pour some acetone or nail polish remover on your palm? The particles gain energy from your palm or surroundings and evaporate, causing the palm to feel cool.

After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface.

Now, why should we wear cotton clothes in summer? During summer, we perspire more because of the mechanism of our body which keeps us cool. During evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body, leaving the body cool. Cotton, being a good absorber of water, helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.

Why do we see water droplets on the outer surface of a glass containing ice-cold water? Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets. This is called condensation.

Now, let us answer some questions about evaporation.

Question 1: Why does a desert cooler cool better on a hot dry day?

A desert cooler, also called a desert air cooler or swamp cooler, works on the principle of evaporation. On a hot dry day, the humidity is low, so the rate of evaporation is high. This causes more cooling. On a humid day, the air already has a lot of water vapour, so evaporation is slow, and the cooling effect is less.

Question 2: How does the water kept in an earthen pot become cool during summer?

An earthen pot has tiny pores in it. Water seeps through these pores and evaporates from the surface of the pot. This evaporation takes heat from the water inside the pot, making it cool. This is similar to how our body cools through perspiration.

Question 3: Why does our palm feel cold when we put some acetone or petrol or perfume on it?

When we put acetone, petrol, or perfume on our palm, it evaporates quickly. The particles of these substances gain energy from our palm and change into vapour. This loss of heat from our palm makes it feel cold.

Question 4: Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

When we pour hot tea or milk into a saucer, the surface area increases. This increases the rate of evaporation, which causes cooling. Also, the shallow layer in the saucer allows the liquid to cool faster due to convection currents. So the tea or milk in a saucer cools faster, and we can sip it more comfortably.

Question 5: What type of clothes should we wear in summer?

We should wear cotton clothes in summer. Cotton is a good absorber of water. It absorbs the sweat from our body and exposes it to the atmosphere for easy evaporation. This helps in cooling our body.

Now students, we have covered all the topics in this chapter. Let me now solve the exercises at the end of the chapter.

Exercise 1: Convert the following temperatures to the celsius scale. (a) 293 K (b) 470 K

To convert Kelvin to Celsius, we subtract 273. So 293 K minus 273 equals 20 degrees Celsius. And 470 K minus 273 equals 197 degrees Celsius.

Exercise 2: Convert the following temperatures to the kelvin scale. (a) 25°C (b) 373°C

To convert Celsius to Kelvin, we add 273. So 25°C plus 273 equals 298 K. And 373°C plus 273 equals 646 K.

Exercise 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.

(a) Naphthalene balls undergo sublimation. They change directly from solid to gas without passing through the liquid state. That is why they disappear with time without leaving any solid.

(b) The particles of perfume diffuse into the air and spread out. They can travel several metres because gases diffuse very fast due to the high speed of their particles and large spaces between them.

Exercise 4: Arrange the following substances in increasing order of forces of attraction between the particles — water, sugar, oxygen.

Oxygen is a gas, so the forces of attraction between its particles are very weak. Water is a liquid, so the forces of attraction are intermediate. Sugar is a solid, so the forces of attraction are very strong. So in increasing order of forces of attraction: oxygen, water, sugar.

Exercise 5: What is the physical state of water at — (a) 25°C (b) 0°C (c) 100°C?

At 25°C, water is in liquid state. At 0°C, water is at its freezing point, so it can exist as both ice and water. At standard atmospheric pressure, 0°C is the melting point of ice, so at 0°C, water can be in liquid or solid state depending on conditions. At 100°C, water is at its boiling point, so it can exist as both water and steam. At standard atmospheric pressure, 100°C is the boiling point of water, so at 100°C, water can be in liquid or gaseous state.

Exercise 6: Give two reasons to justify — (a) water at room temperature is a liquid. (b) an iron almirah is a solid at room temperature.

(a) Water at room temperature is a liquid because: first, it has no fixed shape — it takes the shape of the container. Second, it has a fixed volume. Third, the particles in water are not as closely packed as in solids, but they are held together by moderate forces of attraction.

(b) An iron almirah is a solid at room temperature because: first, it has a definite shape and fixed volume. Second, the particles in iron are closely packed and held together by strong forces of attraction. Third, it is rigid and does not flow.

Exercise 7: Why is ice at 273 K more effective in cooling than water at the same temperature?

Ice at 273 K and water at 273 K are at the same temperature. But ice requires latent heat of fusion to change into water. When ice melts, it absorbs this latent heat. So when we use ice for cooling, it absorbs heat both to raise its temperature (if needed) and to melt. Water at the same temperature does not have this extra absorbed energy. So ice at 273 K is more effective in cooling because it can absorb more heat as it changes state from solid to liquid.

Exercise 8: What produces more severe burns, boiling water or steam?

Steam produces more severe burns than boiling water. This is because steam contains latent heat of vaporisation. When steam condenses to form water, it releases this latent heat. So when steam touches our skin, it first condenses to water, releasing latent heat, and then cools down. This extra heat released by steam causes more severe burns than boiling water at the same temperature.

Exercise 9: Name A, B, C, D, E, and F in the following diagram showing change in its state.

Looking at the diagram described: There is a cycle showing state changes between solid, liquid, and gas. The arrows are labeled A, B, C, D, E, and F. The conditions are given as "Increase heat and decrease pressure" and "Decrease heat and increase pressure."

Let me describe the changes:

A: Solid to Liquid — This is melting or fusion. It occurs when we add heat to a solid.

B: Liquid to Solid — This is freezing or solidification. It occurs when we remove heat from a liquid.

C: Liquid to Gas — This is boiling or vaporisation. It occurs when we add heat to a liquid.

D: Gas to Liquid — This is condensation. It occurs when we remove heat from a gas.

E: Solid to Gas — This is sublimation. It occurs when we add heat to a solid directly, without it passing through liquid state.

F: Gas to Solid — This is deposition. It occurs when we remove heat from a gas directly, without it passing through liquid state.

So students, A is melting, B is freezing, C is boiling, D is condensation, E is sublimation, and F is deposition.

Now, let me give you a summary of everything we have learned in this chapter.

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. 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 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 kilogram 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 kilogram of solid into liquid at its melting point.

Some measurable quantities and their units to remember: Temperature is measured in kelvin (K), length in metre (m), mass in kilogram (kg), weight in newton (N), volume in cubic metre (m³), density in kilogram per cubic metre (kg m⁻³), and pressure in pascal (Pa).

Students, this concludes our lesson on Chapter 1: Matter in Our Surroundings. I hope you have understood all the concepts clearly. Remember, science is all around us — observe, question, and learn. Thank you for your attention, and see you in the next class.