Particulate Nature Of Matter

Namaste students, welcome to today's science class. I am so happy to see you all here, ready to learn something fascinating about the world around us. Today, we are going to study Chapter 7 from your NCERT Science book, and the title is "Particulate Nature of Matter." This is a really important chapter because it will help you understand what everything around you is made of at a very fundamental level. Are you ready to explore? Let's begin!

Now, before we start with the actual content of the chapter, I want you to think about some interesting questions. The chapter begins with something called "Probe and Ponder" - these are questions that make you wonder and think deeply. Let me read them out to you.

First question: Why is it possible to pile up stones or sand, but not a liquid like water? Think about this for a moment. You can definitely make a heap of stones or sand, can't you? But if you try to pile up water, what happens? It just flows away, doesn't it? We'll understand why this happens.

Second question: Why does water take the shape of folded hands but lose that shape when released? This is such a common observation. When you cup water in your hands, it takes the shape of your hands. But as soon as you open your hands or let the water fall, it loses that shape and becomes droplets. Why does this happen?

Third question: We cannot see air, so how does it add weight to an inflated balloon? This is really interesting. When you blow air into a balloon, it becomes heavier. But we can't even see the air! How can something invisible add weight? We'll find out.

Fourth question: Is the air we breathe today the same that existed thousands of years ago? This is a thought-provoking question. The air around us - is it old air or new air? We'll explore this too.

And finally, there's a question about where pebbles, stones, and sand come from. You might have collected pebbles and stones from the sand while playing on a riverbank or a beach. Where do these come from? Let me explain this to you.

In the mountains, rocks gradually break down due to erosion - this is the process where wind, water, and other natural forces wear away rocks over time. Rivers flowing through these regions carry along the eroded rock pieces. As the rivers flow, they continue to break down the rocks further into pebbles, stones, sand, and transport large quantities of them to the plains. The bigger rocks are eventually broken down into finer grains of sand and clay. Now, here comes the really important question: Is this grain the smallest unit of a bigger rock, or can these grains of sand and clay be broken down further? This is exactly what we are going to learn in this chapter. Let's find out!

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## 7.1 What Is Matter Composed of?

Now, let's start with the main question: What is matter composed of? Matter means anything that has mass and occupies space. Everything around you - this book, the desk, the air, water, everything is matter. But what is matter made of? Let's explore this through some activities.

### Activity 7.1: Let us explore

Students, I want you to imagine doing this activity with me. Take a stick of chalk - you can find chalk in your school or at a stationery shop. First, look at the whole piece of chalk. Now, break it into two pieces. What do you get? Two smaller pieces of chalk, right? Continue breaking the chalk till it becomes difficult to break it further by hand. You will have small pieces now. Now, take these small pieces and grind them using a mortar and pestle - you might have seen this in your kitchen or in a science lab. After grinding, observe the fine powder of chalk with a magnifying glass. What do you observe?

You will see that each tiny grain you observe is still a speck of chalk. It looks like chalk, it feels like chalk, everything about it says chalk. So, here's the important question: Is every speck of this fine chalk powder still composed of the same substance, or has it changed into something else on breaking or grinding?

Think about what you learned in your previous grade in the chapter "Changes Around Us: Physical and Chemical." Is grinding chalk a physical change or a chemical change? You learnt that the chalk does not change into a new substance on grinding. It is a physical change in which only the size of each speck of chalk has reduced further. The substance remains chalk, just in smaller pieces.

Now, here's something fascinating. These specks of chalk powder can be broken further into smaller particles by further grinding. Let us imagine that this process of grinding continues. Eventually, we would reach a stage where the chalk particles cannot be broken down any further. The tiny units obtained at this stage are the basic building blocks that the chalk was made up of.

Now, here's another question for you: Are the units of chalk obtained in this manner considered the smallest units of chalk? The answer is yes! This means that one whole piece of chalk was made up of a large number of smaller units. These units are called constituent particles of chalk. A constituent particle is the basic unit that makes up a larger piece of a substance or material. Just like chalk, the grains of sand and clay are not the smallest units of bigger rocks. These are also made up of a large number of their constituent particles. Isn't that amazing?

Let us explore further! Let me ask you another question. Do you remember the dissolution of sugar into water to form a solution? What happens to sugar when it is dissolved in water? Let's do another activity to understand this better.

### Activity 7.2: Let us perform

Now students, this is an activity you should perform under the supervision of a teacher or an adult. Remember, safety first! Never eat or drink anything unless asked to by your teacher or parents.

First, fill a glass tumbler with drinking water. Now, put two teaspoons of sugar into it. Do not stir the water. Taste a small spoonful of water from the top layer. Does the water taste sweet? Most likely, it does not taste sweet at all because the sugar hasn't dissolved yet.

Now, stir the water until the sugar dissolves completely. You will see the sugar gradually disappear into the water. Again, taste a spoonful of water from the top layer. What difference in taste do you notice? Does it taste sweet? Yes, it does! The water now tastes sweet.

Now, here's the interesting part. Since the top layer of water tastes sweet after dissolving sugar, it must be present in the solution. But do you observe any sugar particles in the solution? You cannot see any sugar particles, can you? The sugar has disappeared, but its sweetness is still there. This means the sugar has broken down into such tiny particles that they cannot be seen even with your eyes. Sugar particles can no longer be observed but their presence can be sensed by taste. When sugar dissolves in water, it breaks up into its constituent particles which cannot be broken down further. Each tiny grain of sugar is made up of millions and millions of such constituent particles.

So students, what have Activities 7.1 and 7.2 taught us? These activities support the idea that matter is composed of a large number of extremely small particles. These particles are so small that they cannot be seen even through an ordinary microscope. That's why we call this the "particulate nature of matter" - matter is made of tiny particles!

But now you might ask: But, where did the sugar go? The tiny sugar particles separate and occupy the available spaces between the water particles. These spaces between the particles are known as interparticle spaces. This is a very important concept, so remember it - interparticle spaces are the spaces between the particles of a substance.

Now, here's another question. Chalk and sugar can both be broken down into their constituent particles. But how are the constituent particles held together to form the solid pieces we see? What keeps them together? This is what we are going to learn next.

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## 7.2 What Decides Different States of Matter?

The constituent particles of matter are held together through forces which are attractive in nature. These forces are called interparticle attractions. The strength of these attractions depends on the nature of the substance and the interparticle distance. Even a slight increase in the distance decreases the interparticle forces drastically. The strength of these forces ultimately decides the physical state of the substances. This is a very important concept, so let me explain it once more.

Every substance is made up of tiny particles. These particles attract each other - this attraction is what keeps them together. But the strength of this attraction is not the same for all substances. It depends on two things: first, the nature of the substance itself, and second, how far apart the particles are. If the particles are very close together, the attraction is strong. But if the particles move even a little bit apart, the attraction becomes much weaker. This strength of attraction between particles is what determines whether a substance is a solid, a liquid, or a gas. We'll see how this works in a while.

### Our Scientific Heritage

Now, before we go further, I want to tell you about something really interesting. Do you know that since ancient times, people have been thinking about how far things could be broken down and what matter is made up of? This is not a new idea at all! People have been wondering about this for thousands of years.

In India, there was a philosopher named Acharya Kanad who first spoke about the idea of a Parmanu, which is the ancient Indian word for atom. He believed that matter is made up of tiny, indivisible eternal particles called Parmanu. This idea was written in his work called Vaisheshika Sutras. Acharya Kanad lived a long time ago, but his ideas were so good that they are still relevant today! So students, the concept we are studying today has its roots in ancient Indian philosophy. Isn't that something to be proud of?

Now, let us explore how these attractions vary in different states.

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### 7.2.1 Solid State

How are constituent particles held together in solids? Let's find out through an activity.

#### Activity 7.3: Let us find out

Collect a few solid objects, such as a piece of iron or an iron nail, a piece of rock salt, a stone, a piece of wood, a key, and a piece of aluminium. Observe their shapes and sizes. Try hammering them. In which of the above six objects do you think particles are strongly held together?

You must have noticed that all these objects are solids. They have a definite shape and volume. This is due to the fact that in solids, the particles are tightly packed and the interparticle attractions are very strong. These strong forces of attraction hold the particles in fixed positions, preventing them from moving freely. The particles can only move to and fro about their positions - they can vibrate or oscillate - but they cannot move past each other. Think of it like this: imagine a room full of people standing very close together, holding hands tightly. They can wiggle a little bit, but they cannot move from their positions or pass each other. That's exactly what happens in a solid!

Now, here's an important question: In the solid state, is there any way to move these particles apart? The answer is yes! When solids are heated, their particles vibrate more vigorously. You know that when you heat something, the particles in it move faster, right? This is because heat gives them energy. So, when a solid is heated, its particles start vibrating more vigorously. A stage is reached when these vibrations become so vigorous that the particles start leaving their respective positions. The interparticle forces of attraction get weakened and the solid gets converted into the liquid state. The temperature at which this happens is the melting point of the solid.

So students, when you heat a solid like ice, the particles gain energy, vibrate more strongly, and eventually break free from their fixed positions to become a liquid. That's exactly what happens when ice melts into water!

The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. Different substances have different melting points. Generally, in a liquid state, particles are somewhat farther away from each other as compared to those in the solid state. But remember, ice is an exception - its particles are actually farther apart than those in water! That's why ice floats on water.

Some solids have weak interparticle forces of attraction, so their melting points are low. While others have strong attractive forces and have high melting points. Let me give you some examples from Table 7.1 in your book.

Table 7.1 shows melting points of some solids. Ice melts at 0°C, which is why it melts so easily at room temperature. Urea melts at 133°C, which is much higher than ice. And iron, which is a very hard and strong solid, has a melting point of 1538°C! That's extremely high. This tells us that iron particles are held together with very strong attractive forces, which is why it's so difficult to melt iron.

Now students, here's a question: Solids have a definite volume; what about liquids and gases? We'll find out!

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### 7.2.2 Liquid State

Let's explore the properties of liquids through an activity.

#### Activity 7.4: Let us try and find out

Take three clean and dry containers of different shapes. Label them A, B, and C. Mark the 200 mL level in each container using a marker or by pasting a thin strip of paper. Fill Container A with water up to the marked level. Carefully transfer the water from Container A to Container B without spilling, and observe the shape and level of the water. Now, transfer the same water from Container B to Container C, carefully, and observe the shape and its level again.

What do you notice? You will notice that the water takes the shape of the container into which it is poured. So, we can say that liquids do not have a fixed shape and take the shape of the container they are kept in. This happens because the particles of liquids are free to move. In all three containers, the water level remains at 200 mL and no change in volume is observed. Hence, we can say that liquids have a definite volume. However, if a container is not clean, some water may stick to its walls, causing the water level in the next container to be slightly less than 200 mL after pouring.

So students, Activity 7.4 shows that the particles of liquids can move freely, but only within a limited space. Therefore, we can infer that liquids have no fixed shape but have a fixed volume.

Now let us compare interparticle forces of attraction in liquids and solids. Take some water in a shallow vessel and try to move your finger through it. Are you able to move your finger through the water? Yes, you can! You can move your finger through water without breaking or cutting it permanently, which cannot be done in the case of solids. When you try this, you are temporarily displacing water. As soon as you remove your finger, the position of the water is restored. We can say that in liquids, the interparticle attractions are slightly weaker than in solids, but still strong enough to keep the particles close together.

Now, recall what you learned in your Grade 6 chapter "Temperature and Its Measurement" about the temperature of boiling water. When a liquid is heated, a stage comes when it starts boiling. The temperature at which a liquid boils and turns into vapour at atmospheric pressure is called its boiling point. The movement of particles becomes so vigorous that they move apart from each other, resulting in a decrease in the interparticle forces of attraction. Eventually, the constituent particles can escape from the liquid state. The liquid is converted into vapour or the gaseous state.

At the boiling point, the formation of vapour is very fast and occurs not only at the surface but also within the liquid. This process is observed as bubble formation in the liquid. However, vapour formation occurs at all temperatures, even below the boiling point, though slowly and only at the surface. This slower process is known as evaporation, about which you have learnt in earlier grades.

So students, to summarize: liquids have no fixed shape but have definite volume. Their particles can move around but within a limited space. The interparticle attractions are weaker than in solids but stronger than in gases.

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### 7.2.3 Gaseous State

Now, let's learn about gases. Do gases also have a fixed volume? Let's find out through an activity.

#### Activity 7.5: Let us investigate

Safety first! Take two transparent gas jars or glass tumblers and mark them A and B. Create some smoke by burning an incense stick. Hold the Gas Jar A upside down over the smoke. The gas jar should trap the smoke inside. Turn it over and cover it with a glass plate. Hold another Gas Jar B upside down and gently place it over the glass plate covering the Gas Jar A. Remove the glass plate slowly and ensure that both gas jars are close enough and there is no gap for smoke to escape. Observe how the smoke spreads inside the Gas Jar B.

What do you see? The smoke fills the entire space in the Gas Jar B, indicating that gases do not have a fixed volume and tend to occupy the entire available space. Like liquids, they also acquire the shape of the vessel they are in.

This illustrates that the particles in gases move freely in all directions and the interparticle attractions are negligible. As a result, gases do not have a fixed shape or volume.

In this activity, smoke is used to represent the gaseous state. The tiny particles of smoke suspended in the air are constantly hit by invisible particles of gases, and their movement helps us observe the motion of gas particles.

This activity can also be demonstrated by using iodine vapour instead of smoke from incense sticks. Iodine vapour can be obtained by placing some solid iodine in a closed gas jar for some time. But remember, be careful while using solid iodine. Vapours of iodine can cause irritation.

So students, both liquids and gases flow and do not retain a fixed shape. These properties distinguish them from solids and classify them as fluids. Fluids are substances that can flow - both liquids and gases are fluids.

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## 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

What role does the interparticle spacing play in determining the properties of each state - solid, liquid, and gas? Let us perform the following activities to find answers to these questions.

### Activity 7.6: Let us experiment

Take a syringe without a needle. Pull the plunger of the syringe outwards in a fully extended position. Place your thumb over the open end of the syringe to prevent the air present inside the syringe from escaping. Push the plunger slowly and steadily inward. What do you observe?

As you do this, you will notice that the volume of air inside the syringe decreases. What can we say about the behaviour of gas in the syringe? When you compress the air by pushing the plunger, the particles are forced to come closer. This shows that the gas particles have a lot of space between them in their natural state, and this space can be reduced by applying external pressure. If you stop pushing the plunger, the gas particles spread, and the plunger moves back to its original position. This is because gases are elastic - they try to bounce back to their original volume.

Now, repeat this activity using water and observe. You would observe that water is practically incompressible. You cannot easily reduce its volume by pushing the plunger. This tells us that water particles are already very close together, with very little space between them.

Let us perform another activity to learn about the interparticle spaces in liquids.

### Activity 7.7: Let us observe

Take a glass vessel, fill it about half with water, and mark the level of water A. Add two teaspoons of sugar into it. Mark the new water level on the glass vessel B. Stir the water with a glass rod to dissolve the sugar. Predict whether the water level will increase or decrease with respect to the mark B. Mark this water level again as C.

What difference do you observe in the water levels? You will observe that initially, when sugar is added, the level of water increases, but after dissolution, it may decrease to some extent. Since the volume of the solution is less than the sum of the volumes of water and sugar, it indicates that there is some space between the water particles. The particles of the dissolved substance occupy these spaces.

This is really interesting! When sugar dissolves, its particles fit into the spaces between water particles. That's why the total volume doesn't increase as much as you might expect. In fact, it might even decrease slightly because the sugar particles fill up the empty spaces between water particles.

Now, repeat Activity 7.7 with some other soluble solids, such as common salt or glucose, and insoluble solids, like sand and stone pieces. What do you observe in each case? Do the sand particles dissolve? Does the volume of water in the vessel change after mixing, and why?

Sand is a solid that does not dissolve in water. When added to water, the sand particles settle down and occupy some space in the container, causing the total volume to increase. Unlike sugar, sand particles do not break down into smaller particles that can fit into the spaces between water molecules. They just sit there, taking up space.

Now, what do you think about the interparticle spacing in solids? You learnt earlier that the constituent particles in solids are held together by strong forces of attraction. So, these particles do not move from one place to another and are closely packed. However, despite close packing, some space is left between the particles. You might assume that the space between particles is filled with air, but this is not the case. They contain nothing at all - it's just empty space.

Figure 7.12 in your book summarizes the packing of particles and the interparticle spacing in the three states of matter. In solids, particles are very close together with minimal space. In liquids, particles are a little farther apart than in solids. And in gases, particles are very far apart with lots of empty space between them.

### A Step Further

Now students, I want to tell you something important. Often, we use the term "particle" in different contexts. The meaning of this term changes with the context. For example, while talking about air pollution, the term Suspended Particulate Matter (SPM) is used. This term refers to the tiny dust particles suspended in air and not the constituent particles of matter which are extremely small as compared to the dust particles. In fact, even these tiny dust particles are also made up of a very large number of constituent particles, that is, atoms and molecules. So, when we talk about the particulate nature of matter, we are talking about the tiniest building blocks of matter - atoms and molecules - which are much, much smaller than even dust particles.

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## 7.4 How Particles Move in Different States of Matter?

Let us find out about the movement of particles in the three states of matter.

### Activity 7.8: Let us experiment

Safety first! Do not touch potassium permanganate with your hands. Use a spoon or a spatula to handle it. Take a glass tumbler containing water and put a few grains of potassium permanganate into it. What do you observe?

Initially, you will see some streaks of pink colour spreading out from the grain. With the passage of time, the entire bulk of water will acquire a uniform pink colour. Do you know why this happens?

This happens because the water particles are in constant motion. First, they pull out the particles of potassium permanganate from its grain, and later they hit these particles so that they get spread throughout the liquid. In the case of many substances, the constituent particles are held together so strongly that the water particles are unable to pull these out. Such substances, like sand, are insoluble in water.

This is a beautiful demonstration of how particles move in liquids. The water particles are always moving, and as they move, they bump into the potassium permanganate particles, causing them to spread throughout the water. This process is called diffusion.

## Think Like a Scientist

Now, let me ask you to try something. Take three clean glass tumblers. Pour hot water in one of them, water at room temperature in the second, and ice-cold water in the third. Drop a small grain of potassium permanganate into each of them. Watch carefully and compare. What do you observe?

You will see that water particles move faster in hot water compared to water at room temperature, and even slower in ice-cold water. As a result, the potassium permanganate spreads the fastest in hot water, less quickly in water at room temperature, and the slowest in ice-cold water. Hence, the movement of particles increases when heat is provided. This is because heat gives more energy to the particles, making them move faster.

Now, how can we demonstrate the movement of gas particles that cannot be seen with the naked eye? Let's do another activity.

### Activity 7.9: Let us find out

Light an incense stick in one corner of the room. Wait for a few minutes and observe. Do you notice the fragrance from a distance?

When an incense stick is burnt in one corner of the room, initially, the fragrance is felt only around the incense stick. Shortly, you can smell the fragrance throughout the room. This happens because the particles of the fragrance spread, filling the entire room. This shows that the particles of air are moving constantly. The air particles hit the particles of the fragrance and help them spread throughout the room.

Oh! Now I know why and how the fragrance of perfume reaches us. Can you share a few other real-life situations where you have experienced the movement of gas particles?

Think about it - when someone sprays a perfume in one corner of a room, you can smell it everywhere. When someone cooks food, the aroma spreads throughout the house. When a room is ventilated, the stale air is replaced by fresh air. All of these happen because gas particles are constantly moving and spreading.

## Ever Heard Of...

Now, here's something really interesting. The particulate nature of matter plays a crucial role in many everyday processes. For example, when we wash clothes stained with oil using soap, numerous soap particles surround the oil particles on the fabric. One end of the soap particle attaches to the oil, and the other mixes with water, thus helping lift the oil off and wash it away. This is how soap works at the particle level! The soap molecules have one end that loves water (hydrophilic) and one end that loves oil (hydrophobic). The oil-loving end attaches to the oil stain, and the water-loving end attaches to water, helping wash away the oil.

Based on our learnings from the chapter, we can say that matter is made up of small particles which are held together by the force of attraction. The strength of attractive forces between particles depends on the distance between them, which in turn depends on their thermal (heat) energy. Thus, it is the thermal energy of the particles that determines the physical state of matter. In the solid state, the thermal energy of particles is low, so they remain close to each other and experience strong interparticle attractive forces. This restricts their motion to only small vibrations.

At the melting point, the thermal energy is used to overcome the attractive forces between particles, allowing the solid to change into a liquid. At this stage, the particles can move away from their fixed positions. The interparticle distance increases slightly, reducing the strength of the attractive forces to a level that allows the particles to move around, though still within a limited space. In the gaseous state, the particles have enough energy to overcome the forces of attraction between them and move freely in all directions. You will learn more about these particles that constitute matter in higher grades.

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## Snapshots

Let us wrap up what we have learned with some important points:

- Matter is composed of extremely small particles. - The particles are held together by interparticle forces of attractions. - The interparticle attractions are the strongest in solids, a little weaker in liquids, and the weakest in gases. - Solids have a fixed shape and size due to strong interparticle attraction, minimum interparticle space, and no free movement of the constituent particles. - The interparticle attraction in liquids is slightly weaker than in solids, enabling the particles to move within a particular space and providing them with a little more interparticle spacing. Therefore, liquids have a definite volume but no fixed shape. - The interparticle attractions in gases are negligible, making their particles completely free to move from one place to another and resulting in maximum interparticle space. Therefore, gases have no fixed shape and volume.

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## Keep the Curiosity Alive

Now students, it's time to solve some questions. I want you to listen carefully as I explain each question.

### Question 1

Choose the correct option. The primary difference between solids and liquids is that the constituent particles are:

(i) closely packed in solids, while they are stationary in liquids. (ii) far apart in solids and have fixed position in liquids. (iii) always moving in solids and have fixed position in liquids. (iv) closely packed in solids and move past each other in liquids.

Let me analyze each option:

Option (i): In solids, particles are closely packed, but in liquids, they are not stationary - they can move within a limited space. So this is incorrect.

Option (ii): In solids, particles are closely packed, not far apart. So this is incorrect.

Option (iii): In solids, particles are not always moving - they only vibrate in their positions. They are not moving freely like in liquids. So this is incorrect.

Option (iv): This is correct! In solids, particles are closely packed, while in liquids, particles can move past each other within a limited space. That's the primary difference between solids and liquids.

So the correct answer is (iv).

### Question 2

Which of the following statements are true? Correct the false statements.

(i) Melting ice into water is an example of the transformation of a solid into a liquid.

This is TRUE. Ice is a solid, and when it melts, it becomes water, which is a liquid.

(ii) Melting process involves a decrease in interparticle attractions during the transformation.

This is TRUE. During melting, the particles gain enough energy to overcome the strong attractive forces that held them in fixed positions in the solid state. So the interparticle attractions decrease.

(iii) Solids have a fixed shape and a fixed volume.

This is TRUE. Solids maintain their shape and volume because their particles are tightly packed and held strongly together.

(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.

This is TRUE. In solids, particles are closely packed with very little space between them, and the attractive forces are very strong.

(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.

This is TRUE. Camphor, when heated, vaporizes and its particles spread throughout the room due to the movement of gas particles.

(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.

This statement needs correction. The smell is not "released" as energy. What happens is that when camphor is heated, it gains thermal energy and changes from solid to gas directly (this process is called sublimation). These gas particles then spread through the air, and when they reach our nose, we detect them as a smell. So the correct version would be: "On heating, we are adding energy to the camphor, which changes it into gas, and the gas particles spread to create the smell."

### Question 3

Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?

(i) Nothing will change. (ii) The chair will weigh less due to lost particles. (iii) Nothing of the chair will remain.

Let me think about this carefully. A chair is made of matter, and matter is made of constituent particles. If we could remove all the constituent particles from a chair, what would be left? Nothing! The chair would cease to exist. So the correct answer is (iii) Nothing of the chair will remain.

The justification is that the chair is made up of particles. If we remove all the particles, there will be nothing left - no matter, no chair. This is like saying if you remove all the bricks from a wall, nothing of the wall remains.

### Question 4

Why do gases mix easily, while solids do not?

This is a great question! Gases mix easily because their particles are far apart and move freely in all directions. When two different gases come together, their particles can easily intermingle and spread throughout the available space. This process is called diffusion, and it happens quickly in gases.

On the other hand, in solids, particles are tightly packed and cannot move freely. They are held in fixed positions by strong attractive forces. Therefore, solids cannot mix easily. If you put two solid substances together, they might react chemically over a long time, but they don't simply mix like gases do. Even if you grind two solids together, they don't truly mix at the particle level - they just become a mixture of small pieces.

### Question 5

When spilled on the table, milk in a glass tumbler, flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.

This is a wonderful observation! Let me explain why this happens.

Milk is a liquid. Liquids do not have a fixed shape - they take the shape of the container they are in. When milk is spilled on the table, there is no container holding it, so it flows and spreads out, taking the shape of the flat surface of the table. This is because the particles in a liquid can move freely within a limited space.

On the other hand, the glass tumbler is a solid. Solids have a fixed shape and volume. The particles in a solid are tightly packed and held in fixed positions. They cannot move freely or change the shape of the solid. That's why the glass tumbler stays in the same shape even when milk is spilled on the table next to it.

So, the key difference is that liquids can flow and change shape, while solids maintain their shape.

### Question 6

Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.

Now students, I cannot draw an actual diagram here since this is an audio lesson, but I will describe it in detail so you can draw it yourself or understand what the diagram should look like.

First, let's talk about ice (solid water). In ice, water particles are arranged in a specific pattern called a crystal lattice. The particles are closely packed together and can only vibrate in their positions. They are held together by strong attractive forces.

As ice melts and turns into water (liquid), the particles gain energy and start moving more freely. They break free from their fixed positions in the crystal lattice. The interparticle distance increases slightly - in fact, ice is an exception because its particles are actually farther apart than liquid water! As heat is added, the ice structure breaks down, and water particles can move around within a limited space.

As water is heated further and turns into water vapour (gas), the particles gain even more energy. They move very fast and far apart from each other. The interparticle distance becomes very large, and the attractive forces become negligible. The particles move freely in all directions and fill the entire available space.

So in your diagram, you should show: - For ice: particles closely packed in a regular pattern, only vibrating - For water: particles somewhat loosely packed, moving within limited space - For water vapour: particles far apart, moving freely in all directions

### Question 7

Draw a picture representing particles present in the following: (i) Aluminium foil (ii) Glycerin (iii) Methane gas

Again, I will describe these for you so you can draw them.

(i) Aluminium foil: Aluminium is a metal, so it is a solid. In your picture, you should show many aluminium atoms closely packed together in a regular arrangement. The particles should be very close to each other with minimal space between them. They should be shown vibrating in their positions but not moving around.

(ii) Glycerin: Glycerin is a liquid. It is a viscous, thick liquid. In your picture, you should show glycerin molecules that are somewhat close together but not as close as in solids. They should be shown moving within a limited space, sliding past each other. The interparticle spacing should be a bit more than in solids.

(iii) Methane gas: Methane is a gas - it is the main component of natural gas. In your picture, you should show methane molecules that are far apart from each other. They should be moving rapidly in all directions. There should be lots of empty space between the particles. The particles should be shown bouncing off each other and the walls of the container.

### Question 8

Observe Figure 7.16a which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Figure 7.16b showing the arrangement of particles.

Now students, I don't have the actual figures to show you, but let me explain what this question is about.

When a candle burns, the heat melts the wax. The wax near the flame is in liquid state. The wax in the middle part of the candle, which hasn't been heated much, is still solid. And the wax vapour that rises from the flame is in gaseous state.

So in Figure 7.16a, you would see: - The solid part of the candle (still in its original shape) - The liquid wax pool around the wick - The wax vapour rising up (which might look like smoke)

In Figure 7.16b, you would see particle arrangements: - For solid wax: closely packed particles that can only vibrate - For liquid wax: somewhat loosely packed particles that can move within limited space - For gas wax: far apart particles moving freely

You need to match these correctly.

### Question 9

Why does the water in the ocean taste salty, even though the salt is not visible? Explain.

This is a great question! You all know that ocean water is salty, but if you look at ocean water, you cannot see any salt crystals in it. Why is that?

The answer lies in the particulate nature of matter. Salt (sodium chloride) dissolves in water. When salt dissolves, it breaks down into its constituent particles - sodium ions and chloride ions. These particles are so tiny that they cannot be seen with the naked eye. They spread out and occupy the spaces between water particles.

So, even though we cannot see the salt, its particles are still there, dispersed throughout the water. When we taste ocean water, our taste buds detect these salt particles, which is why it tastes salty. This is similar to how sugar disappears in water but still makes the water sweet.

### Question 10

Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.

This is a tricky question! At first glance, it might seem like rice and rice flour are liquids because they take the shape of the container. But that's not correct.

Rice grains and rice flour are actually solids. Here's why: when you pour rice into a jar, it settles at the bottom and takes the shape of the jar only in the sense that the top surface becomes flat. But each individual grain of rice maintains its own shape - it doesn't flow like a liquid. The grains just pile up on top of each other.

The key difference is that in a liquid, the particles can move freely and flow. In rice, the particles (grains) cannot move freely - they are stuck in place once they settle. They don't flow or spread out on their own. If you tilt the jar, the rice doesn't flow like water would - it just slides or rolls slowly.

So rice and rice flour are solids, not liquids, even though they might appear to take the shape of the container. They are granular solids, and their behavior is different from true liquids.

---

## Discover, Design, and Debate

Now let's look at some activities and discussion points from the chapter.

### Activity 1

Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what will happen?

When you put a bottle with a balloon on its neck into hot water, several things happen. First, the air inside the bottle gets heated. When air is heated, its particles gain energy and move faster, spreading apart. This causes the air to expand. Since the balloon is flexible, it will inflate as the warm air pushes into it. This demonstrates that gases expand when heated - this is called thermal expansion.

### Activity 2

Design and create simple models to represent particles of solids, liquids, and gases showing interparticle spacing using clay balls, beads, etc.

This is a fun activity! You can use clay balls or beads to represent particles. For a solid, pack the balls tightly together in a container - they should be touching each other. For a liquid, put the balls in a container but leave some space between them - they should be loosely packed. For a gas, put just a few balls in a very large container so they are far apart. This will help you visualize the difference in interparticle spacing.

### Activity 3

Pretend to be particles of solids, liquids, and gases, at different temperatures - create and perform a role-play/dance showing particles in motion.

This is a creative activity! You can have students represent particles. At low temperature (cold), students should stand close together and only wiggle slightly (representing solid). At medium temperature, they can move around within a small area (representing liquid). At high temperature, they can run around freely in the entire room (representing gas). This will help you understand how temperature affects particle movement.

### Activity 4

Debate in the class - "Gases can spread and fill all the available space". Is this property of gases beneficial or harmful?

This is an interesting debate topic. Let me give you some points to consider.

Benefits of gases filling available space: - We can smell perfumes and food aromas because gas particles spread and reach our nose - Air fills our lungs completely, allowing us to breathe - Gases help in spreading seeds, pollen, and spores for reproduction - The atmosphere surrounds the Earth, protecting us from harmful radiation

Harmful aspects: - Air pollution spreads easily because pollutants can travel long distances - Toxic gases can spread and affect large areas - Greenhouse gases spread throughout the atmosphere, contributing to global warming - Foul smells spread quickly

---

## A Step Further

Now students, I want to tell you about something more advanced that you'll learn in higher grades.

The tiny particles that make up all matter are atoms and molecules. For example, iron is made up of atoms of iron, and gold is made up of atoms of gold. Atoms of many elements like hydrogen, oxygen, and sulfur are not able to exist independently. In such cases, a certain number of atoms of the same element combine to form a molecule. For example, two atoms of hydrogen combine and form a stable particle called a molecule of hydrogen. A water molecule is made up of two hydrogen atoms and one oxygen atom. You will learn about atoms and molecules in higher grades.

So, the constituent particles we have been talking about in this chapter are actually atoms and molecules. This is the foundation for understanding chemistry, which you will study in detail in Classes 9 and 10.

---

## Summary

Now students, we have come to the end of this chapter. Let me give you a complete summary of everything we have learned today.

We started with the question: What is matter composed of? And we learned that matter is made up of extremely small particles called constituent particles. These particles are so small that they cannot be seen even through an ordinary microscope.

We learned about three states of matter: solid, liquid, and gas.

In solids, particles are closely packed with minimum interparticle spacing. The interparticle attraction is maximum, and particles can only vibrate in their positions. Solids have definite shape and volume.

In liquids, particles are somewhat loosely packed with a little more interparticle spacing than in solids. The interparticle attraction is slightly weaker than in solids, allowing particles to move within a limited space. Liquids have definite volume but no fixed shape - they take the shape of the container.

In gases, particles are far apart with maximum interparticle spacing. The interparticle attraction is minimum or negligible, allowing particles to move freely in all directions. Gases have no fixed shape or volume - they fill the entire available space.

We learned about interparticle spaces - the empty spaces between particles. These spaces are minimum in solids, more in liquids, and maximum in gases.

We learned about how particles move in different states. In solids, particles only vibrate. In liquids, particles move within a limited space. In gases, particles move freely in all directions. We also learned that the movement of particles increases with temperature - this is why hot water diffuses faster than cold water.

We learned about melting point - the temperature at which a solid changes to liquid - and boiling point - the temperature at which a liquid changes to gas.

We also learned about evaporation - the slow formation of vapour at temperatures below the boiling point, occurring only at the surface.

We explored how dissolution works - when sugar dissolves in water, its particles break down into tiny particles that occupy the spaces between water particles.

We learned about diffusion - the spreading of particles from an area of higher concentration to lower concentration. This happens in liquids and gases due to the movement of particles.

We learned about the ancient Indian philosopher Acharya Kanad who first spoke about Parmanu (atoms), showing that Indian scientists thought about these concepts thousands of years ago.

We also learned about some real-life applications, like how soap works to clean oil stains, and why ocean water tastes salty.

This chapter forms the foundation for understanding the particle nature of matter, which is essential for learning chemistry in higher classes. The concepts of atoms, molecules, and the three states of matter will be built upon in your future studies.

Thank you for listening so attentively. Keep curious, keep questioning, and keep learning. Namaste!