Changes Around Us: Physical And Chemical

Hello students, welcome to today's science class. I am so happy to see you all here, ready to learn something new and exciting about the world around us. Today, we are going to study Chapter 5 from your science textbook, and the title of this chapter is "Changes Around Us: Physical and Chemical". This is a very interesting chapter because it talks about the changes that happen all around us, every single day, in our homes, in our schools, in nature, everywhere! So students, let us begin our journey of understanding these changes.

Now, students, before we start with the formal definitions and concepts, let me ask you a question. Have you ever kept an ice cube in a glass and left it for some time? What happens? Yes, the ice cube melts and becomes water. Similarly, have you ever seen a bud on a rose plant? One day it is a small bud, and the next day it has bloomed into a beautiful flower. And what about a bottle of cold water kept in a room for some time? Yes, it becomes warm after some time. And have you ever kept a banana for a few days? Initially it is yellow and good, but after some time, brown spots appear on it, and it starts to smell. These are all examples of changes that happen around us.

Now students, let us think about these changes more carefully. In each of these cases, something is changing. The ice cube changes from solid to liquid. The bud changes into a flower. The cold water becomes warm. The banana changes its colour and smell. These changes can be in the size, shape, smell, colour, or other properties of the substance or object. Can you think of some other changes that happen in your surroundings? Yes, there are many. Let us explore some of them together.

Now students, I want you to look at Table 5.1 in your textbook. This table lists some changes that we observe around us. Let me go through each one of them with you and think about what we observe in each case.

First, let us talk about melting ice cubes. When you keep ice cubes out of the freezer, they absorb heat from the surroundings and melt into water. The ice cube changes from a solid state to a liquid state. What do we observe? We observe that the solid ice becomes liquid water. The shape changes, the state changes, but does the substance itself change? No, ice and water are both made of the same substance, H2O. So the material remains the same, only its appearance changes.

Second, chopping vegetables. When you chop vegetables like carrots or potatoes, you cut them into smaller pieces. What do we observe? The vegetables change their shape and size, but are they still the same vegetables? Yes, they are still carrots or potatoes, just in smaller pieces. No new substance is formed.

Third, boiling water. When you heat water, it becomes steam or water vapour. This is a change from liquid to gas. The water changes its state, but it is still water. When the steam cools down, it can condense back into liquid water. So this change can be reversed.

Fourth, making popcorn from corn. When you heat corn kernels in a popcorn maker or a pan, they explode and become popcorn. This is an interesting change! The corn kernel changes completely in appearance, texture, and taste. Can you get the original corn kernel back from the popcorn? No, you cannot. This is a change that cannot be reversed.

Fifth, cutting a piece of paper. When you cut paper into smaller pieces, you change its shape and size, but it is still paper. However, can you join the pieces back together to get the original paper? Well, you can stick them together, but it will not be exactly the same as the original piece of paper. So this change is not easily reversible.

Sixth, adding beetroot extract to water. When you add beetroot extract to water, the water gets coloured. This is because the pigments from the beetroot dissolve in the water. But is any new substance formed? No, the beetroot extract and water are still the same substances, just mixed together. This is a physical mixing, not a chemical change.

Seventh, burning wood. When you burn wood, it produces ash, smoke, and gases. Can you get the original wood back from the ash? No, you cannot. This is a change that produces new substances. This is a chemical change.

Eighth, drying wet clothes. When you hang wet clothes in the sun, the water evaporates and the clothes become dry. The water changes from liquid to gas, but the water itself does not change. This is a physical change, and it can be reversed by adding water to the dry clothes.

Ninth, making small balls of dough. When you mix flour and water and knead it, you get dough. You can then make small balls from this dough. What is happening here? The flour and water are mixing together, and their physical properties change. But can you get back the original flour and water from the dough? Not easily, because when you mix flour and water, some chemical interactions also happen. But primarily, this is a physical change in terms of shape and size.

Tenth, rolling small balls of dough into chapatis. When you take a ball of dough and roll it flat and cook it on a tawa, you get a chapati. This involves both physical and chemical changes. The rolling is a physical change, but the cooking involves chemical changes because the heat causes chemical reactions in the dough, and new substances are formed.

So students, as you can see, there are many different types of changes happening around us. Some changes are simple, like melting ice or drying clothes. Some changes are more complex, like burning wood or cooking food. Now the question is, can we categorize these changes into different groups? Yes, we can! That is exactly what we are going to learn in this chapter.

Now students, let us move on to the first main concept of this chapter. We are going to learn about physical changes.

So students, let us do Activity 5.2 from your textbook. This activity will help us understand what physical changes are.

Part A of the activity is about creating some objects with paper. Take a few sheets of paper and fold them to create new objects, like paper boats, paper airplanes, or any other shape. Now, the question is, do you get the same paper back when you unfold these objects? Yes, you do! The paper is still paper. Only its shape has changed. No new substance has been formed. The paper is still the same paper, just in a different shape.

Part B is about playing with a balloon. Take a balloon and inflate it. Now, loosen your grip and let the air escape out. What happens? The balloon deflates. Do you get the uninflated balloon back? Yes, you do! The balloon is still the same rubber balloon, just in a different shape. Now take another balloon, inflate it, and grip the opening tightly. Now prick it with a pin. What happens? The balloon bursts! Will you be able to get the uninflated balloon back? No, you cannot. The balloon is broken and cannot be repaired. But wait, in the first case where the air just escaped, the balloon came back to its original shape. So what is the difference? In the first case, only the shape of the balloon changed temporarily. In the second case, the balloon was permanently damaged.

Part C is about crushing a piece of chalk. Take a small piece of chalk and crush it into powder. Can you get the chalk piece back from the powder? No, you cannot. The chalk has been broken into smaller pieces, but it is still chalk. However, you cannot join the powder back into a solid piece of chalk. So this change is not easily reversible.

Now students, is there any similarity in the changes listed in A, B, and C? Yes, there is! In all these changes, the materials remained the same. The paper is still paper. The rubber of the balloon is still rubber. The chalk is still chalk. Only their appearances changed. Only their physical properties like shape, size, or state changed. No new substance was formed.

You might also remember from your Grade 6 science textbook that water can exist in three different states: solid, liquid, and gas. Ice is solid water, water is liquid water, and steam is gaseous water. Water can change from one state to another when heat is added or removed. But in all these states, the substance is still water, H2O. No new substance is formed.

So students, let us define what a physical change is. A physical change is a change in which only the physical properties of a substance change, such as its shape, size, colour, or state. No new substance is formed in a physical change. The original substance can be recovered, at least in some cases. Examples of physical changes include melting of ice, boiling of water, cutting of paper, folding of cloth, and so on.

Now students, let us move on to the next concept. We are going to learn about chemical changes, which are very different from physical changes.

Now students, let us do Activity 5.3. This is a very interesting activity that will help us understand chemical changes.

Take two glass tumblers or small transparent bottles. Mark them A and B. Fill one-fourth of glass tumbler A with tap water and one-fourth of glass tumbler B with lime water. Now, blow air into each glass tumbler, one at a time, using separate straws. Be careful not to suck the water or lime water while doing this.

What do you observe in glass tumbler A? When you blow air into tap water, you see bubbles forming. These are just air bubbles. There is no change in the appearance of the water. The water remains clear. So no new substance is formed here. This is just a physical process of mixing air into water.

Now, what do you observe in glass tumbler B? When you blow air into lime water, bubbles form too, but something else happens as well. The lime water turns milky or cloudy! If you leave it for some time, a white substance settles at the bottom of the glass tumbler. This is very interesting! What does this tell us? It tells us that something new has formed. The lime water has reacted with the carbon dioxide in the air we exhaled, and a new substance has been created. This new substance is calcium carbonate, which is white in colour and does not dissolve in water. That is why the lime water appears milky. Along with calcium carbonate, a small amount of water is also formed.

So students, this is a chemical change! In a chemical change, one or more new substances are formed. The original substances react with each other and produce new substances with different properties.

This change can be represented as a chemical equation. Let me write it for you:

Calcium hydroxide plus Carbon dioxide reacts to form Calcium carbonate plus Water.

In simpler terms, lime water plus carbon dioxide gives calcium carbonate plus water.

Now students, the turning of lime water milky is also used as a test for carbon dioxide. If you want to test whether a gas contains carbon dioxide, you can pass it through lime water. If the lime water turns milky, then the gas contains carbon dioxide. This is a very useful test in chemistry.

Now students, let us do another activity to understand chemical changes better. This is Activity 5.4.

Take a teaspoonful of vinegar or lemon juice in a test tube. Add a pinch of baking soda to it. What do you observe? You would hear a fizzing bubbling sound and see gas bubbles forming. This is because a chemical reaction is happening between the vinegar and the baking soda. The vinegar contains acetic acid, and the baking soda is sodium hydrogen carbonate. When they react, they produce carbon dioxide gas, which appears as bubbles.

Now, pass this gas through freshly prepared lime water kept in another test tube. What do you observe? The lime water turns milky! What does this tell us? It tells us that the gas formed by mixing vinegar and baking soda is carbon dioxide. Because we know that carbon dioxide turns lime water milky.

So students, since a new substance, carbon dioxide, is formed in this reaction, we say that a chemical change has occurred. This reaction can be represented as:

Vinegar plus Baking soda gives Carbon dioxide plus Other substances.

Now students, I want you to think about this. What would happen if you mix baking soda with plain water instead of vinegar? Would you see any bubble formation? Let me tell you, if you mix baking soda with water alone, you will not see much bubbling. That is because baking soda needs an acid to react with to produce carbon dioxide. So this is a chemical change only when an acid is present.

Now students, let us summarize what we have learned so far. Physical changes are those in which only the physical properties of a substance change, and no new substance is formed. Chemical changes are those in which one or more new substances are formed through a chemical reaction.

Now, let us learn about some more examples of chemical changes.

Students, you must have seen that iron objects, like nails, gates, or bicycles, often get a brownish coating on them. This is called rust, and the process is called rusting. In the rusting of iron, which you might have studied in the chapter about metals and non-metals, a new brown-coloured substance called rust is formed. This rust is actually iron oxide. So rusting is a chemical change because it involves the formation of a new substance. The iron reacts with oxygen and moisture in the air to form iron oxide, which is rust.

Now students, let us talk about combustion. Combustion is another important type of chemical change. You might have seen a magnesium ribbon burning. When you burn a magnesium ribbon, it produces a bright white flame and a white powder. This white powder is magnesium oxide. So the burning of magnesium is a chemical change because a new substance, magnesium oxide, is formed. Heat and light are also produced along with the formation of the new substance.

The burning of magnesium can be represented as:

Magnesium plus Oxygen gives Magnesium oxide plus Heat plus Light.

Now students, what is combustion? Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that undergo combustion are called combustible substances. Examples of combustible substances include wood, paper, cotton, kerosene, LPG, and so on.

Now students, let us do an activity to understand whether oxygen is essential for combustion. This is Activity 5.5.

Place two identical candles on two separate petri dishes and light them. Now, cover one of these candles with a glass tumbler. What happens to the candle flames in the two cases?

The candle that is not covered continues to burn normally. But the candle that is covered with a glass tumbler stops burning after some time. Why does this happen? Because when the candle is covered, the oxygen inside the glass tumbler gets used up, and no fresh oxygen can enter. Once the oxygen is exhausted, the flame cannot continue burning. So oxygen is essential for combustion.

Now students, you can test for the presence of carbon dioxide in the glass tumbler. How? By adding a small amount of lime water to the petri dish. You will notice that the lime water turns milky. This carbon dioxide was formed by the carbon from the wax and the oxygen from the air. So we can conclude that oxygen is required for combustion.

Now students, I want to tell you something important about fire safety. If a person's clothes catch fire, what is the best way to extinguish the fire? You should wrap a blanket or cloth around the person. This cuts off the supply of air, and the fire gets extinguished. But be careful! You should never use synthetic blankets or cloths because they can melt and stick to the skin, causing more harm.

Now students, let me tell you a fascinating fact about nature. Have you ever seen insects emitting light in a garden or a field in the evenings? These insects are called fireflies. Their light is produced by a chemical change! This type of light production without heat in living organisms is called bioluminescence. Isn't that amazing?

Now students, let us think about this. We learned that combustible substances and oxygen are necessary for combustion. But is that enough? What about paper? Paper is a combustible substance, but we can keep it in the air for any length of time without it catching fire. Why is that? Because paper needs to be heated to a certain temperature before it can catch fire. Just having oxygen and a combustible substance is not enough. We need to provide heat to raise the temperature of the substance to its ignition temperature.

Now students, let us do Activity 5.6 to understand this better.

Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it. What happens? It quickly catches fire. This is because the temperature of the matchstick flame is higher than the ignition temperature of the paper.

Now take another piece of paper. Using a magnifying glass, focus the sunrays to make the smallest and brightest spot on the paper. Hold it there for some time. What do you observe? The paper starts to emit smoke, and then it catches fire. This is because focusing the sunrays heats the paper. The temperature of the paper increases with time. After some time, the paper becomes so hot that it reaches its ignition temperature and catches fire.

So students, what is ignition temperature? The ignition temperature is the minimum temperature at which a substance can catch fire. It is also called the flash point. Different substances have different ignition temperatures. For example, the ignition temperature of paper is lower than that of wood.

Now students, we can conclude that for combustion to occur, three things are necessary:

First, a combustible substance, also called fuel. Second, oxygen. Third, heat that allows the fuel to reach its ignition temperature. All three are essential for combustion to take place.

Now students, let us learn about an interesting question. Can physical and chemical changes occur in the same process? The answer is yes! Let me explain with an example.

What happens when a candle is lit? Let us think about this carefully. When you light a candle, the heat of the flame melts the wax near the wick. This melted wax is then drawn up the wick by capillary action. The heat of the flame vaporizes this liquid wax, and the vapour burns to produce a flame. So what is happening here?

The melting of wax is a physical change because the wax changes from solid to liquid, but it is still wax. The vaporization of wax is also a physical change because the liquid wax changes to gas, but it is still wax. The solidification of wax when it cools is also a physical change.

However, the burning of wax vapour is a chemical change because new substances are formed, like carbon dioxide, water vapour, and other gases. So the burning of a candle involves both physical and chemical changes. The physical changes are melting, vaporization, and solidification. The chemical change is the combustion of the wax vapour.

This is a very interesting example that shows that physical and chemical changes can occur in the same process.

Now students, let us talk about whether changes are permanent or not. Once something has undergone a change, can we get it back in its original form? Let us think about this.

Look at Table 5.2 in your textbook. It lists some changes and asks whether the original state can be brought back or not.

Melting ice cubes: Yes, we can refreeze the water to get ice back. So this change can be reversed.

Chopping vegetables: No, we cannot get the original vegetables back. The vegetables have been cut into pieces, and we cannot join them back. So this change cannot be reversed.

Boiling water: Yes, we can condense the steam to get water back. So this change can be reversed.

Making popcorn from corn: No, we cannot get the original corn kernel back from popcorn. So this change cannot be reversed.

Now students, what about the other changes? Let me explain a few more.

Stitching cloth to a shirt: This is a change that can be reversed. You can unstitch the shirt to get the cloth back. But it might not be exactly the same as before.

Twisting of straight string: This can be reversed. You can untwist the string to get it back to straight.

Making idlis from batter: This cannot be reversed. Once the batter is steamed to make idlis, you cannot get the batter back.

Dissolving sugar in water: This can be reversed. You can evaporate the water to get the sugar back.

Drawing water from a well: This can be reversed. You can put the water back into the well.

Ripening of fruits: This cannot be reversed. Once a fruit ripens, it does not become unripe again. Although we can slow down the ripening process, we cannot reverse it.

Boiling water in an open pan: This can be reversed. You can condense the steam to get water back.

Rolling up a mat: This can be reversed. You can unroll the mat.

Grinding wheat grains to flour: This cannot be reversed. Once the wheat is ground into flour, you cannot get the grains back.

Forming of soil from rocks: This cannot be reversed in a short time. It takes thousands of years for rocks to break down into soil.

So students, changes can be either reversible or irreversible. Reversible changes are those where we can get back the original substance or object. Irreversible changes are those where we cannot get back the original substance or object.

Now students, let us think about whether all changes are desirable or not. Many useful changes happen in our daily life. For example, the changing of milk into curd is a desirable change because we get curd from milk. Ripening of fruits is desirable because we eat ripe fruits. Cutting of fruits is desirable because it makes them easier to eat. Cooking of food is desirable because it makes food edible and tasty. All these are desirable changes.

But some changes may be undesirable. For example, rusting of iron is undesirable because it damages iron objects. Decay of food during storage is undesirable because the food becomes inedible. But wait, a change that is undesirable in some situations may be desirable in other situations. For example, decomposition of food is undesirable when we want to store food, but it is very useful in converting food waste into compost, which can be used as fertilizer for plants. So it depends on the context.

Some changes occurring over the years due to human activity can have a long-term environmental impact. For example, the increased consumption of fuels in cars, trains, and airplanes is increasing the amount of carbon dioxide in the atmosphere. This is leading to global warming. Drying of paint on walls, doors, and furniture releases many substances through evaporation, causing atmospheric pollution. So we need to be careful about the changes we cause.

Now students, let us learn about some slow natural changes that happen over long periods of time.

First, let us talk about weathering of rocks. Have you seen heaps of sand, soil, and stones lying at the base of mountains? These are called sediments. How are these formed? These are formed by physical changes that break up large rocks into smaller pieces. How does this happen? Temperature changes due to climatic conditions cause the rocks to expand and contract. Over time, this weakens the rocks and they break apart. The growing roots of trees can also break rocks. When water freezes in cracks in rocks, it expands and pushes the rocks apart. All these are physical changes.

Water or chemicals present in water can also react with the rocks and cause chemical changes in their composition. For example, there is a rock called basalt, which is black in colour and contains iron. When this rock is exposed to water or air containing water vapour for a long time, the iron in it reacts with oxygen and moisture to form iron oxide, which is red in colour. So the rock changes from black to red. This is a chemical change. These physical and chemical changes in rocks are collectively called weathering. Weathering eventually leads to the formation of soil.

Now students, let us talk about erosion. Have you noticed fine sand collecting on riverbeds or in lakes? This sand is formed when rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water. This process is called erosion. Erosion during a landslide is an example of a physical change. River rocks and pebbles often appear smoother due to the constant erosion caused by flowing water. When the speed of the water or wind decreases, such as in an ocean or a lake, the material transported during erosion settles down at the bottom. These sediments harden over time and become new rocks. Most of these changes take place over thousands of years and cannot be reversed.

Now students, let me summarize what we have learned in this chapter:

A physical change is one in which a substance or object undergoes a change in its physical properties, and no new substance is formed. Examples include melting of ice, boiling of water, cutting of paper, and so on.

A chemical change is one in which one or more new substances are formed. It involves a chemical reaction and can be represented by a chemical equation. Examples include burning, cooking, rusting, and so on.

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that undergo combustion are called combustible substances.

The lowest temperature at which a substance can catch fire is called its ignition temperature.

Some changes can be reversed, and some cannot be reversed.

Some changes are desirable, and some are not.

Rocks undergo physical and chemical changes due to weathering to form soil.

Erosion caused by flowing water and wind is a physical change.

Now students, let us move on to the exercises at the end of the chapter. I am going to solve each question for you, step by step.

Question 1: Which of the following statements are the characteristics of a physical change?

(i) The state of the substance may or may not change. (ii) A substance with different properties is formed. (iii) No new substance is formed. (iv) The substance undergoes a chemical reaction.

Options: (a) (i) and (ii) (b) (ii) and (iii) (c) (i) and (iii) (d) (iii) and (iv)

Now students, let us analyze each statement:

(i) The state of the substance may or may not change. In a physical change, the state can change, like ice melting to water, but it may not change, like cutting paper. So this is a characteristic of physical change.

(ii) A substance with different properties is formed. This is NOT true for physical changes. In physical changes, no new substance is formed, so no new properties are created. This is characteristic of chemical changes.

(iii) No new substance is formed. This is TRUE for physical changes. This is one of the main characteristics of physical changes.

(iv) The substance undergoes a chemical reaction. This is NOT true for physical changes. Chemical reactions happen in chemical changes, not in physical changes.

So the correct statements for physical changes are (i) and (iii). Therefore, the answer is option (c).

Question 2: Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these?

(i) Stitching cloth to a shirt (ii) Twisting of straight string (iii) Making idlis from a batter (iv) Dissolving sugar in water (v) Drawing water from a well (vi) Ripening of fruits (vii) Boiling water in an open pan (viii) Rolling up a mat (ix) Grinding wheat grains to flour (x) Forming of soil from rocks

Now students, let me answer each one:

(i) Stitching cloth to a shirt: This can be reversed. You can cut the stitches and separate the cloth from the shirt.

(ii) Twisting of straight string: This can be reversed. You can untwist the string to make it straight again.

(iii) Making idlis from a batter: This cannot be reversed. Once the batter is steamed, it becomes idli, and you cannot get the batter back.

(iv) Dissolving sugar in water: This can be reversed. You can evaporate the water to get the sugar back.

(v) Drawing water from a well: This can be reversed. You can pour the water back into the well.

(vi) Ripening of fruits: This cannot be reversed. Once a fruit ripens, it does not become unripe again.

(vii) Boiling water in an open pan: This can be reversed. You can condense the steam to get water back.

(viii) Rolling up a mat: This can be reversed. You can unroll the mat.

(ix) Grinding wheat grains to flour: This cannot be reversed. Once the wheat is ground into flour, you cannot get the grains back.

(x) Forming of soil from rocks: This cannot be reversed in a short time. It takes thousands of years for rocks to break down into soil, and we cannot reverse this process quickly.

Question 3: State whether the following statements are True or False. In case a statement is False, write the correct statement.

(i) Melting of wax is necessary for burning a candle.

This is True. For a candle to burn, the wax needs to melt first. The melted wax is drawn up the wick and then vaporizes and burns. So melting of wax is necessary for burning a candle.

(ii) Collecting water vapour by condensing involves a chemical change.

This is False. Condensation is a physical change. Water vapour changing to liquid water is a change in state, not a chemical change. No new substance is formed.

(iii) The process of converting leaves into compost is a chemical change.

This is True. When leaves decompose, they break down due to the action of bacteria and fungi. This involves chemical reactions that produce new substances. So it is a chemical change.

(iv) Mixing baking soda with lemon juice is a chemical change.

This is True. When baking soda reacts with lemon juice, which contains citric acid, carbon dioxide gas is produced. This is a chemical change because a new substance is formed.

Question 4: Fill in the blanks in the following statements:

(i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ______, and this is a ______ change.

The brown deposits are due to rusting, and this is a chemical change. Rusting is the formation of iron oxide on the surface of iron objects.

(ii) Folding a handkerchief is a ______ change and can be ______.

Folding a handkerchief is a physical change and can be reversed. You can unfold the handkerchief to get it back to its original shape.

(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called ______, and this is a ______ change.

A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.

(iv) Magnesium, when burnt in air, produces a substance called ______. The substance formed is ______ in nature. Burning of magnesium is a ______ change.

Magnesium, when burnt in air, produces a substance called magnesium oxide. The substance formed is basic in nature. Burning of magnesium is a chemical change.

Question 5: Are the changes of water to ice and water to steam, physical or chemical? Explain.

Students, both water to ice and water to steam are physical changes. Let me explain why.

When water freezes to form ice, it changes from liquid state to solid state. But the substance is still water, H2O. No new substance is formed. The chemical properties remain the same. This is a physical change.

Similarly, when water boils to form steam, it changes from liquid state to gaseous state. But again, the substance is still water, H2O. No new substance is formed. This is also a physical change.

Both of these changes involve a change in the physical state of water, but the chemical composition remains the same. So they are physical changes.

Question 6: Is curdling of milk a physical or chemical change? Justify your statement.

Students, curdling of milk is a chemical change. Let me justify this.

When milk curdles, it separates into curds and whey. The proteins in milk, casein, undergo a chemical change and form a solid substance called curds. New substances are formed. The process involves a chemical reaction, usually triggered by the addition of an acid like lemon juice or vinegar, or by the action of bacteria in milk. The properties of the new substances are different from the original milk. Therefore, curdling of milk is a chemical change.

Question 7: Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?

Students, the formation of soil from rocks involves both physical and chemical changes. Let me explain.

Physical changes: Wind, rain, and temperature changes can break rocks into smaller pieces. This is a physical change because the composition of the rocks does not change; they are just broken into smaller pieces.

Chemical changes: Rainwater can react with the minerals in the rocks and cause chemical changes. For example, water and carbon dioxide in the air can react with rocks to form new substances. This is a chemical change.

So the formation of soil from rocks involves both physical and chemical changes. These changes are collectively called weathering.

Question 8: Read the following story titled 'Eco-friendly Prithvi', and tick the most appropriate option(s) given in the brackets. Provide a suitable title of your choice for the story.

Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change). His efforts are appreciated by all his family members.

Now students, let me identify the changes:

Chopping vegetables, peeling potatoes, and cutting fruits: These are physical changes because the shape and size change, but no new substances are formed.

Collecting the seeds, fruits, and vegetable peels into a clay pot: This is a physical change because it is just collecting and storing, no chemical reaction happens.

Decomposition of fruits, vegetable peels, and other materials to form compost: This is a chemical change because bacteria and fungi break down the materials into new substances.

Germination of seeds and growth of plants into flowers: This is a chemical change because new substances are formed during the growth process, and biological processes are involved.

So the answers are:

Chopping vegetables, peeling potatoes, and cutting fruits: physical changes

Collecting the seeds, fruits, and vegetable peels into a clay pot: physical change

Decomposition to form compost: chemical change

Germination and growth of plants: chemical change

Now students, you need to provide a suitable title for the story. Some good titles could be "Eco-friendly Prithvi", "Turning Waste into Wealth", "Prithvi's Compost Garden", or "From Kitchen Waste to Beautiful Flowers".

Question 9: Some changes are given here. Write physical changes in the area marked 'A' and chemical changes in the area marked 'B'. Enter the changes which are both physical and chemical in the area marked 'C'.

Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.

Now students, let me categorize these changes:

Physical changes: Tearing of paper, Melting of ice, Folding of clothes

Chemical changes: Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar

Both physical and chemical: Process of burning a candle

Let me explain why burning a candle involves both physical and chemical changes. The melting of wax is a physical change, and the burning of wax vapour is a chemical change. So it involves both.

Question 10: The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why?

The experiments are:

(a) Vinegar and baking soda with lime water (b) Lemon juice and vinegar with lime water (c) Vinegar and common salt with lime water (d) Lemon juice and baking soda with lime water

Now students, lime water turns milky when carbon dioxide is passed through it. So we need to identify which of these combinations produce carbon dioxide.

(a) Vinegar and baking soda: Vinegar contains acetic acid, and baking soda is sodium hydrogen carbonate. When they react, they produce carbon dioxide gas. So when this gas is passed through lime water, it will turn milky.

(b) Lemon juice and vinegar: Both are acids. When mixed together, they do not produce carbon dioxide. So the lime water will not turn milky.

(c) Vinegar and common salt: Vinegar is acetic acid, and common salt is sodium chloride. When mixed, they do not produce carbon dioxide. So the lime water will not turn milky.

(d) Lemon juice and baking soda: Lemon juice contains citric acid, and baking soda is sodium hydrogen carbonate. When they react, they produce carbon dioxide gas. So when this gas is passed through lime water, it will turn milky.

So lime water turns milky in cases (a) and (d). This is because in both these cases, an acid reacts with baking soda to produce carbon dioxide, which turns lime water milky.

Now students, we have completed all the exercises. Let me also tell you about some exploratory projects that you can try at home.

First, you can write a message on a piece of paper using lemon juice as ink and let it dry. The message will become invisible. Now use a warm iron over the paper, or hold the paper over the flame of a candle, taking care that it does not catch fire. The invisible letters turn dark brown as the paper gets warm. This is because the heat causes the lemon juice to undergo a chemical change and turn brown. Can any of these changes be reversed? No, once the letters appear, they cannot be made invisible again. So this is an irreversible change.

Second, you can discuss what steps we can take to reduce landslides and rock erosion in hilly regions. Some measures include planting trees to hold the soil, avoiding construction on steep slopes, and building retaining walls.

Third, observe the activities going on in the kitchen and note any changes that can be reversed. Are these physical or chemical changes? For example, melting of butter is a physical change, cooking of food is a chemical change, freezing of water is a physical change, and so on.

Fourth, you can do the experiment with yeast. Yeast is added while baking bread to make it fluffy and soft. Yeast is a fungus that produces carbon dioxide gas through fermentation. When you mix yeast with sugar and water, the yeast feeds on the sugar and produces carbon dioxide gas. This gas makes the dough rise and become fluffy. When you pass this gas through lime water, it turns milky, confirming that carbon dioxide is produced. This experiment involves both physical and chemical changes. The production of gas is a physical change, and the fermentation process is a chemical change.

Finally, chameleons change colour to blend in with their surroundings and also when they are angry or sense danger. This is a change that can be reversed because the chameleon can change its colour back and forth. This is a biological process controlled by special cells called chromatophores.

Now students, we have covered the entire chapter thoroughly. Let me give you a complete summary of everything we have learned in this lesson.

In this chapter, we learned about changes around us. We started by observing various changes in our daily life, such as melting ice, chopping vegetables, boiling water, and so on. We learned that these changes can be categorized into physical changes and chemical changes.

Physical changes are those in which only the physical properties of a substance change, such as shape, size, colour, or state. No new substance is formed. Examples include melting of ice, boiling of water, cutting of paper, folding of clothes, and so on. Some physical changes can be reversed, like freezing water into ice, and some cannot be reversed, like cutting paper.

Chemical changes are those in which one or more new substances are formed through a chemical reaction. Examples include burning, rusting, cooking, curdling of milk, and so on. Chemical changes cannot be reversed to get the original substances back.

We learned about combustion, which is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. We learned that for combustion to occur, three things are necessary: a combustible substance, oxygen, and heat to reach the ignition temperature.

We learned that some processes involve both physical and chemical changes, like the burning of a candle. The melting of wax is a physical change, and the burning of wax vapour is a chemical change.

We learned about reversible and irreversible changes. Reversible changes can be reversed to get the original substance back, like melting and freezing of water. Irreversible changes cannot be reversed, like burning wood or cooking food.

We learned about desirable and undesirable changes. Some changes are useful to us, like cooking food and ripening of fruits. Some changes are harmful, like rusting and decay of food.

Finally, we learned about slow natural changes like weathering and erosion, which help in the formation of soil from rocks over thousands of years.

Students, this is the end of our lesson. I hope you have understood all the concepts clearly. Remember to revise this chapter thoroughly and practice the exercises. Thank you for listening attentively. See you in the next class!