NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes

NCERT Solutions for Class 7 Science Chapter 6

Class 7 Science includes three main branches: Biology, Chemistry, and Physics. Among these, Physics often feels a bit difficult for students because it involves understanding various concepts related to physical and chemical changes, their reactions, and examples. One such important chapter that students need to focus on is Chapter 6, Physical and Chemical Changes, as it builds the base for higher classes and helps develop a clear understanding of how substances change in nature. 

The CBSE Class 7 Science Chapter 6 Physical and Chemical Changes includes topics such as differences between physical and chemical changes, reversible and irreversible changes, rusting of iron, crystallisation, and various examples from daily life that make learning interesting. This chapter helps students observe and relate science to real-life situations. 

That is why it becomes very important to first study the NCERT textbook thoroughly and then solve the NCERT questions carefully, because most of the exam questions are based directly on these concepts. Go through the provided article, as PW has brought NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes for students preparing for the CBSE Class 7 exam in an easy and simple way, so that everyone can understand the chapter properly and score well in their exams.

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NCERT Solutions for Class 7 Science Chapter 5 Exercise 1

Question 1: Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminium foil

(f) Digestion of food

Answer: (a) Photosynthesis : Chemical change. Photosynthesis involves the conversion of carbon dioxide and water into glucose and oxygen, a transformation of substances into entirely new ones through chemical reactions.

(b) Dissolving sugar in water : Physical change. This process involves the sugar particles dispersing throughout the water but does not change the chemical makeup of sugar itself.

(c) Burning of coal : Chemical change. Burning coal involves a reaction with oxygen, resulting in the release of heat, light, carbon dioxide, and other byproducts, fundamentally altering the composition of coal.

(d) Melting of wax : Physical change. When wax melts, it changes from a solid to a liquid state without altering its chemical composition.

(e) Beating aluminum to make aluminum foil : Physical change. The process of beating aluminum alters its shape and form but doesn't change its chemical composition. However, the process of making aluminum foil involves rolling and thinning, which can be classified as a type of chemical change due to mechanical and thermal actions affecting the metal.

(f) Digestion of food : Chemical change. Digestion involves the breakdown of complex food molecules into simpler ones through chemical reactions, transforming food into nutrients that can be absorbed by the body.

Question 2: State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

(a) Cutting a log of wood into pieces is a chemical change. (True/False)

(b) Formation of manure from leaves is a physical change. (True/ False)

(c) Iron pipes coated with zinc do not get rusted easily. (True/False)

(d) Iron and rust are the same substances. (True/False)

(e) Condensation of steam is not a chemical change. (True/False)

Answer: (a) False

Cutting a log of wood into pieces is a physical change.

(b) False

Formation of manure from leaves is a chemical change.

(c) True

(d) False

Iron and rust are different substances. Rust is iron oxide Fe2O3

(e) True

Question 3: Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.

(b) The chemical name of baking soda is _________.

(c) Two methods by which rusting of iron can be prevented are _________ and ____ ____.

(d) Changes in which only _____ __ properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called _________changes.

Answer: (a) When carbon dioxide is passed through limewater, it turns milky due to the formation of calcium carbonate (CaCO 3 ) .

(b) The chemical name of baking soda is sodium hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are galvanization and painting.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called chemical changes.

Question 4: When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Answer: When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. The bubbles are formed due to the evolution of carbon dioxide gas.

This is a chemical change. In this change, citric acid contained in lemon juice reacts with sodium hydrogen carbonate which results in the evolution of carbon dioxide gas. Lemon juice + Baking soda → Carbon dioxide + Other substances Citric acid Sodium Hydrogen carbonate

Question 5: When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Answer: When a candle burns, both physical and chemical changes take place.

Physical change → Melting of wax Chemical change → Burning of wax Eating of food is another example where both physical and chemical changes occur simultaneously. Physical change → Breaking down of larger food particles into smaller particles Chemical change → Digestion of food

Question 6: How would you show that setting of curd is a chemical change?

Answer: Once the curd is formed, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.

Question 7: Explain why burning of wood and cutting it into small piece are considered as two different types of changes.

Answer: When we burn wood, a new substance, coal, is formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance is formed. Therefore, it is a physical change.

Question 8: Describe how crystals of copper sulphate are prepared.

Answer: Crystals of copper sulphate are prepared by the method of crystallization. The process is as follows. A cupful of water is taken in a beaker. Few drops of dilute sulphuric acid are added to this. The water is then heated and when it starts boiling, copper sulphate powder is added with stirring. Copper sulphate powder should be added on till the solution becomes saturated. It is then filtered into a china dish and allowed to cool. The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.

Question 9: Explain how painting of an iron gate prevents it from rusting.

Answer: Rusting is aided by both moisture (water) and air (oxygen). By painting an iron gate, we prevent its contact from the air and moisture present in the atmosphere. Hence, rusting is prevented.

Question 10: Explain why rusting of iron objects is faster in coastal areas than in deserts.

Answer: Both air and moisture are required for rusting to take place. In coastal areas, the quantity of moisture present in air is more than that in deserts. In desert areas, the amount of moisture in air is even lower. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

Read More: NCERT Solutions for Class 7 Science Chapter 1

Question 11: The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change − A) then it burns (Change − B). The following statements pertain to these changes. Choose the correct one.

(i) Process − A is a chemical change.

(ii) Process − B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Answer: (ii) Process − B is a chemical change.

Question 12: Anaerobic bacteria digest animal waste and produce biogas (Change − A). The biogas is then burnt as fuel (Change − B). The following statements pertain to these changes. Choose the correct one.

(i) Process − A is a chemical change.

(ii) Process − B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Answer: (iii) Both processes A and B are chemical changes.

Extended  Learning

Question 1: Describe two changes that are harmful. Explain why you consider them harmful? How can you prevent them?

Answer: (i) Rusting: If a piece of iron is open for some time, it acquires a film of brownish substance. This substance is called  rust and the process is called rusting.

Rusting is harmful because it destroys the iron objects. Iron is the most  widely used metal and so rusting is such a serious problem. Prevention of rusting: Rusting can be prevented by preventing iron articles from coming in contact with oxygen, or water, or both. • One simple way is to apply a coat of paint or grease. In fact, these coats should be applied  regularly to prevent rusting . • Another way is galvanization, i.e., to deposit a layer of metal like chromium or zinc on iron.

(ii) Spoilage of food: Food item when kept carelessly, get spoiled. This is a chemical change and obviously harmful for us.

Food is spoiled by microorganisms.

Prevention of food spoilage: Microorganisms do not survive at high or low  temperature. So, food items stored in refrigerator do not spoil. Also we should keep them covered so that microorganisms do not get any chance to enter and spoil them.

Question 2: Take three glass bottles with wide mouths. Label them A,B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same   boiled water and of the amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.

Answer: The nails in bottles B rust  a little, nails in A are the most rusted and that in C remain unchanged. For rusting both water and oxygen are necessary. Both the factors are present in the bottle A since oxygen is dissolved  in water. In bottle B, water is boiled and hence dissolved air is removed. Due to lack of oxygen, iron nails do not rust much. In bottle C, the layer of oil present prevents dissolving of atmospheric air in the water and hence no rusting occurs.

Question 3: Prepare crystals of alum.

Answer: A cupful of water  taken in beaker and a few drops of dilute sulphuric acid are added into it. The water is heated. When it starts boiling alum powder is added slowly while stirring continuously. Alum powder is added continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of alum slowly form at the bottom of the beaker.

Question 4: Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why ?

Answer: The different fuels used for cooking are wood, charcoal, cow-dung cake, kerosene, biogas, LPG, etc. Among all these, biogas and  LPG are least polluting. Both of these burn completely and do not give smoke. Also they do not  leave any residue (ash, unburnt part, etc.)

Read More: NCERT Solutions for Class 7 Science Chapter 2

Important Topics to Learn in Class 7 Science Chapter 6

Class 7 Science Chapter 6, Physical and Chemical Changes, is one of the most interesting chapters in the syllabus. It helps students understand how different materials around us change their form or nature. Some changes are temporary while others are permanent. 

To understand the chapter easily, students should study the textbook carefully and then practise the Class 7 Science Chapter 6 question answer section from their NCERT book.

1. Introduction to Changes Around Us

This topic explains that changes happen everywhere around us. For example, the melting of ice, the tearing of paper, or the rusting of iron. Some of these can be reversed, while others cannot. 

The chapter teaches how these changes can be divided into two types: physical changes and chemical changes. This part is also useful while practising the Class 7th Science Chapter 6 Physical and Chemical Changes question answer section.

2. Physical Changes

A physical change happens when a substance changes its form, shape, size, or state but does not form a new substance. For example, water turning into ice or steam is a physical change because the substance remains water.

 Similarly, cutting wood or dissolving salt in water are also physical changes. This topic forms the base for many questions in Class 7 Science Chapter 6 Question Answer Physical and Chemical Changes, so learning it properly is important.

3. Chemical Changes

A chemical change is a change where one or more new substances are formed. For instance, burning of wood, cooking of food, or rusting of iron. These changes cannot be reversed easily. 

The chapter also explains that chemical changes are often noticed by changes in colour, smell, or the release of gas. For example, when vinegar reacts with baking soda, a gas is released, which shows that a chemical change has occurred.

4. Difference Between Physical and Chemical Changes

The difference between physical and chemical changes is one of the key points in this chapter.

• In a physical change, no new substance is formed, and it is usually reversible.

• In a chemical change, a new substance is formed, and it is usually irreversible.

5. Rusting of Iron

Rusting is one of the most important examples of a chemical change. When iron comes in contact with air and moisture, it forms a reddish-brown layer called rust. The chapter explains that both air and water are necessary for rusting to occur.

It also discusses methods to prevent rusting, such as painting, oiling, or galvanising metal surfaces. This topic is frequently asked in exams, so students must prepare it well from the Class 7 Science Chapter 6 question answer Physical and Chemical Changes section.

6. Crystallisation

Crystallisation is another key topic in this chapter. It is the process of forming pure crystals from a solution. For example, salt crystals can be obtained by evaporating saltwater. The process helps to remove impurities and is used in industries for purifying substances. 

7. Changes in Everyday Life

This section connects the concepts with daily experiences. For example, ripening of fruits, rusting of iron objects, melting of wax, or burning a candle are all examples of different types of changes.

8. Activities and Experiments

The NCERT book includes several simple experiments that make learning interesting. Activities like burning a magnesium ribbon, dissolving substances in water, and observing rusting help students see both physical and chemical changes in action.

Check Out: PW Stationery

Class 7 Science Chapter 6 FAQs

Q1. What is the main topic of Class 7 Science Chapter 6?

Class 7 Science Chapter 6 is about Physical and Chemical Changes. It explains how different substances change their form or composition and helps students understand the difference between temporary and permanent changes.

Q2. What is a physical change?

A physical change is a change in which no new substance is formed. Only the state, shape, or appearance of a substance changes. 

Q3. What is a chemical change?

A chemical change is a change in which one or more new substances are formed. For example, burning of paper, rusting of iron, and cooking of food.

Q4. What is the difference between a physical and a chemical change?

In a physical change, the original substance remains the same, and the change can usually be reversed. In a chemical change, a new substance is formed, and the change cannot be easily reversed.

Q5. What are the important topics covered in Class 7 Science Chapter 6?

The important topics include introduction to changes, physical changes, chemical changes, differences between them, rusting of iron, crystallisation, and examples from daily life.