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

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

Welcome to the Chapter 6 - Physical and Chemical Changes, Class 7 Science NCERT Solutions page. Here, we provide detailed question answers for Chapter 6 - Physical and Chemical Changes. The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.

Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics Physical and Chemical Changes and excel in their exams. By going through these Physical and Chemical Changes question answers, you can strengthen your foundation and improve your performance in Class 7 Science. Whether you’re revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.

Exercise 1

Q 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

(a) Photosynthesis - Chemical change
Those changes in which new substances are formed, are called chemical changes. Photosynthesis is a chemical change because chemical reaction occur in it which is completed in the presence of sunlight.

(b) Dissolving sugar in water - Physical change
The physical changes are temporary changes which can be easily reversed to form the original substances. Physical changes are those changes which can be easily reversed by physical methods: evaporization and condensation.

(d) Melting of wax - Physical change

(e) Beating aluminium to make aluminium foil - Physical change
(f) Digestion of food - Chemical change

Q 10:

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

Rusting of iron occurs faster in coastal area as compared to the deserts because in deserts less moisture is present.

Q 11:

The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist 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.

(ii) Process - B is a chemical change

LPG is present in liquid form in the cylinder and when it comes in the contact of environment it converts into gaseous form which is a physical change. But When it burns, it converts in the from of heat and light, then this is a chemical change; this cannot be reversed.

Q 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.

(iii) Both process A and B are chemical change.
Anaerobic bacteria are those which works in the absence of oxygen. Burning the biogas as fuel is a chemical change because it is an irreversible process. Anaerobic bacteria digest animal waste and produce biogas as the final product in the absence of oxygen. Biogas is a natural gas, which is produced by bacteria.

Q 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)

(a) False
This staement is false because cutting the log of wood into pieces is a physical change. Here, no new substances are formed.

(b) False
This staement is false because the formation from manure from leaves is not a physical change. It is the chemical change which involves decomposers and some microorganisms.

(c) True
This staement is true because the coating of zinc protect iron pipe from rusting. It protects the iron pipes to come in contact with air and moisture.

(d) False
This staement is false because iron and rust are the different substances, because rusting occurs when iron is oxidized. Iron is (Fe) and rust is (Fe₂O₃).

(e) True
This staement is true because condensation of steam is a physical change. Here, no new substances are formed.

Q 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 re called physical changes.
(e) Changes in which new substances are formed are called _________ changes.

(a) Calcium carbonate (CaCO₃)
CO₂ that is Carbon dioxide reacts with Ca(OH)₂ (Calcium hydroxide) that is lime water, to form a white precipitate of CaCO₃ (Calcium carbonate).

(b) Sodium hydrogen carbonate
The chemical name of baking soda is (NaHCO₃) Sodium hydrogen carbonate (Sodium bicarbonate) .Sodium bicarbonate comes with sodium, acid and hydrogen. Sodium carbonate is diprotic and a strong base compound.

(c) Painting and galvanization Metals which cannot wear easily are melted into their molten form and are used to cover metals which rust easily. The surface of iron or metal are coated with paint or grease. Galvanization is the process of applying a protective layer of zinc on a metal. It is a very common method of preventing the rusting of iron.

(d) Physical
The change in which physical properties of the substance or a material change, this is called physical change. In physical changes, no new substances are formed.

(e) Chemical
The changes in which cannot be seen by naked eyes are called chemical changes. The chemical changes cannot be reversed.

Q 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.

When baking soda (Sodium bicarbonate) is mixed with lemon juice (citric acid), bubbles formed because of evolution of carbon dioxide gas. When baking soda is mixed with citric acid (lemon juice), formation of bubble is occur. These bubbles are formed due to the presence of carbon dioxide presence and this shows the chemical change.

Lemon Juice + Baking soda (Sodium bicarbonate) → Carbon dioxide + other substances

Q 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.

When a candle burns both physical and chemical changes takes place. When a candle is burnt, melting of wax occurs; this is a physical change. And, the burning of wax shows the chemical change.

Digestion of food is the familiar process which shows both chemical and physical changes. In starting, breaking down of large food particles into the smaller ones shows the physical change. The process of digestion of food is a chemical change in which many new substances are formed.

Q 6:

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

Setting of curd is a chemical change because this process cannot be reversed, we cannot obtain milk from this again. Both have different nature. Setting of curd is done by the Lacto Bacillus bacteria, this shows setting of the curd is a chemical change.

Q 7:

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Burning of wood and cutting it into small pieces are two different type of changes. When we burn wood, the new substance 'coal' is formed; it is a chemical change and this is not reversible. Whereas, cutting of wood into small pieces is considered as physical change because there is no formation of new substance occurs. Physical change means change in the physical properties of the substance. Here, cutting of wood into small pieces changes the size and the shape of the wood.

Q 8:

Describe how crystals of copper sulphate are prepared.

Crystals of copper sulphate can be obtained by crystallization method. The formation of crystals are as follows:

1. Take some water in beaker.

2. Now add few drops of sulphuric acid to it.

3. Heat the water until its boiling point.

4. Now add copper sulphate powder to it with constant stirring.

5. Add the copper sulphate powder till the saturation point is obtained.

6. Now, filter the solution into a china dish and allowed it to cool.

7. The solution should be left undisturbed for 24 hours.

8. After 24 hours, separate the crystals carefully.

Q 9:

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

Painting prevents rusting of iron. The process of depositing a layer of zinc on iron called galvanization. Painting prevents iron from the moisture and oxygen present in the air. When iron comes in contact with the oxygen and the moisture from the environment. it gets oxidised and makes ferric oxide (Fe₂O₃).

Frequently Asked Questions about Physical and Chemical Changes - Class 7 Science

- 1. How many questions are covered in Physical and Chemical Changes solutions?
- All questions from Physical and Chemical Changes are covered with detailed step-by-step solutions including exercise questions, additional questions, and examples.
- 2. Are the solutions for Physical and Chemical Changes helpful for exam preparation?
- Yes, the solutions provide comprehensive explanations that help students understand concepts clearly and prepare effectively for both board and competitive exams.
- 3. Can I find solutions to all exercises in Physical and Chemical Changes?
- Yes, we provide solutions to all exercises, examples, and additional questions from Physical and Chemical Changes with detailed explanations.
- 4. How do these solutions help in understanding Physical and Chemical Changes concepts?
- Our solutions break down complex problems into simple steps, provide clear explanations, and include relevant examples to help students grasp the concepts easily.
- 5. Are there any tips for studying Physical and Chemical Changes effectively?
- Yes, practice regularly, understand the concepts before memorizing, solve additional problems, and refer to our step-by-step solutions for better understanding.

Exam Preparation Tips for Physical and Chemical Changes

The Physical and Chemical Changes is an important chapter of 7 Science. This chapter’s important topics like Physical and Chemical Changes are often featured in board exams. Practicing the question answers from this chapter will help you rank high in your board exams.

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