Class 7 Science Chapter 1 – Physical and Chemical Changes Around Us – All Textual Solutions | ASSEB Assam (English Medium)

Class 7 Science Chapter 1 Physical and Chemical Changes Around Us Solutions | ASSEB Assam | English Medium

Class 7 Science (Amar Bijan) Chapter 1 – Physical and Chemical Changes Around Us Complete Textual Question Answers and Solutions (ASSEB Assam – English Medium)

The first chapter of the Class 7 Science (English Medium) syllabus, ‘Physical and Chemical Changes Around Us,’ is a foundational topic for students. Based on the new ASSEB (Assam State School Education Board) guidelines and the latest National Education Policy (NEP) framework according to the current syllabus, we have prepared comprehensive textual question answers for this chapter. This special guide includes very short answers (VSA), short questions, long descriptive answers, multiple-choice questions (MCQs), and additional questions. Brought to you by Ospin Academy, these solutions are accurate, easy to understand, and highly exam-oriented.

In ‘Physical and Chemical Changes Around Us’, students explore the vital differences between reversible physical changes and irreversible chemical reactions. Understanding concepts like rusting of iron, crystallization, and the properties of different substances is crucial for exam preparation. Our textual solutions not only cover the textbook exercises but also provide extra important questions and MCQs to ensure students have a thorough grasp of the topic.

What you will learn and get from these textual solutions:

• Clear definitions and examples distinguishing physical and chemical changes.
• 1-mark objective questions (VSA), MCQs, and detailed 2-3 mark short questions tailored for exams.
• Step-by-step explanations for 4-5 mark long questions, including chemical equations and processes like crystallization.
• A wide range of additional questions and MCQs specifically designed according to the new syllabus pattern.
• Comprehensive insights into everyday chemical changes, such as rusting and digestion, explained in simple English.

Special Features of Ospin Academy’s Notes:

• Prepared strictly according to the new ASSEB syllabus and current educational guidelines.
• 100% accurate and written in simple, easy-to-memorize English (Class 7 Science Notes).
• Includes quick revision notes, highly beneficial for last-minute exam preparation.
• Exclusive compilation of extra important questions and MCQs to maximize your scores.

Aiming for academic excellence, Ospin Academy provides these comprehensive textual solutions to aid students in their studies. Start your preparation today and secure the highest marks in Class 7 Science.

 SCERT Assam, Class 7: Science

Lesson-1                                                                                   Ospin Academy

Physical and Chemical Changes Around Us

EXERCISE 

1. Darken the circle of the correct answer.
(i) Which of the following is an example of a physical change?
(a) Burning of wood
(b) Rusting of iron
(c) Melting of ice
(d) Milk turning sour
Answer: (c)

(ii) Which of the following shows a chemical change?
(a) Stretching a rubber band
(b) Boiling of water
(c) Burning of magnesium ribbon
(d) Cutting paper into pieces
Answer: (c)

(iii) Which of the following changes cannot be reversed?
(a) Freezing of water
(b) Dissolving sugar in water
(c) Boiling of water
(d) Burning of paper
Answer: (d)

(iv) Which of the following can prevent or slow down rusting of iron objects?
(a) Storing in a humid place
(b) Painting or oiling the surface
(c) Washing with water often
(d) Exposing to rain
Answer: (b)

2. Which of the following statements are the characteristics of chemical change?
(i) New substance are formed
(ii) No new substance is formed
(iii) It is usually irreversible
(iv) Only the state of matter changes
(a) (i) and (iii) 
(b) (ii) and (iv) 
(c) (i) and (ii)
(d) (iii) and (iv)
Answer: (a) (i) and (iii) 

3. Which of the following changes can be reversed and which cannot be.
(a) Melting of ice cubes
(b) Burning of paper
(c) Inflating a balloon
(d) Digestion of food
(e) Evaporation of water from wet clothes
(f) Rusting of iron
(g) Stretching of a rubber band
(h) Baking a cake
(i) Sharpening a pencil
(j) Freezing water to make ice
Answer:
Reversible changes : (a), (c), (e), (g), (j)
Irreversible changes : (b), (d), (f), (h), (i)

4. State whether the following are True or False. Correct the false one.
(a) Freezing of water is a chemical change.
Answer: False The transformation of water into ice is considered a physical change.

(b) Burning of wood produces new substances like ash, carbon dioxide, and water vapour.
Answer: True 

(c) Tearing of paper is a physical change.
Answer: True 

(d) Boiling water to form steam is a chemical change.
Answer: False 

The conversion of boiling water into steam represents a physical change. 

5. Fill in the blanks.
(a) Rusting of iron is a ____ change and cannot be ____
Answer: chemical, reversed 

(b) Dissolving salt in water is a ____ change because no ____ substance is formed
Answer: physical, new 

(c) The process of cooking rice is a ____ change.
Answer: chemical

(d) Melting of butter is a ____ change, but burning butter is a ____ change.
Answer: physical, chemical 

6. Is tearing paper and burning paper the same kind of change? Explain with reasons.
Answer: No, these two processes represent entirely different types of changes. When a piece of paper is torn, its physical appearance changes but no new substance is created, making it a physical change. Conversely, burning paper generates entirely new substances such as ash, carbon dioxide, and water vapour, which categorizes it as a chemical change.

7. Can rusted iron be changed back to iron? Is rusting a reversible or irreversible change? Explain.
Answer: No, it is impossible to convert rusted iron back into its original metallic iron form, making rusting an irreversible process. During this process, iron reacts with atmospheric oxygen and moisture to produce a completely new compound called iron oxide, commonly known as rust. Therefore, rusting represents a chemical change that cannot be undone. 

8. When a candle is lit, both melting of wax and burning of wax take place. Which of these is a physical change and which is a chemical change? Why?
Answer: The melting of wax represents a physical change, whereas the burning of wax is categorized as a chemical change. When wax melts, it merely transitions from a solid to a liquid state without producing any new matter. On the other hand, the combustion of wax results in the formation of entirely new substances, specifically carbon dioxide and water vapour. 

9. Read the following short story and answer the questions.
A farmer collects fresh sugarcane juice (physical change/chemical change). He boils the juice to make jaggery (physical change/chemical change). Later, he grinds the jaggery to powder form (physical change/chemical change). The farmer uses some jaggery to make sweets, where jaggery is heated and mixed with peanuts to form chikki (physical change/chemical change).

Give a suitable title to the story and tick the correct option in brackets.
Answer: Story Title: The Journey of Sugarcane to Chikki. The sequence of correct choices is: physical change, chemical change, physical change, and chemical change. 

10. Some changes are given here. Write physical changes in the area marked ‘A’, chemical changes in the area marked ‘B’, and both physical & chemical changes in the area marked ‘C’.

(i) Freezing of milk into ice cream 

(ii) Photosynthesis in plants 

(iii) Melting of gold 

(iv) Burning of paper 

(v) Breaking of a glass bottle 

(vi) Condensation of water vapour 

(vii) Cooking rice 

(viii) Lighting of matchstics 

Answer:
(i) Freezing of milk into ice cream – A
(ii) Photosynthesis in plants — B
(iii) Melting of gold — A
(iv) Burning of paper — B
(v) Breaking of a glass bottle — A
(vi) Condensation of water vapour — A
(vii) Cooking rice — B
(viii) Lighting of matchstics — B

11. Ravi wanted to test which substances release carbon dioxide when mixed. He tried the following :
(a) Baking soda + lemon juice
(b) Sugar + water
(c) Vinegar + washing soda
(d) Salt + water

He passed the gas released in each through lime water. In which cases will the lime water turn milky? Write the reason with a simple chemical reaction.
Answer: The lime water will turn milky in scenarios (a) and (c). This happens because a chemical reaction takes place in both instances, leading to the evolution of carbon dioxide gas.

(a) Baking soda + Lemon juice → Carbon dioxide + Other substances
(c) Vinegar + Washing soda → Carbon dioxide + Other substances

When this released carbon dioxide gas is passed through lime water, it reacts to form calcium carbonate, making the lime water appear milky. 

The chemical equation is:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Multiple Choice Questions (MCQs)

1. Which of the following is a physical change?
(A) Rusting of iron
(B) Digestion of food
(C) Tearing of paper
(D) Burning of coal
Answer: (C)

2. What new substance is formed when carbon dioxide reacts with lime water?
(A) Calcium hydroxide
(B) Calcium carbonate
(C) Calcium chloride
(D) Oxygen
Answer: (B)

3. Which change involves the absorption or release of heat?
(A) Physical change
(B) Chemical change )
(C) Both A and B
(D) None of the above
Answer: (B)

4. Rusting of iron occurs in the presence of:
(A) Only oxygen
(B) Only water
(C) Both oxygen and water
(D) Nitrogen
Answer: (C)

5. Which of the following processes is a chemical change?
(A) Melting of wax
(B) Boiling of water
(C) Photosynthesis
(D) Dissolving salt in water
Answer: (C)

6. A change in which one or more new substances are formed is called a:
(A) Physical change
(B) Chemical change
(C) Reversible change
(D) Periodic change
Answer: (B)

7. Galvanisation is a process used to prevent the rusting of which metal?
(A) Copper
(B) Zinc
(C) Iron
(D) Aluminium
Answer: (C)

8. When a piece of magnesium ribbon is burned, it produces:
(A) Magnesium oxide
(B) Magnesium chloride
(C) Magnesium carbonate
(D) Magnesium sulphate
Answer: (A)

9. Which of the following is NOT a chemical change?
(A) Souring of milk
(B) Baking a cake
(C) Breaking a glass
(D) Ripening of fruits
Answer: (C)

10. Crystallisation is an example of a:
(A) Physical change
(B) Chemical change
(C) Biological change
(D) None of the above
Answer: (A)

11. The brown layer formed on an iron nail left in the open is called:
(A) Dust
(B) Rust
(C) Copper
(D) Silver
Answer: (B)

12. What gas is released when vinegar reacts with baking soda?
(A) Oxygen
(B) Nitrogen
(C) Carbon dioxide
(D) Hydrogen
Answer: (C)

13. Freezing of water is a:
(A) Reversible change
(B) Irreversible change
(C) Chemical change
(D) Undesirable change
Answer: (A)

14. The chemical formula for rust is closely related to:
(A) Iron sulphate
(B) Iron oxide
(C) Iron chloride
(D) Iron carbonate
Answer: (B)

15. Turning milk into curd is an example of:
(A) Physical change
(B) Chemical change
(C) Temporary change
(D) Reversible change
Answer: (B)

16. Which process is used to obtain pure crystals of copper sulphate from an impure sample?
(A) Evaporation
(B) Crystallisation
(C) Filtration
(D) Distillation
Answer: (B)

Q17. Stretching a rubber band is:
(A) A chemical change
(B) A physical change
(C) An irreversible change
(D) Both A and C
Answer: (B)

18. Burning of any substance is fundamentally a/an:
(A) Physical change
(B) Irreversible physical change
(C) Chemical change
(D) Biological change
Answer: (C)

19. Painting iron gates prevents:
(A) Melting
(B) Rusting
(C) Freezing
(D) Evaporating
Answer: (B)

20. When ice melts, it changes its:
(A) Chemical composition
(B) State of matter
(C) Colour
(D) Mass
Answer: (B)

Long Questions with Answers

1. Explain the difference between a physical change and a chemical change with examples.
Answer: A physical change occurs when the form, shape, or state of a substance changes, but no new chemical substance is created. Examples include the melting of ice and the tearing of paper. A chemical change occurs when substances combine or break apart to form entirely new substances with different properties. Examples include the rusting of iron and the burning of wood.

2. How does the process of rusting damage iron objects?
Answer: Rusting happens when iron continuously reacts with atmospheric oxygen and moisture. This reaction forms a flaky, fragile layer of iron oxide, known as rust, on the surface. Over time, the rust peels off, exposing fresh iron beneath, which then rusts as well, eventually deteriorating the entire object and making it weak.

3. Why is the burning of magnesium considered a chemical change?
Answer: The combustion of a magnesium ribbon is a chemical change because it reacts with oxygen in the air while burning to produce an intense white light and ash. This ash is a newly formed chemical compound called magnesium oxide, proving that the original substance has been completely altered.

4. Describe the change that occurs when you mix baking soda with vinegar.
Answer: Mixing baking soda and vinegar causes a distinct chemical reaction. The two substances react rapidly, creating a fizzing sound and producing bubbles as carbon dioxide gas escapes. Along with the gas, other new substances are left behind in the container, establishing it as a clear chemical change.

5. What are the common methods to prevent iron from rusting?
Answer: Iron can be protected from rusting by limiting its exposure to moisture and air. Standard methods include applying a layer of paint, grease, or oil over the iron surface. Another highly effective industrial method is galvanisation, which involves coating the iron with a thin layer of zinc.

6. Why do cut apples or eggplants turn brown when exposed to air?
Answer: Cut fruits and vegetables like apples and eggplants turn brown due to a chemical reaction between the enzymes in the plant cells and the oxygen present in the air. This oxidation process creates new brownish pigments on the cut surface, marking it as a chemical change.

7. Explain the process of crystallisation with an example.
Answer: Crystallisation is a physical process used to extract large, pure crystals of a solid substance from its concentrated solution. For instance, obtaining pure copper sulphate crystals involves dissolving impure copper sulphate powder in boiling dilute sulphuric acid and allowing the filtered hot solution to cool slowly.

8. Is the digestion of food a physical or chemical change? Justify your answer.
Answer: The digestion of food is a chemical change. During digestion, complex food molecules are broken down by various enzymes and digestive juices in the body into entirely simpler, distinct chemical substances that the body can absorb for energy and growth.

9. Distinguish between boiling of water and boiling of an egg in terms of types of changes.
Answer: Boiling water simply changes it from a liquid to water vapour, making it a reversible physical change. In contrast, boiling an egg permanently alters the chemical structure of its proteins, turning the liquid inside into a hard solid. Thus, boiling an egg is an irreversible chemical change.

10. How would you prove that the curdling of milk is a chemical change?
Answer: The curdling of milk is considered a chemical change because the final product, curd, has completely different physical properties and a sour taste compared to the original milk. Additionally, the process is irreversible, meaning the curd cannot be turned back into milk.

11. Write the chemical equation when carbon dioxide is passed through lime water.
Answer: When carbon dioxide passes through lime water, it forms an insoluble solid called calcium carbonate along with water. The chemical equation is:
Calcium hydroxide [Ca(OH)₂] + Carbon dioxide [CO₂] → Calcium carbonate [CaCO₃] + Water [H₂O].

12. Can a physical and chemical change happen simultaneously? Explain with an example.
Answer: Yes, both changes can occur simultaneously. A common example is lighting a matchstick or burning a candle. When a candle burns, the solid wax melts into a liquid state (physical change), while the wick and wax vapor combust to produce heat, light, carbon dioxide, and water vapor (chemical change).

13. What is galvanisation and why is it useful?
Answer: Galvanisation is the mechanical process of applying a protective zinc coating to iron or steel components. This technique is extremely useful because the layer of zinc acts as a barrier, preventing oxygen and moisture from reaching the underlying iron, thereby significantly reducing the risk of rusting.

14. Why is cutting a piece of wood a physical change, but burning it a chemical change?
Answer: Cutting a piece of wood only modifies its size and dimensions; no new material is produced, making it a physical change. However, when that wood is burned, it undergoes a combustion reaction, producing entirely new substances such as smoke, ash, and carbon dioxide, which represents a chemical change.

15. Describe the role of the ozone layer and the type of change involved when it absorbs ultraviolet radiation.
Answer: The ozone layer acts as a critical protective shield for Earth by absorbing harmful ultraviolet (UV) radiation from the sun. During this process, UV light hits the ozone (O₃) molecules, breaking them down into separate oxygen molecules (O₂). Since a new substance is formed from the breakdown, it is considered a chemical change.