NCERT Solutions for Class 7 Science Curiosity Chapter 5: Changes Around Us: Physical and Chemical Changes by PW

Changes occur around us in many forms, but identifying whether a change is physical or chemical can sometimes be difficult. Changes such as melting, folding, rusting, burning and curdling may look different, and understanding whether a new substance is formed is important for classifying them correctly. Some changes can also be reversed, while others cannot.

The Class 7 Science Curiosity Chapter 5 Solutions explain the textbook questions from Changes Around Us: Physical and Chemical Changes with clear answers and reasoning. They cover physical and chemical changes, reversible and irreversible processes, changes of state, combustion, rusting, decomposition and everyday chemical reactions.

Class 7 Science Chapter 5 NCERT Solutions

The questions in this chapter use familiar situations and experiments to help distinguish physical changes from chemical changes.

Q1. 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.

Answer: (c) (i) and (iii)

Explanation:
A physical change may involve a change in the state, shape or size of a substance, but no new substance is formed.

Therefore:

• (i) Correct: The state of a substance may change during a physical change.

• (ii) Incorrect: A new substance with different properties is not formed.

• (iii) Correct: No new substance is formed.

• (iv) Incorrect: A chemical reaction is not involved in a physical change.

Answer: (c) (i) and (iii)

Q2. Predict which of the following changes can be reversed and which cannot be reversed.

Explanation:
A reversible change can be brought back to its original state, while an irreversible change cannot be easily reversed.

For example, water vapour formed by boiling can condense back into liquid water. However, once fruits ripen or wheat grains are ground into flour, the original form cannot be obtained easily.

Q3. State whether the following statements are True or False. If false, write the correct statement.

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

Answer: True

When a candle burns, the wax melts first. The liquid wax then moves towards the flame, where it burns.

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

Answer: False

It is a physical change.

During condensation, water vapour changes into liquid water. No new substance is formed.

Water vapour → Liquid water

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

Answer: True

During decomposition, microorganisms break down the leaves and convert them into new substances with different properties. Therefore, making compost involves a chemical change.

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

Answer: True

Baking soda reacts with lemon juice and produces carbon dioxide gas along with other products. Since new substances are formed, it is a chemical change.

Q4. Fill in the blanks.

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

Answer: The brown deposits are due to rusting, and this is a chemical change.

Rusting produces a new substance with properties different from those of iron.

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

Answer: Folding a handkerchief is a physical change and can be reversed.

Folding changes only the shape of the handkerchief. No new substance is formed.

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

Answer: A chemical process in which a substance reacts with oxygen with the 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.

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

Q5. Are the changes of water to ice and water to steam physical or chemical? Explain.

Answer: Both changes are physical changes.

• Water → Ice: Water changes from liquid to solid.

• Water → Steam: Water changes from liquid to gas.

Only the state of water changes in both cases. No new substance is formed, and the original water can be obtained again.

Therefore, both changes are physical changes.

Q6. Is curdling of milk a physical or chemical change? Justify your statement.

Answer: Curdling of milk is a chemical change.

The reasons are:

• A new substance, curd, is formed.

• The taste and smell of milk change.

• The change cannot be easily reversed.

• During curdling, microorganisms help convert milk components and lactic acid is produced.

Therefore, curdling of milk is a chemical change.

Q7. Natural factors such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?

Answer: Soil formation involves both physical and chemical changes.

Physical change:
Wind, rain and changes in temperature can break rocks into smaller pieces. No new substance necessarily forms during this process.

Chemical change:
Weathering can also cause chemical changes in the minerals present in rocks, leading to the formation of new substances.

Therefore, the formation of soil from rocks involves both physical and chemical changes.

Q8. Read the story titled "Eco-friendly Prithvi" and identify the physical and chemical changes.

The story describes Prithvi preparing a meal and making compost.

Suitable title: Prithvi's Eco-Kitchen

Q9. Classify the following changes as physical, chemical, or both physical and chemical.

Why is burning of a candle both physical and chemical?

When a candle burns, two changes occur:

• Wax melts: This is a physical change because the state of wax changes.

• Wax burns: This is a chemical change because new substances are formed.

Therefore, burning of a candle involves both physical and chemical changes.

Q10. The experiments shown in Fig. 5.11 (a), (b), (c) and (d) were performed. Find out in which case(s) did lime water turn milky and why?

The lime water turns milky when carbon dioxide gas is passed through it. Carbon dioxide reacts with lime water to form insoluble calcium carbonate, which gives the solution a milky appearance.

In cases (a) and (d), baking soda reacts with an acidic substance and produces carbon dioxide gas. When this gas is passed through lime water, calcium carbonate is formed, making the lime water appear milky.

Answer: Cases (a) and (d)

NCERT Solutions for Class 7 Science Curiosity Chapter 5 PDF

Quick revision becomes easier when the important questions and their answers are available together. The Class 7 Science Curiosity Chapter 5 Solutions PDF covers questions related to physical and chemical changes, reversible and irreversible processes, reactions and everyday examples.

What Can You Learn from NCERT Class 7 Science Chapter 5 Solutions by PW?

The NCERT Class 7 Science Chapter 5 Solutions by PW explain physical and chemical changes through textbook questions, everyday examples and simple experiments. Important learning areas include:

• Differentiate Physical and Chemical Changes: Identify whether a change forms a new substance or only affects the state, shape or size of a substance.

• Understand Reversible and Irreversible Changes: Learn how some changes can return to their original state, while others cannot be easily reversed.

• Identify Chemical Changes in Daily Life: Recognise processes such as rusting, curdling, ripening and decomposition as chemical changes.

• Understand Changes of State: Learn why melting, freezing, boiling and condensation are physical changes.

• Explore Combustion: Understand how a substance reacts with oxygen and releases heat during combustion.

• Identify Formation of New Substances: Understand how reactions such as baking soda reacting with an acid produce new substances and gases.

• Interpret Experimental Observations: Use observations such as lime water turning milky to identify the presence of carbon dioxide.

• Relate Changes to Daily Life: Connect physical and chemical changes with cooking, composting, candle burning, soil formation and other familiar processes.

Physical and chemical changes occur in many familiar situations, from melting ice and boiling water to rusting, curdling and decomposition. The NCERT Class 7 Science Chapter 5 Solutions explain how these changes can be identified based on new substance formation, reversibility and observable effects. Practising these questions can strengthen understanding of the concepts and their everyday applications.

Related Resources

• CBSE Class 7 Science Syllabus

• CBSE Class 7 Science NCERT Solutions

• CBSE Class 7 Science Sample Papers

• CBSE Class 7 Science Notes