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.

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

Accepted Answer

The correct answer is (c) (i) and (iii).
Explanation:
- Physical changes may or may not involve a change in the state of the substance (e.g., melting, freezing).
- No new substance is formed during a physical change.
- Statements (ii) and (iv) refer to chemical changes where new substances are formed and chemical reactions occur. — Physical changes involve change in state or form without formation of new substances. Hence, (i) and (iii) are characteristics of physical change.

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

Accepted Answer

Solutions:
(i) Stitching cloth to a shirt - Cannot be reversed easily because the cloth is permanently joined.
(ii) Twisting of straight string - Can be reversed by untwisting.
(iii) Making idlis from a batter - Cannot be reversed as chemical changes occur during cooking.
(iv) Dissolving sugar in water - Can be reversed by evaporation.
(v) Drawing water from a well - Can be reversed by putting water back.
(vi) Ripening of fruits - Cannot be reversed as chemical changes occur.
(vii) Boiling water in an open pan - Can be reversed by condensation.
(viii) Rolling up a mat - Can be reversed by unrolling.
(ix) Grinding wheat grains to flour - Cannot be reversed as physical structure is changed.
(x) Forming of soil from rocks - Cannot be reversed as chemical and physical changes occur over time.

Some changes are difficult to be sure about because they involve both physical and chemical processes or irreversible transformations. — Reversible changes are mostly physical changes where the original state can be restored. Irreversible changes involve chemical changes or permanent physical changes.

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. (True/False)
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
(iii) The process of converting leaves into compost is a chemical change. (True/False)
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)

Accepted Answer

(i) False. Melting of wax is not necessary for burning a candle; burning involves combustion which is a chemical change.
(ii) False. Collecting water vapour by condensing is a physical change.
(iii) True. Converting leaves into compost involves decomposition, a chemical change.
(iv) True. Mixing baking soda with lemon juice produces carbon dioxide gas, a chemical change. — Melting is a physical change and not necessary for combustion. Condensation is physical. Composting and mixing baking soda with acid cause chemical changes.

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.

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

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

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

Accepted Answer

(i) The brown deposits are due to rusting, and this is a chemical change.
(ii) Folding a handkerchief is a physical change and can be reversed.
(iii) 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 magnesium oxide. The substance formed is basic in nature. Burning of magnesium is a chemical change. — Rusting is chemical change forming new substance. Folding is physical and reversible. Combustion involves oxygen and heat, a chemical change. Burning magnesium forms magnesium oxide, a new compound.

Question 5

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

Accepted Answer

Both changes are physical changes because the chemical composition of water (H2O) remains the same in ice, water, and steam. Only the state changes (solid to liquid to gas) without formation of any new substance. — Physical changes involve change in state without change in chemical identity. Melting and boiling are physical changes.

Question 6

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

Accepted Answer

Curdling of milk is a chemical change because it involves the breakdown of milk proteins by bacteria, forming new substances like lactic acid and curd. This change is irreversible. — Chemical changes produce new substances and are usually irreversible, as in curdling.

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?

Accepted Answer

The formation of soil from rocks involves both physical and chemical changes. Physical weathering breaks rocks into smaller pieces, and chemical weathering alters the minerals to form new substances. Hence, it is a chemical change because new substances are formed. — Chemical weathering changes the composition of rocks, forming soil, which is a chemical change.

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.

Accepted Answer

Answers:
- Chopping vegetables, peeling potatoes, and cutting fruits are physical changes.
- Collecting seeds, fruits, and peels into a clay pot is a physical change.
- Decomposition forming compost is a chemical change.
- Germination and growth of plants is a chemical change.

Suitable title: "Eco-friendly Prithvi: Understanding Physical and Chemical Changes in Nature" — Physical changes involve change in form without new substances. Decomposition and growth involve chemical changes.

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.

Accepted Answer

Area A (Physical changes): Tearing of paper, Melting of ice, Folding of clothes
Area B (Chemical changes): Burning of candle, Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar
Area C (Both physical and chemical changes): None of the given changes clearly fall into both categories simultaneously. — Physical changes involve change in form or state without new substances. Chemical changes involve formation of new substances.

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?

Accepted Answer

Lime water turns milky in cases where carbon dioxide gas is present because it reacts with lime water (calcium hydroxide) to form calcium carbonate, which is insoluble and appears milky.

In the experiments:
- (a) Lime water turns milky if carbon dioxide is produced.
- (b) Lime water turns milky if carbon dioxide is produced.
- (c) Lime water turns milky if carbon dioxide is produced.
- (d) Lime water does not turn milky if no carbon dioxide is produced.

Therefore, lime water turns milky in those experiments where carbon dioxide is released during the reaction. — Carbon dioxide reacts with lime water to form insoluble calcium carbonate, causing milky appearance.

Question 11

Which of the following is an example of a physical change?

Accepted Answer

Melting of ice — Physical changes involve a change in the form or appearance of a substance without altering its chemical composition. Melting of ice is a physical change because water molecules remain the same, only their arrangement changes from solid to liquid.

Question 12

What is a chemical change? Give one example from daily life.

Accepted Answer

A chemical change is a change in which one or more new substances with different properties are formed. For example, burning of wood is a chemical change because it produces ash, carbon dioxide, and water vapor which are different from wood. — A chemical change involves breaking and forming of chemical bonds resulting in new substances. Burning of wood is a common example where wood reacts with oxygen producing new substances.

Question 13

Which of the following changes is irreversible?

Accepted Answer

Burning of paper — Irreversible changes cannot be undone by simple means. Burning of paper produces ash and smoke which cannot be changed back to paper, making it an irreversible change.

Question 14

Explain why melting of ice is considered a physical change.

Accepted Answer

Melting of ice is a physical change because it involves a change in state from solid to liquid without changing the chemical composition of water. The water molecules remain the same, only their arrangement changes. — Physical changes do not alter the chemical identity of a substance. In melting, only the state changes while the substance remains water.

Question 15

During rusting of iron, which substances combine to form rust?

Accepted Answer

Iron, oxygen, and moisture — Rusting is a chemical change where iron reacts with oxygen and moisture in the air to form iron oxide, commonly called rust.

Changes Around Us

Changes Around Us — Study Notes

Introduction

Physical Changes

Chemical Changes

Practice Questions — Changes Around Us

All 12 Chapters in Curiosity