Class 7 Science

Chapter 5 Solutions — Changes Around Us: Physical and Chemical

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Solutions

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Step-by-step NCERT solutions for Changes Around Us: Physical and Chemical (Chapter 5, NCERT Class 7 Science) — every question and answer worked out in full, not just the final result. You can also read the Changes Around Us: Physical and Chemical textbook chapter.

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All 10 questions in Changes Around Us: Physical and Chemical are solved in the PDF. Here's what's inside, exercise by exercise:

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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)
2.
Predict which of the following changes can be reversed and which cannot be reversed.
- (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
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.
- (ii) Collecting water vapour by condensing involves a chemical change.
- (iii) The process of converting leaves into compost is a chemical change.
- (iv) Mixing baking soda with lemon juice is a chemical change.
4.
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.
- (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)…
Are the changes of water to ice and water to steam, physical or chemical? Explain.
Is curdling of milk a physical or chemical change? Justify your statement.
Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Read the story 'Eco-friendly Prithvi' and tick the most appropriate option(s): Prithvi 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…
Sort the following changes: write physical changes in area 'A', chemical changes in area 'B', and changes that are both physical and chemical in area 'C'. Changes: Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium; Mixing baking soda with vinegar.
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?
- (a) Vinegar and baking soda → gas passed into lime water.
- (b) Lemon juice and vinegar → gas passed into lime water.
- (c) Vinegar and common salt → gas passed into lime water.
- (d) Lemon juice and baking soda → gas passed into lime water.

Sample

Worked examples

2 questions from Changes Around Us: Physical and Chemical, solved step by step.

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

Both changes — water to ice and water to steam — are physical changes.

Water to ice (freezing): Water changes from the liquid state to the solid state. Only the state of matter changes; the substance remains water (H₂O) in both cases. No new substance is formed. The change is reversible — ice can melt back to water.

Water to steam (vaporisation/boiling): Water changes from the liquid state to the gaseous state (steam). Again, only the state of matter changes; the substance is still water (H₂O). No new substance is formed. The change is reversible — steam can condense back to liquid water.

In both cases, only the physical property (state of matter) changes and no chemical reaction occurs. Therefore, both are physical changes.

Answer: Both are physical changes. Only the state of matter (solid/liquid/gas) changes; the substance remains water (H₂O) with no new substance formed, and both changes are reversible.

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

Curdling of milk is a chemical change.

Justification:

When milk curdles, the proteins in the milk (mainly casein) react chemically with an acid (such as lactic acid produced by bacteria, or lemon juice/vinegar if added deliberately). This reaction causes the proteins to coagulate and precipitate out, forming curd (solids) and whey (liquid).

New substances are formed — the composition of the curd is different from that of the original milk.

The change cannot be reversed — curd cannot be converted back to the original milk.

The formation of new substances with different properties confirms that curdling is a chemical change.

Answer: Curdling of milk is a chemical change. New substances (curd and whey) with different properties are formed, and the change cannot be reversed.

All 10 questions in Changes Around Us: Physical and Chemical are solved step by step in the Solutions PDF above.

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Chapter 5 Solutions — Changes Around Us: Physical and Chemical

Overview

What these solutions cover

Let Us Enhance Our Learning

Worked examples

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

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

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