Chapter 10 of 23

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Tamil Nadu (TNBSE)Class 8 Science★ 4-7 marks in the TN Class 8 annual exam through book-back MCQs, true/false and short answers

Changes Around Us

Samacheer Kalvi (TN State Board) Class 8 Science — Chemistry Chapter 10, Changes Around Us. Physical and chemical changes and their classification, the effects of chemical reactions (colour, light, heat), rusting and the browning of fruits, and real processes like brine, limestone, the Haber process, acid rain and global warming. Notes with the actual book-back questions, common mistakes and quick revision.

⏱ 16 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Distinguish physical and chemical changes and classify them
2Identify the effects of chemical reactions (colour, light, heat)
3Explain rusting and the browning of fruits (melanin)
4Recall real processes — brine, limestone, Haber process
5Link acid rain and global warming to their causes

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Why this chapter matters

Changes Around Us explains physical and chemical changes and links them to real processes — rusting, the Haber process, acid rain and global warming. These specific facts are directly tested book-back content in the TN Class 8 exam.

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Class 7 — Physical and chemical changes

Basic idea of change.

Changes Around Us — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Chemistry — Chapter 10. Physical and chemical changes around us.

1. About this chapter

This chapter explains physical and chemical changes and their classification, the effects of chemical reactions, and several real chemical processes.

2. Types of change

A physical change alters only shape, size or state — no new substance, usually reversible (melting ice).
A chemical change forms a new substance; it is permanent (burning paper, rusting).
Changes are also reversible/irreversible, periodic/non-periodic, desirable/undesirable, and exothermic/endothermic (the formation of slaked lime from quicklime is exothermic).

3. Effects of chemical reactions

Colour change: the browning of apples is due to the pigment melanin.
Light: the burning of a matchstick gives out light.
Gas/precipitate formation and temperature change also signal a chemical reaction.

4. Some real chemical processes

Rusting: iron + air + moisture → rust.
Brine is a concentrated solution of sodium chloride; limestone is mainly calcium carbonate.
Electrolysis is induced by electricity; the Haber process (with an iron catalyst) makes ammonia.
Acid rain is caused by dissolved sulphur dioxide and nitrogen oxides; carbon dioxide is mainly responsible for global warming; CFCs are pollutants.

5. Worked examples

Example 1. Is the burning of paper a physical or chemical change? Chemical — a new substance forms and it is permanent.

Example 2. Which pigment causes the browning of apples? Melanin.

Example 3. Name the catalyst used in the Haber process. Iron.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

Burning of paper is a — (a) physical / (b) chemical change. Ans: (b) chemical.
The metal that undergoes rusting is — (a) gold / (b) iron. Ans: (b) iron.
The pigment responsible for browning of apples is — (a) tannin / (b) melanin. Ans: (b) melanin.
Brine is a concentrated solution of — (a) sugar / (b) sodium chloride. Ans: (b) sodium chloride.
Limestone mainly contains — (a) sodium chloride / (b) calcium carbonate. Ans: (b) calcium carbonate.
______ is mainly responsible for global warming — (a) carbon dioxide / (b) nitrogen. Ans: (a) carbon dioxide.

II. True or False 7. A chemical reaction is a temporary reaction. — False (it is permanent). 8. Formation of slaked lime from quicklime is an endothermic reaction. — False (it is exothermic). 9. CFC is a pollutant. — True.

7. Common mistakes

Mistake: Calling melting of ice a chemical change. Fix: It is a physical change (no new substance).
Mistake: Saying browning of apples is due to tannin. Fix: It is due to melanin.
Mistake: Saying quicklime → slaked lime is endothermic. Fix: It is exothermic (gives out heat).

8. Quick revision

Chemistry Ch 10 · physical/chemical change, effects, real processes.
Physical: no new substance, reversible; Chemical: new substance, permanent.
Effects: colour (browning = melanin), light (matchstick), heat (quicklime→slaked lime exothermic).
Rusting = iron + air + moisture; brine = NaCl; limestone = CaCO₃.
Haber process (iron catalyst); acid rain from SO₂/NOₓ; CO₂ → global warming; CFC pollutant.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Physical vs chemical change

no new substance (reversible) vs new substance (permanent)

Chemical change is permanent.

Effects of reaction

colour (melanin), light (matchstick), heat

Quicklime → slaked lime is exothermic.

Real processes

brine = NaCl; limestone = CaCO₃; Haber catalyst = iron

Named TN examples.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Calling melting of ice a chemical change

✓ It is a physical change (no new substance).

WATCH OUT

✗ Saying browning of apples is due to tannin

✓ It is due to melanin.

WATCH OUT

✗ Saying quicklime → slaked lime is endothermic

✓ It is exothermic (gives out heat).

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on changes, rusting, melanin, brine, limestone and global warming.

Questions

True or False

Permanence of chemical reactions, exothermic reactions and CFCs.

Questions

Answer briefly

Effects of chemical reactions; the Haber process; acid rain.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

Burning of paper is a ____ change.

▶ Show solution

chemical.

Q2EASY· MCQ

The pigment responsible for the browning of apples is ____.

▶ Show solution

melanin.

Q3EASY· MCQ

Brine is a concentrated solution of ____.

▶ Show solution

sodium chloride.

Q4EASY· MCQ

____ is mainly responsible for global warming.

▶ Show solution

Carbon dioxide.

Q5EASY· True/False

True or False: The formation of slaked lime from quicklime is endothermic.

▶ Show solution

False — it is exothermic.

Q6EASY· Recall

Name the catalyst used in the Haber process.

▶ Show solution

Iron.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Chemistry Chapter 10 of Samacheer Kalvi Class 8 Science.
•Physical: no new substance, reversible; Chemical: new substance, permanent.
•Effects: colour (browning = melanin), light (matchstick), heat.
•Rusting = iron + air + moisture; brine = NaCl; limestone = CaCO₃.
•Haber process uses an iron catalyst; acid rain from SO₂/NOₓ.
•CO₂ → global warming; CFCs are pollutants; quicklime → slaked lime is exothermic.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-7 marks across book-back MCQ, true/false and short answers

Question type	Marks each	Typical count	What it tests
MCQ / True-False	1	3-5	Type of change, real processes, global warming
Short Answer	2-3	1-2	Effects of reactions, Haber process, acid rain

Prep strategy

Separate physical and chemical changes
Learn the named examples (brine, limestone, Haber)
Remember melanin (browning) and exothermic quicklime
Link acid rain and global warming to causes

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Preventing rust

Understanding rusting helps protect tools and structures.

Industry

The Haber process makes ammonia for fertilisers.

Environment

Knowing acid rain and global warming guides pollution control.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Decide if a new substance is formed
Quote the named TN examples (brine, limestone, Haber)
Remember melanin and exothermic quicklime
Link causes to acid rain and global warming

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Classify ten changes as physical or chemical with reasons.
Explain how the Haber process combines nitrogen and hydrogen.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 8 Annual ExamHigh

Foundation / NMMS ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Iron reacts with oxygen and moisture to form a new substance, rust (a hydrated iron oxide), with different properties, and it cannot easily be reversed.

Gases such as sulphur dioxide and nitrogen oxides dissolve in rainwater to form acids, which fall as acid rain.

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