Chapter 11 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, fill-ups, match and short answers

Air

Samacheer Kalvi (TN State Board) Class 8 Science — Chemistry Chapter 11, Air. The composition of air, the properties and uses of oxygen, nitrogen and carbon dioxide, dry ice and the Solvay process, combustion and rusting, and the greenhouse effect. 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…

1State the composition of air
2Describe the uses of oxygen, nitrogen and carbon dioxide
3Explain dry ice and the Solvay process
4Explain combustion and rusting
5Describe the greenhouse effect and global warming

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Why this chapter matters

Air explains the gases we breathe and use — oxygen, nitrogen (azote) and carbon dioxide — plus dry ice, the Solvay process and the greenhouse effect. These are directly tested book-back topics in the TN Class 8 exam.

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A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 7 — Air and water

Properties of air.

Air — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Chemistry — Chapter 11. The mixture of gases that surrounds us.

1. About this chapter

This chapter covers the composition of air, the gases of air and their uses, dry ice and the Solvay process, combustion and rusting, and the greenhouse effect.

2. Composition of air and its gases

Air is a mixture: about 78% nitrogen, 21% oxygen, and small amounts of carbon dioxide, argon, water vapour and dust.
Oxygen supports burning and respiration — it is called "vital air".
Nitrogen does not support burning; it is also called azote, is lighter than air, and is used in fertilisers.
Carbon dioxide is used by plants in photosynthesis, in fire extinguishers and in aerated water; CO₂ dissolved in water forms a weak acid that turns blue litmus red.

3. Dry ice, the Solvay process, combustion and rusting

Dry ice is solid carbon dioxide, used as a refrigerant and to make fog for stage effects.
The Solvay process is used to manufacture sodium carbonate (washing soda).
Combustion is burning in oxygen, giving heat and light.
Rusting is the conversion of iron into a hydrated oxide (rust) in the presence of air and moisture.

4. Greenhouse effect and pollution

Global warming is the increased greenhouse effect caused by a rise in air pollutants, raising the average temperature of the atmosphere.
Air pollution is reduced by using clean fuels, public transport and planting trees.

5. Worked examples

Example 1. Which gas supports burning? Oxygen.

Example 2. What is dry ice? Solid carbon dioxide, used as a refrigerant.

Example 3. What is the Solvay process used for? To manufacture sodium carbonate (washing soda).

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

Oxygen — (a) puts out fire / (b) supports burning. Ans: (b) supports burning.
Aerated water contains — (a) nitrogen / (b) carbon dioxide. Ans: (b) carbon dioxide.
The Solvay process is used to manufacture — (a) ammonia / (b) sodium carbonate. Ans: (b) sodium carbonate.
Carbon dioxide with water turns blue litmus — (a) red / (b) green. Ans: (a) red.
Nitrogen is also known as — (a) vital air / (b) azote. Ans: (b) azote.

II. Fill in the blanks 6. Oxygen is called vital air. 7. Dry ice is solid carbon dioxide used as a refrigerant. 8. Rusting is the conversion of iron into a hydrated oxide.

III. Match the following 9. Nitrogen → Fertiliser; Oxygen → Respiration; Carbon dioxide → Fire extinguisher; Dry ice → Refrigerator.

7. Common mistakes

Mistake: Thinking air is mostly oxygen. Fix: Air is about 78% nitrogen, only 21% oxygen.
Mistake: Saying nitrogen supports burning. Fix: Nitrogen does not support burning (it is azote).
Mistake: Calling dry ice frozen water. Fix: Dry ice is solid carbon dioxide.

8. Quick revision

Chemistry Ch 11 · air, gases, dry ice, Solvay, rusting.
Air ≈ 78% N₂, 21% O₂, plus CO₂, argon, water vapour.
Oxygen = vital air (supports burning); nitrogen = azote (no burning).
CO₂ in aerated water; CO₂ + water turns blue litmus red; dry ice = solid CO₂.
Solvay process → sodium carbonate; rusting → hydrated oxide; excess greenhouse gases → global warming.

Key formulas & results

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

Composition of air

~78% N₂, 21% O₂, rest CO₂/argon/water vapour

Air is a mixture.

Oxygen / nitrogen

oxygen = vital air; nitrogen = azote

Oxygen supports burning; nitrogen does not.

Dry ice / Solvay

dry ice = solid CO₂; Solvay → sodium carbonate

Refrigerant; washing soda.

Rusting

iron → hydrated oxide (air + moisture)

A chemical change.

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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

✗ Thinking air is mostly oxygen

✓ Air is about 78% nitrogen, only 21% oxygen.

WATCH OUT

✗ Saying nitrogen supports burning

✓ Nitrogen does not support burning (it is azote).

WATCH OUT

✗ Calling dry ice frozen water

✓ Dry ice is solid carbon dioxide.

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 oxygen, aerated water, Solvay process and azote.

Questions

Fill in the blanks

Vital air, dry ice and rusting.

Questions

Match the following

Gases with their uses (fertiliser, respiration, fire extinguisher, refrigerator).

Questions

Practice problems

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

Q1EASY· MCQ

Aerated water contains ____.

▶ Show solution

carbon dioxide.

Q2EASY· MCQ

The Solvay process is used to manufacture ____.

▶ Show solution

sodium carbonate (washing soda).

Q3EASY· MCQ

Nitrogen is also known as ____.

▶ Show solution

azote.

Q4EASY· Fill in the blanks

Dry ice is solid ____ used as a refrigerant.

▶ Show solution

carbon dioxide.

Q5EASY· Match

Match: Nitrogen, Oxygen, Carbon dioxide, Dry ice with their uses.

▶ Show solution

Nitrogen → fertiliser; Oxygen → respiration; Carbon dioxide → fire extinguisher; Dry ice → refrigerator.

Q6EASY· Concept

What is rusting?

▶ Show solution

The conversion of iron into a hydrated oxide (rust) in the presence of air and moisture.

5-minute revision

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

•Chemistry Chapter 11 of Samacheer Kalvi Class 8 Science.
•Air ≈ 78% N₂, 21% O₂, plus CO₂, argon, water vapour.
•Oxygen = vital air (supports burning); nitrogen = azote (no burning).
•CO₂ in aerated water; CO₂ + water turns blue litmus red.
•Dry ice = solid CO₂; Solvay process → sodium carbonate.
•Rusting → hydrated oxide; excess greenhouse gases → global warming.

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, fill-ups, match and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	3-5	Gases, dry ice, Solvay, rusting
Match	2	0-1	Gas–use matching
Short Answer	2-3	1	Greenhouse effect, combustion

Prep strategy

Memorise the composition of air
Link each gas to its uses and other names
Remember dry ice (solid CO₂) and the Solvay process
Understand rusting and the greenhouse effect

Where this shows up in the real world

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

Breathing and burning

Oxygen supports life and combustion.

Refrigeration and effects

Dry ice keeps things cold and creates stage fog.

Industry

The Solvay process makes washing soda; nitrogen makes fertilisers.

Exam strategy

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

Quote the air composition accurately
Link each gas to its use and alternate name
Recall dry ice = solid CO₂ and Solvay → sodium carbonate
Define rusting precisely

Going beyond the textbook

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

Explain how the percentage of CO₂ in air is rising and why it matters.
Describe a test to distinguish oxygen and carbon dioxide.

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.

Nitrogen (azote) is unreactive and does not support burning or spoilage, so it keeps packaged food fresh.

Carbon dioxide dissolves in water to form a weak acid (carbonic acid), which turns blue litmus red.

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