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Tamil Nadu (TNBSE)Class 6 Science★ 6-10 marks in the TN Class 6 Term 2 exam through book-back MCQs, fill-ups and short answers

Air

Samacheer Kalvi (TN State Board) Class 6 Science — Term 2 Chapter 4, Air. The composition of air (78% nitrogen, oxygen and small amounts of carbon dioxide, water vapour and other gases), oxygen as the active component that supports combustion, the role of air in photosynthesis, and tests for the gases, with the actual book-back questions, common mistakes and quick revision.

⏱ 14 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
2Identify oxygen as the active component
3Explain combustion
4Describe the role of air in photosynthesis
5Recall tests for the gases

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Why this chapter matters

Air explains the gases we breathe and depend on for life and burning. The composition of air, oxygen's role and photosynthesis are directly tested book-back content in the TN Class 6 Term 2 exam.

Before you start — revise these

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

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Class 5 — Air Around Us

Air and breathing.

Air — Class 6 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 6 Science, Term 2 — Chapter 4. What air is made of and why it matters.

1. About this chapter

This chapter covers the composition of air, the properties of its gases, combustion, and the role of air in photosynthesis.

2. Composition of air

Air is a mixture of gases. Its major component is nitrogen — about 78% (nearly four-fifths) of air.
The second component is oxygen. Together, nitrogen and oxygen make up about 99% of air.
Air also has small amounts of carbon dioxide, water vapour and other gases (argon, helium).

3. Oxygen and combustion

Oxygen is the active component of air; it supports combustion (burning). Burning is the reaction of a substance with oxygen, releasing heat and light.
Oxygen is given to patients with breathing problems.

4. Air, photosynthesis and tests

In photosynthesis, plants use carbon dioxide and water in the presence of sunlight to make food; the gas given out is oxygen. Gases are exchanged in a leaf through the stomata.
Tests: carbon dioxide turns lime water milky; dust particles can be seen moving in a beam of sunlight in a dark room (showing air contains dust).

5. Worked examples

Example 1. Which gas makes up most of the air? Nitrogen (about 78%).

Example 2. Which gas supports burning? Oxygen.

Example 3. Which gas turns lime water milky? Carbon dioxide.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

The percentage of nitrogen in air is — (a) 78% / (b) 21%. Ans: (a) 78%.
Gas exchange in plants takes place through the — (a) roots / (b) stomata. Ans: (b) stomata.
The constituent of air that supports combustion is — (a) nitrogen / (b) oxygen. Ans: (b) oxygen.
The two gases that together make about 99% of air are — (a) nitrogen and oxygen / (b) CO₂ and argon. Ans: (a) nitrogen and oxygen.

II. Fill in the blanks 5. The active component of air is oxygen. 6. The gas given out during photosynthesis is oxygen. 7. Oxygen is given to patients having breathing problems. 8. Carbon dioxide gas turns lime water milky.

III. Answer briefly 9. What is combustion? — The burning of a substance in the presence of oxygen, releasing heat and light. 10. How do we show that air contains dust? — Dust particles are seen moving in a beam of sunlight in a dark room.

7. Common mistakes

Mistake: Saying oxygen is the most abundant gas in air. Fix: Nitrogen (78%) is the most abundant; oxygen is about 21%.
Mistake: Thinking nitrogen supports burning. Fix: Oxygen supports combustion; nitrogen does not.
Mistake: Saying CO₂ is given out and taken in only. Fix: In photosynthesis, plants take in CO₂ and give out oxygen.

8. Quick revision

Term 2 · Ch 4 · air.
Air = mixture; nitrogen 78% (major), oxygen ~21% (together ~99%), plus CO₂, water vapour, argon.
Oxygen = active component, supports combustion, given to patients; CO₂ turns lime water milky.
Photosynthesis: plants use CO₂ + water + sunlight → food, releasing oxygen (gas exchange via stomata).

Key formulas & results

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

Composition

nitrogen 78%, oxygen ~21% (together ~99%)

Plus CO₂, water vapour, argon.

Oxygen

active component; supports combustion

Given to patients.

Photosynthesis

CO₂ + water + sunlight → food + oxygen

Gas exchange via stomata.

Tests

CO₂ turns lime water milky; dust seen in sunlight

Identify gases/dust.

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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

✗ Saying oxygen is the most abundant gas in air

✓ Nitrogen (78%) is the most abundant; oxygen is about 21%.

WATCH OUT

✗ Thinking nitrogen supports burning

✓ Oxygen supports combustion; nitrogen does not.

WATCH OUT

✗ Saying plants only release CO₂

✓ In photosynthesis, plants take in CO₂ and give out oxygen.

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 nitrogen %, stomata, oxygen and the 99% pair.

Questions

Fill in the blanks

Active component, photosynthesis gas, patients and lime water.

Questions

Answer briefly

Combustion and showing air contains dust.

Questions

Practice problems

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

Q1EASY· MCQ

The percentage of nitrogen in air is ____.

▶ Show solution

78%.

Q2EASY· MCQ

The constituent of air that supports combustion is ____.

▶ Show solution

oxygen.

Q3EASY· Fill in the blanks

The gas given out during photosynthesis is ____.

▶ Show solution

oxygen.

Q4EASY· Fill in the blanks

Carbon dioxide gas turns lime water ____.

▶ Show solution

milky.

Q5EASY· MCQ

Gas exchange in plants takes place through the ____.

▶ Show solution

stomata.

Q6MEDIUM· Answer briefly

What is combustion?

▶ Show solution

Combustion is the burning of a substance in the presence of oxygen, releasing a large amount of heat and light.

5-minute revision

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

•Term 2 Chapter 4 of Samacheer Kalvi Class 6 Science.
•Air is a mixture; nitrogen (78%) is the major component, oxygen is the second (together ~99%).
•Oxygen is the active component and supports combustion; it is given to patients with breathing problems.
•Burning (combustion) is the reaction of a substance with oxygen, releasing heat and light.
•In photosynthesis, plants use CO₂, water and sunlight to make food and give out oxygen.
•Carbon dioxide turns lime water milky; dust in air is seen in a sunlight beam.

Tamil Nadu (TNBSE) marks blueprint

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

Typical chapter weightage: 6-10 marks across book-back MCQ, fill-ups and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	5-7	Composition, oxygen, photosynthesis
Short Answer	2	1-2	Combustion, dust in air

Prep strategy

Memorise nitrogen 78% and oxygen ~21%
Remember oxygen supports combustion
Note CO₂ turns lime water milky
Recall photosynthesis releases oxygen

Where this shows up in the real world

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

Breathing

We need oxygen from air to live.

Burning

Oxygen lets fuels burn for cooking and energy.

Plants

Air's CO₂ is used by plants to make food.

Exam strategy

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

Quote nitrogen 78%, oxygen ~21%
Name oxygen as the active component
State CO₂ turns lime water milky
Describe photosynthesis briefly

Going beyond the textbook

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

Explain why a burning candle goes out when covered with a jar.
Describe the give-and-take of gases between plants and animals.

Where else this chapter is tested

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

TN Class 6 Term 2 ExamHigh

NMMS / Foundation ScienceMedium

School unit testsHigh

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Questions students ask

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

Nitrogen makes up about 78% of air but is fairly unreactive, so although we breathe it in we use the oxygen (about 21%); oxygen is the active, life-supporting part of air.

Pass the gas through lime water — if it turns the clear lime water milky, the gas is carbon dioxide.

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