मुख्य सामग्री पर जाएं
REET Level 2 · Science

Combustion and Flame — REET Level 2 Science Notes

इसे हिंदी में पढ़ें →

This page covers combustion and flame for REET Level 2 Science — ignition temperature, types of combustion, the structure of a flame, and the calorific value of major fuels — in exam-focused form. REET 2024 asked a direct question comparing the calorific values of different fuels.

  • 1 question in REET 2024
  • Class 6-8 level content
  • 6 min read
Verified 17 September 2026

How REET actually asks this topic

REET 2024 asked a question comparing the calorific value (in kJ/kg) of different fuels — which of the given options has the lowest calorific value. In other words, REET often builds questions around remembering the order of calorific values of various fuels, as well as around the core concepts of combustion (ignition temperature, flame structure).

What is combustion? Ignition temperature

Combustion is the chemical process in which a substance reacts with oxygen to produce heat (and usually light). A substance that produces heat when burnt is called a fuel — such as wood, coal, petrol, and LPG.

The minimum temperature at which a substance starts to burn is called its ignition temperature; below this temperature, a substance cannot catch fire, even if oxygen is available. Substances like petrol, alcohol and LPG, which have a very low ignition temperature, are called inflammable, and they need to be stored away from fire and handled with care.

A good fuel should have certain qualities — a high calorific value, a suitable ignition temperature (neither too high nor too low), low smoke and pollution when burnt, easy storage and transport, and an affordable price. Different fuels are compared against one another based on exactly these qualities.

Types of combustion

Rapid combustion
Ordinary burning, with a quick flame — such as LPG burning
Spontaneous combustion
Catching fire suddenly, without any external flame — such as coal dust in coal mines
Explosive combustion
Combustion that happens extremely fast, along with heat, light and sound — such as firecrackers

Three conditions required for combustion

ComponentRole
Fuel (the substance that burns)The substance that burns
Oxygen / airEssential for combustion — it cannot happen without it
Heat up to the ignition temperatureNecessary to heat the substance to its ignition temperature

Removing any one of these three puts out a fire — this is the basic principle behind a fire extinguisher.

Common ways of putting out a fire

Pouring water
Cools the substance below its ignition temperature and cuts off the oxygen supply; suitable only for ordinary solid-fuel fires
Using carbon dioxide (CO₂)
Because it is heavier than air, it forms a layer over the fuel and cuts off the oxygen supply; suitable for electrical and oil fires
Throwing sand or soil
Puts out a fire by cutting off the oxygen supply, especially useful for oil fires
Never pouring water on an electrical fire
Water conducts electricity, so there's a risk of the current spreading
Never pouring water on an oil/petrol fire
Oil is lighter than water and floats on top of it, so the burning oil can spread further and the fire can grow

The structure of a candle flame

A candle flame has three zones — the innermost black/dark zone (unburnt vapour, the least hot), the middle bright yellow zone (partial combustion, bright but only moderately hot), and the outermost blue zone (complete combustion, the hottest but least bright). This structure shows that the more oxygen is available in a flame, the more complete the combustion is, and the greater the heat produced.

Calorific value of major fuels

FuelCalorific value (approx., kJ/kg)
Cow-dung cakes6,000 – 8,000
Wood17,000 – 22,000
Coal25,000 – 33,000
Biogas35,000 – 40,000
Petrol / kerosene / dieselapproximately 45,000
CNG (methane)approximately 50,000
LPGapproximately 55,000
Hydrogenapproximately 1,50,000

Among the options given in the REET 2024 question (coal, petrol, LPG, hydrogen), coal has the lowest calorific value — that was the correct answer.

How to teach this to children

Classroom activity

Light a candle and have children carefully observe the three layers of the flame (black, yellow, blue), and have them label each part. You can also show, by holding a cold spoon briefly in different parts of the flame, which part deposits the most soot.

Teaching aids

Cover a burning candle with a glass to show that the flame goes out as soon as the oxygen runs out — demonstrating the role of one of the three conditions for combustion (oxygen). You can also cover it with glasses of different sizes to show that the candle burns longer inside a bigger glass.

Common misconception

Students often think the brightest flame is the hottest one; make it clear that the outermost blue (less bright) part of a candle flame is actually the hottest, because combustion is complete there. Also clarify that calorific value and ignition temperature are two different quantities that shouldn't be mixed up.

Assessment idea

Ask students, 'why shouldn't water be used to put out an electrical fire?' — a correct answer checks understanding of the danger of electrical conduction. You can also ask them to arrange various fuels in increasing order of calorific value.

Memory trick

Remember the order of calorific value — 'cow-dung cake < wood < coal < biogas < petrol/diesel < CNG < LPG < hydrogen' — that is, calorific value generally increases as you move from traditional solid fuels toward gaseous fuels, with hydrogen highest of all. For the three colours of the flame, remember — 'black is the coolest, yellow is in between, blue is the hottest'.

REET 2024 question (with solution)

Question 148

The fuel having lowest calorific value in kJ/kg unit is

  1. Aकोयला
  2. Bपैट्रोल
  3. Cएलपीजी
  4. Dहाइड्रोजन

Explanation

NCERT के अनुसार ईंधनों के ऊष्मीय मान लगभग इस प्रकार हैं: कोयला 25000–33000 kJ/kg, पेट्रोल लगभग 45000 kJ/kg, एलपीजी लगभग 55000 kJ/kg और हाइड्रोजन लगभग 150000 kJ/kg। अतः दिए गए विकल्पों में सबसे कम ऊष्मीय मान कोयले का है, जबकि हाइड्रोजन का सबसे अधिक।

Practice questions

Question 1

The minimum temperature at which a substance starts to burn is called its

  1. Aगलनांक
  2. Bक्वथनांक
  3. Cज्वलन ताप
  4. Dसामान्य ताप

Explanation

Ignition temperature is the minimum temperature at which a substance starts to burn in air.

Question 2

Calorific value is measured in

  1. Aजूल प्रति सेकंड में
  2. Bकिलोजूल प्रति किलोग्राम (kJ/kg) में
  3. Cडिग्री सेल्सियस में
  4. Dपास्कल में

Explanation

Calorific value is the heat produced by the complete combustion of 1 kilogram of fuel, measured in kJ/kg.

Question 3

Which of the following fuels has the highest calorific value?

  1. Aकोयला
  2. Bपेट्रोल
  3. CLPG
  4. Dहाइड्रोजन

Explanation

Hydrogen's calorific value is approximately 1,50,000 kJ/kg, the highest among the given options.

Question 4

Which of the following fuels has the lowest calorific value?

  1. Aहाइड्रोजन
  2. BLPG
  3. Cकोयला
  4. DCNG

Explanation

Coal's calorific value is approximately 25,000-33,000 kJ/kg, the lowest among the given options (the REET 2024 pattern).

Question 5

The outermost region of a candle flame is called

  1. Aकाला क्षेत्र (अदहित वाष्प)
  2. Bपीला क्षेत्र (आंशिक दहन)
  3. Cनीला क्षेत्र (पूर्ण दहन, सबसे गर्म)
  4. Dकोई क्षेत्र नहीं होता

Explanation

The outermost blue zone of a candle flame is the hottest, because combustion is complete there, even though it's the least bright.

Question 6

The three conditions necessary for combustion are

  1. Aईंधन, ऑक्सीजन, ज्वलन ताप तक ऊष्मा
  2. Bकेवल ईंधन
  3. Cकेवल ऑक्सीजन
  4. Dईंधन व जल

Explanation

Combustion needs three things — fuel, oxygen (air), and heat up to the ignition temperature.

Question 7

A fire breaking out in coal mines without any external flame is an example of which type of combustion?

  1. Aतीव्र दहन
  2. Bस्वतः दहन
  3. Cविस्फोटक दहन
  4. Dकोई दहन नहीं

Explanation

Catching fire suddenly, without any external flame, is called spontaneous combustion, as happens with coal dust in coal mines.

Question 8

A teacher explains to children the reason a burning candle goes out when covered with a glass as follows

  1. Aईंधन खत्म हो गया
  2. Bऑक्सीजन की आपूर्ति बंद हो गई
  3. Cज्वलन ताप बढ़ गया
  4. Dमोमबत्ती पिघल गई

Explanation

Covering it with a glass cuts off the air (oxygen) supply, and since that's one of the essential conditions for combustion, the flame goes out.

Question 9

Water should not be used to put out an electrical fire because

  1. Aपानी ज्वलनशील है
  2. Bपानी विद्युत का चालक है और करंट फैला सकता है
  3. Cपानी ठंडा नहीं होता
  4. Dपानी ऑक्सीजन देता है

Explanation

Water conducts electricity, so pouring water on an electrical fire can spread the current and increase the danger.

Question 10

Which of the following fuels is considered as clean (low-polluting) as CNG and LPG?

  1. Aकोयला
  2. Bलकड़ी
  3. Cबायोगैस
  4. Dकंडा (गोबर)

Explanation

Biogas, like CNG and LPG, is a relatively clean fuel that produces less pollution, while coal, wood and cow-dung cakes produce more smoke and pollutants.

Frequently asked questions

What is ignition temperature?+

The minimum temperature at which a substance starts to burn is called its ignition temperature; below this temperature, a substance won't burn even in the presence of oxygen.

Which part of a candle flame is the hottest?+

The outermost blue zone, where combustion is complete, is the hottest — even though it's the least bright.

In what unit is calorific value measured?+

Calorific value is measured in kilojoules per kilogram (kJ/kg) — it's the amount of heat produced by the complete combustion of 1 kilogram of fuel.

Why are CNG and LPG considered clean fuels?+

CNG and LPG produce much less smoke and far fewer pollutants when burnt compared to coal and diesel, which is why they're considered relatively clean fuels.

How does REET ask questions on combustion and flame?+

Around comparing calorific values and around the structure of a flame, as seen in the REET 2024 question, which asked you to compare the calorific values of coal, petrol, LPG and hydrogen.

What's the difference between spontaneous combustion and rapid combustion?+

In rapid combustion, a substance starts burning from an external flame, while in spontaneous combustion, a substance catches fire on its own without any external flame, as happens with coal dust in coal mines.

Why shouldn't water be poured on an oil or petrol fire?+

Because oil is lighter than water, it floats on top of it, which can spread the burning oil further and make the fire worse; such fires should be put out with sand or CO₂ instead.

Read next

Coal and Petroleum

Previous topic

The Oxygen and Nitrogen Cycles

Next topic

REET Syllabus

See the complete REET syllabus in detail

REET 2024 Solved Paper

Read all REET 2024 questions with solutions

Change Log

  • 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked