Combustion and Flame
Welcome to HSLC Guru! In this lesson, we explore Class 8 General Science Chapter 5 — Combustion and Flame as per the ASSEB (Assam State School Education Board) syllabus. This chapter introduces students to one of the most important chemical reactions in our daily lives — combustion. From cooking food on a gas stove to running vehicles and generating electricity, combustion is everywhere. We will study what combustion is, the different types of combustion, the conditions necessary for combustion to occur, ignition temperature, inflammable substances, how fires are controlled, the structure of a flame, the calorific value of fuels, and the harmful effects of burning fuels. This well-organised study guide includes a clear summary, complete textbook question-answers, additional MCQs, fill-in-the-blanks, true/false statements, and a glossary — perfectly tailored for ASSEB Class 8 students.
Summary
Combustion is a chemical process in which a substance reacts with oxygen to give off heat and usually light. The substance that undergoes combustion is called a combustible substance or fuel. Examples include wood, coal, kerosene, petrol, LPG, and natural gas. Substances such as iron, glass, stone, and water do not burn and are called non-combustible substances. There are three main types of combustion. Rapid combustion occurs when a gas burns quickly producing heat and light, as when a matchstick is brought near LPG. Spontaneous combustion happens without any external heat, for example, the burning of phosphorus at room temperature or forest fires caused by the heat of the sun. Explosive combustion occurs when a sudden reaction releases heat, light, sound, and gases, as in fireworks or dynamite. Three conditions are essential for combustion to take place: a combustible substance, a supporter of combustion (usually oxygen from air), and the attainment of ignition temperature — the lowest temperature at which a substance catches fire.
Substances that have very low ignition temperatures and can easily catch fire with a flame are called inflammable substances. Examples include petrol, alcohol, LPG, and CNG. To control fire, we must remove at least one of the three essential requirements of combustion. The most common fire extinguisher is water, which cools the burning substance below its ignition temperature and also cuts off the air supply. However, water cannot be used for fires involving oil and petrol because oil floats on water and continues to burn. Water is also dangerous on electrical fires because it conducts electricity. For oil and electrical fires, carbon dioxide (CO₂) is the best extinguisher — it is heavier than air and forms a blanket around the fire, cutting off oxygen, and it does not conduct electricity. Foam-type extinguishers are used for oil fires, while sand can also smother small fires.
When a substance burns, it may produce a flame. Substances that vaporise during burning give a flame (e.g., wax, kerosene, LPG), while non-vaporising substances such as charcoal glow without a flame. Flames are of two types — luminous flame, which is bright yellow due to unburnt carbon particles (e.g., a candle flame), and non-luminous flame, which is blue and produces complete combustion (e.g., the LPG burner). A candle flame has three zones: the innermost dark zone (unburnt wax vapour, lowest temperature), the middle luminous yellow zone (partial combustion, moderately hot), and the outermost non-luminous blue zone (complete combustion, hottest part of the flame, used by goldsmiths to melt gold and silver).
A fuel is a substance that produces heat or energy on combustion. Fuels can be solid (wood, coal), liquid (kerosene, petrol, diesel), or gaseous (LPG, CNG, biogas). The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value, expressed in kilojoules per kilogram (kJ/kg). A good fuel must be cheap, easily available, easy to store and transport, have a high calorific value, a moderate ignition temperature, and produce minimum smoke and harmful residues. Hydrogen has the highest calorific value (about 1,50,000 kJ/kg). Incomplete combustion of fuels has serious harmful effects: it produces carbon monoxide (CO), a poisonous gas that can cause death; releases carbon dioxide (CO₂), which contributes to global warming; and produces oxides of sulphur and nitrogen, which dissolve in rainwater to form acid rain, damaging buildings, soil, and aquatic life.
Textbook Questions and Answers
1-Mark Questions
Q1. What is combustion?
Answer: Combustion is a chemical reaction in which a substance reacts with oxygen to produce heat and usually light.
Q2. Define ignition temperature.
Answer: Ignition temperature is the lowest temperature at which a substance catches fire and starts to burn.
Q3. What is a combustible substance? Give one example.
Answer: A combustible substance is one that catches fire and burns in the presence of oxygen. Example: wood.
Q4. Name two non-combustible substances.
Answer: Iron and glass are non-combustible substances.
Q5. What is the supporter of combustion in air?
Answer: Oxygen is the supporter of combustion in air.
Q6. Define calorific value.
Answer: Calorific value is the amount of heat energy produced on complete combustion of 1 kg of a fuel, measured in kJ/kg.
Q7. Which zone of a candle flame is the hottest?
Answer: The outermost non-luminous blue zone is the hottest part of a candle flame.
Q8. Which fuel has the highest calorific value?
Answer: Hydrogen has the highest calorific value (about 1,50,000 kJ/kg).
Q9. Name the gas produced by incomplete combustion of carbon fuels that is poisonous.
Answer: Carbon monoxide (CO) is the poisonous gas produced by incomplete combustion.
Q10. Why should water not be used to extinguish electrical fires?
Answer: Water conducts electricity and may give a fatal shock; therefore, it should not be used on electrical fires.
2 to 3-Mark Questions
Q1. What are the three conditions necessary for combustion?
Answer: The three conditions necessary for combustion are: (i) the presence of a combustible substance (fuel), (ii) the presence of a supporter of combustion (oxygen), and (iii) the attainment of ignition temperature. If any one of these is absent, combustion will not take place.
Q2. Differentiate between rapid combustion and explosive combustion.
Answer: In rapid combustion, a substance burns quickly producing heat and light, e.g., LPG burning in a stove. In explosive combustion, a sudden reaction takes place releasing heat, light, sound, and a large amount of gas, e.g., bursting of crackers and dynamite. Explosive combustion is much faster and more violent than rapid combustion.
Q3. Why is CO₂ a good fire extinguisher?
Answer: Carbon dioxide is a good fire extinguisher because it is heavier than air and forms a blanket around the burning substance, cutting off the supply of oxygen. It does not support combustion and does not conduct electricity, so it can be safely used on oil and electrical fires.
Q4. What is an inflammable substance? Give two examples.
Answer: Inflammable substances are substances that have very low ignition temperatures and catch fire very easily with a flame. They are highly dangerous and need to be stored carefully. Examples: petrol and LPG.
Q5. Differentiate between a luminous and a non-luminous flame.
Answer: A luminous flame is bright yellow in colour and is produced when combustion is incomplete; unburnt carbon particles glow and emit light, e.g., a candle flame. A non-luminous flame is blue and is produced when combustion is complete with a sufficient supply of oxygen, e.g., the flame of an LPG stove. Non-luminous flames are hotter than luminous flames.
Q6. Why does a paper cup filled with water not burn?
Answer: When a paper cup filled with water is heated, the heat is absorbed by the water. The temperature of the paper does not reach its ignition temperature because the heat is continuously transferred to the water. As a result, the paper cup does not catch fire.
5 to 6-Mark Questions
Q1. Describe the structure of a candle flame with the three zones.
Answer: A candle flame has three distinct zones:
- Innermost dark zone: This zone is around the wick. It contains unburnt wax vapours and has the lowest temperature. No combustion takes place here due to lack of oxygen.
- Middle luminous yellow zone: This is the moderately hot zone where partial combustion of wax vapour occurs. Unburnt carbon particles glow and emit a bright yellow light, giving the flame its luminosity.
- Outermost non-luminous blue zone: This zone receives plenty of oxygen, leading to complete combustion. It is the hottest part of the flame and is almost colourless or pale blue. Goldsmiths use this zone to melt gold and silver because of its high temperature.
Q2. What are the characteristics of a good fuel?
Answer: A good fuel should possess the following characteristics:
- It should be readily available and cheap.
- It should have a high calorific value, producing a large amount of heat per unit mass.
- It should have a moderate ignition temperature so that it can be ignited easily but does not catch fire on its own.
- It should burn smoothly without producing harmful gases or excessive smoke.
- It should be easy to store and safe to transport.
- It should leave little or no residue after burning.
Examples of good fuels are LPG and CNG, which have high calorific values and are smokeless.
Q3. Explain the harmful effects of burning fuels.
Answer: Burning of fuels has several harmful effects on the environment and human health:
- Carbon monoxide poisoning: Incomplete combustion of carbon-based fuels produces carbon monoxide (CO), a colourless and odourless poisonous gas. It can cause headaches, dizziness, and even death when inhaled in closed rooms.
- Global warming: Combustion of fossil fuels releases carbon dioxide (CO₂), a greenhouse gas. Increased levels of CO₂ in the atmosphere trap heat and raise the Earth’s average temperature, causing climate change, melting of polar ice, and rising sea levels.
- Acid rain: Burning of coal and petroleum produces oxides of sulphur (SO₂) and nitrogen (NO₂). These oxides dissolve in rainwater to form sulphuric acid and nitric acid, which fall as acid rain. Acid rain damages buildings, monuments, soil fertility, plants, and aquatic life.
- Air pollution: Smoke and unburnt particles released during combustion pollute the air and cause respiratory diseases such as asthma and bronchitis.
Q4. Explain how a fire can be controlled by removing each of the three requirements of combustion.
Answer: Combustion needs three things — fuel, oxygen, and ignition temperature. Removing even one of these stops the fire.
- Removing fuel: If the source of fuel can be cut off, the fire dies out. For example, when LPG cylinder valves are closed, the gas supply stops and the flame goes off.
- Cutting off oxygen supply: Covering a small fire with a thick blanket or sand stops the air supply, smothering the fire. CO₂ extinguishers form a blanket of gas around the fire, blocking oxygen.
- Lowering the temperature below ignition point: Spraying water cools the burning substance below its ignition temperature, and the fire stops. Water also produces steam, which further cuts off oxygen.
By understanding these principles, fire safety measures can be effectively planned at home, in industries, and in public places.
Q5. Compare LPG and wood as fuels for cooking.
Answer: LPG is far superior to wood as a domestic fuel for the following reasons:
- Calorific value: LPG has a much higher calorific value (about 55,000 kJ/kg) than wood (17,000–22,000 kJ/kg), so a small amount of LPG produces a large amount of heat.
- Smoke and pollution: LPG burns with a clean blue flame and produces almost no smoke; wood produces a lot of smoke that pollutes the air and harms health.
- Convenience: LPG is supplied in cylinders and can be turned on and off easily; wood must be collected, stored, and lit using kindling.
- Residue: LPG leaves no ash, while wood leaves ash and unburnt charcoal.
- Environmental impact: Wood-burning contributes to deforestation; though LPG is a fossil fuel, it pollutes much less per unit of heat produced.
Additional Multiple Choice Questions (MCQ)
Q1. Which of the following is NOT a combustible substance?
(a) Wood (b) Paper (c) Iron (d) Coal
Answer: (c) Iron
Q2. The supporter of combustion in air is:
(a) Nitrogen (b) Oxygen (c) Carbon dioxide (d) Hydrogen
Answer: (b) Oxygen
Q3. The hottest zone of a candle flame is:
(a) Innermost dark zone (b) Middle luminous zone (c) Outermost non-luminous zone (d) The wick
Answer: (c) Outermost non-luminous zone
Q4. Which fuel has the highest calorific value?
(a) Coal (b) Wood (c) LPG (d) Hydrogen
Answer: (d) Hydrogen
Q5. Calorific value is measured in:
(a) kg/kJ (b) kJ/kg (c) J/g (d) cal/sec
Answer: (b) kJ/kg
Q6. Which of these is an inflammable substance?
(a) Sand (b) Water (c) Petrol (d) Glass
Answer: (c) Petrol
Q7. The poisonous gas produced by incomplete combustion is:
(a) CO₂ (b) CO (c) SO₂ (d) NO₂
Answer: (b) CO
Q8. Which fire extinguisher is best for electrical fires?
(a) Water (b) Foam (c) CO₂ (d) Sand alone
Answer: (c) CO₂
Q9. A flame is produced by substances that:
(a) Vaporise on burning (b) Do not vaporise (c) Are solids only (d) Contain water
Answer: (a) Vaporise on burning
Q10. Acid rain is caused mainly by oxides of:
(a) Carbon and hydrogen (b) Sulphur and nitrogen (c) Oxygen and nitrogen (d) Sodium and potassium
Answer: (b) Sulphur and nitrogen
Fill in the Blanks
Q1. The lowest temperature at which a substance catches fire is called __________.
Answer: ignition temperature
Q2. __________ is a substance that supports combustion.
Answer: Oxygen
Q3. The yellow part of a candle flame is called the __________ zone.
Answer: luminous
Q4. The amount of heat produced by complete combustion of 1 kg of fuel is called its __________.
Answer: calorific value
Q5. __________ rain is caused by oxides of sulphur and nitrogen dissolving in rainwater.
Answer: Acid
True or False
Q1. Combustion always produces a flame.
Answer: False (substances like charcoal glow without producing a flame).
Q2. Water is the best extinguisher for oil fires.
Answer: False (oil floats on water and continues to burn).
Q3. The blue zone of a candle flame is the hottest part.
Answer: True
Q4. Carbon monoxide is a poisonous gas produced during incomplete combustion.
Answer: True
Q5. CNG is more polluting than petrol and diesel.
Answer: False (CNG is much cleaner than petrol and diesel).
Glossary
| Term | Meaning |
|---|---|
| Combustion | A chemical reaction in which a substance reacts with oxygen, producing heat and light. |
| Combustible substance | A substance that burns in the presence of oxygen, e.g., wood, coal. |
| Non-combustible substance | A substance that does not burn, e.g., iron, glass, stone. |
| Ignition temperature | The lowest temperature at which a substance catches fire. |
| Inflammable substance | A substance with a very low ignition temperature, easily catching fire, e.g., petrol, LPG. |
| Rapid combustion | Combustion in which a gas burns quickly producing heat and light. |
| Spontaneous combustion | Combustion that occurs without an external source of heat, e.g., phosphorus. |
| Explosive combustion | Sudden combustion producing heat, light, sound, and gases, e.g., crackers. |
| Luminous flame | A bright yellow flame produced by incomplete combustion. |
| Non-luminous flame | A blue flame produced by complete combustion. |
| Calorific value | The heat produced by complete combustion of 1 kg of fuel, expressed in kJ/kg. |
| Carbon monoxide | A poisonous, colourless, odourless gas formed by incomplete combustion. |
| Acid rain | Rain containing dissolved acids formed from oxides of sulphur and nitrogen. |
| Global warming | Rise in Earth’s average temperature due to increased greenhouse gases like CO₂. |
| Fire extinguisher | A device or substance used to put out fires (water, foam, CO₂). |
That brings us to the end of Class 8 General Science Chapter 5 — Combustion and Flame for the ASSEB syllabus. Keep revising regularly with HSLC Guru and stay tuned for more chapter-wise notes, question-answers, and practice resources designed especially for Assam Board students.