Combustion and Flame | Class 8 Science | Chapter 6

Table of Contents

Combustion and Flame

Combustion: A chemical process where a substance reacts with oxygen, releasing heat and light. Example: burning wood.

Combustion and Flame

Combustible and Non-Combustible Substances: Combustible substances easily catch fire (e.g., paper, coal), while non-combustible substances don’t readily catch fire (e.g., sand, water).

Fuel: Substances that, upon combustion, produce usable energy. Fuels can be solid, liquid, or gas, and can be natural or artificial.

Ignition Temperature: The lowest temperature at which a substance catches fire in the presence of air.

Inflammable Substances: Substances with low ignition temperatures that can easily catch fire with a flame (e.g., diesel, LPG).

Fire: A chemical combustion reaction between oxygen and fuel, its duration depends on available fuel and oxygen.

Fire Triangle: Fire requires three elements: fuel, heat source, and oxygen. Removing any of these elements controls the fire.

Fire Triangle images

Flame: The visible, gaseous part of the fire, produced by the light energy released during combustion.

Zones of Candle Flame: Candle flame has three zones: outermost (hottest, blue, non-luminous), middle (moderately hot, yellow, partial combustion), innermost (least hot, black, unburnt wax vapors).

Smoke: Solid (unburnt particles) dispersed in gas (air). Black color due to unburnt carbon particles.

Matchstick: Contains red phosphorus, which turns into white phosphorus on heating, igniting the matchstick.

Working of Matchstick:

A matchstick works by striking its head against a rough surface, which generates friction and heats up the head. Red phosphorus in the head converts to white phosphorus due to the heat, and white phosphorus spontaneously ignites upon contact with air. The ignited flame then spreads to the wooden stem, causing it to burn as long as there is enough fuel and oxygen.

Types of Combustion: Rapid combustion (quick release of heat and light, e.g., LPG) and spontaneous combustion (substances catch fire without external heat, e.g., forest fires).

Fire Extinguisher: Device used to control fires by cutting off oxygen supply or reducing fuel temperature.

Calorific Value: The amount of heat energy produced by the complete combustion of 1 kg of fuel, measured in kJ/kg.

Ideal Fuel: Cheap, readily available, highly combustible, high calorific value, and minimal harmful emissions.

Pollution: Burning fuels like wood, coal, and petroleum releases harmful carbon particles, carbon monoxide, and carbon dioxide, causing respiratory problems, global warming, and acid rain. Pollution caused by burning fuels has severe consequences for both human health and the environment. When fuels like wood, coal, and petroleum products are burned, harmful by-products are released, contributing to various issues:

    1. Unburnt Carbon Particles: Combustion of carbon-based fuels releases fine particles of unburnt carbon into the air. These particles, also known as particulate matter, are hazardous pollutants that can be inhaled deep into the lungs, leading to respiratory problems, particularly for individuals with asthma or other respiratory conditions.
    2. Carbon Monoxide (CO) Emission: Incomplete combustion of fuels produces carbon monoxide, a colorless and odorless gas that is highly toxic. If carbon monoxide accumulates in closed spaces, such as poorly ventilated rooms, it can be deadly, as it interferes with the body’s ability to transport oxygen.
    3. Carbon Dioxide (CO2) and Global Warming: The burning of fuels releases carbon dioxide into the atmosphere. This greenhouse gas traps heat in the Earth’s atmosphere, contributing to the phenomenon known as global warming. As the average temperature of the planet rises, it leads to adverse effects like the melting of polar ice caps and changes in rainfall patterns, which can disrupt ecosystems and cause environmental imbalances.
    4. Acid Rain: Burning fuels also releases gases like sulphur dioxide and nitrogen oxide. These gases react with water molecules in the atmosphere to form acids, leading to the formation of acid rain. Acid rain can be harmful to plants, soil, aquatic life, and even infrastructure. It can lead to the acidification of water bodies, causing damage to aquatic ecosystems and affecting the health of fish and other aquatic organisms.

Acid Rain

CNG – The Clean Fuel: Compressed Natural Gas (CNG) is replacing diesel and petrol in vehicles due to its lower pollution levels.

Read More: Coal and Petroleum Class 8 Notes | Chapter 5


(a) Burning of wood and coal causes pollution of air.

(b) A liquid fuel, used in homes is kerosene.

(c) Fuel must be heated to its ignition temperature before it starts burning.

(d) The fire produced by oil cannot be controlled by water.

The use of CNG in automobiles reduces pollution in cities due to lower emissions of harmful pollutants, greenhouse gases, and toxic substances. It improves air quality, reduces smog formation, and promotes sustainable energy sources.

LPG (Liquefied Petroleum Gas):

  1. Source: LPG is a fossil fuel derived from natural gas processing and petroleum refining.
  2. State: LPG is a gaseous fuel at room temperature but can be easily liquefied under moderate pressure.
  3. Calorific Value: LPG has a high calorific value, which means it produces more energy per unit mass compared to wood.
  4. Combustion: LPG combustion is more efficient and cleaner as it results in minimal emissions of particulate matter and soot.
  5. Convenience: LPG is readily available in cylinders and can be easily transported and stored for domestic and industrial use.


  1. Source: Wood is a renewable fuel obtained from trees and plant matter.
  2. State: Wood is typically used in its solid form, such as logs or wood pellets.
  3. Calorific Value: Wood has a lower calorific value compared to LPG, which means it produces less energy per unit mass.
  4. Combustion: Wood combustion produces more emissions of particulate matter and smoke, making it less efficient and contributing to air pollution.
  5. Availability: Wood availability depends on forests and sustainable forestry practices, which can be influenced by factors like deforestation and climate change.

(a) Water is not used to control fires involving electrical equipment because water is a good conductor of electricity. If water is applied to an electrical fire, it can conduct the electricity and lead to electrocution or cause the fire to spread further.

(b) LPG is a better domestic fuel than wood because:

  • LPG has a higher calorific value, providing more energy per unit mass, making it more efficient for cooking and heating purposes.
  • LPG burns cleanly with minimal emissions of smoke and particulate matter, reducing air pollution and health hazards.
  • LPG is readily available in cylinders and easy to handle, while wood requires storage space and can be affected by moisture and pests.
  • LPG is a controlled and convenient fuel source, while wood may vary in quality and burn unevenly.

(c) Paper by itself catches fire easily due to its low ignition temperature and combustible nature. When wrapped around an aluminum pipe, the metal acts as a heat sink, absorbing and dissipating heat quickly. This prevents the paper from reaching its ignition temperature, making it less likely to catch fire.

The calorific value of a fuel is expressed in the unit "kilojoules per kilogram" (kJ/kg).

CO2 controls fires by suffocating the flames. When released, it displaces oxygen, leaving the fire without enough air to burn. The lack of oxygen effectively smothers the fire, leading to its suppression. CO2's cooling effect further aids in extinguishing the fire by reducing the fuel's temperature.

Burning a heap of green leaves is difficult due to the presence of moisture in the leaves. The water content acts as a cooling agent, absorbing heat and making it challenging for the leaves to reach their ignition temperature. On the other hand, dry leaves have little to no moisture, allowing them to readily reach their ignition temperature when exposed to a heat source, causing them to catch fire easily. The absence of moisture in dry leaves facilitates the combustion process, making them highly combustible compared to green leaves.

A goldsmith uses the outermost zone of a flame, known as the non-luminous blue zone, for melting gold and silver. This zone has the highest temperature among all zones of the flame.

To calculate the calorific value of the fuel, we can use the formula:

Calorific Value = Heat Produced / Mass of Fuel

Given: Mass of fuel (m) = 4.5 kg Heat produced (Q) = 180,000 kJ

Calorific Value (CV) = Q / m Calorific Value (CV) = 180,000 kJ / 4.5 kg

Now, let's calculate the calorific value:

CV = 40,000 kJ/kg

Therefore, the calorific value of the fuel is 40,000 kJ/kg.

No, rusting cannot be called combustion. Combustion is a rapid reaction with oxygen, producing heat and light, consuming the original substance. Rusting is a slow oxidation process, forming rust on the metal's surface, without releasing significant energy and not consuming the entire metal.

Ramesh's water in the beaker will get heated in a shorter time.  because the outermost part of the candle flame, which is the blue and non-luminous zone, is the hottest part of the flame. This zone has the highest temperature, and heat is transferred most efficiently here. Placing the beaker in this zone allows for faster and more effective heating of the water compared to the yellow part of the flame.

Leave a Comment

Verified by MonsterInsights