NCERT Science Notes - Class 8
Chapter 4 - Combustion and Flame

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 4 - Combustion and Flame. The chapter delves into the fascinating world of burning substances. It explains the essential conditions for combustion, types of fuels, and the concept of ignition temperature. Students learn about different zones of a flame and the characteristics of ideal fuels. The chapter also highlights the environmental impact of combustion and the importance of using cleaner fuels like CNG and LPG. Through engaging examples and experiments, this chapter provides a comprehensive understanding of how combustion plays a crucial role in our daily lives

NCERT Science Notes - Class 8 Chapter 4 - Combustion and Flame, ajslearning, ajs chalo seekhen

NCERT Science Notes - Class 8
Chapter 4 - Combustion and Flame

Different types of fuels and their uses:

Difference Between Burning of a Candle and Coal

• Candle:
  • Burns with a flame.
  • Produces light and heat.
• Coal:
  • Does not burn with a visible flame.
  • Produces heat but may emit smoke or pollutants.
    
    

• Combustion: The chemical process of burning that occurs when a fuel reacts with oxygen.
• Types of Flame: Different materials produce different types of flames based on their combustion properties.

4.1 - What is Combustion?

• Definition: Combustion is a chemical process in which a substance reacts with oxygen to produce heat and often light.
• Combustible Substances: The substances that undergo combustion are called combustible substances or fuels. Fuels can be solid, liquid, or gas.
• Examples of Combustion:
  • Magnesium Ribbon: Burns to form magnesium oxide, releasing heat and light.
  • Charcoal: Burns in air, producing carbon dioxide, heat, and light.
  • Coal: Also burns in air, producing carbon dioxide, heat, and light.
• Observation: When performing combustion activities (like burning magnesium or charcoal), heat and light are typically observed.
• Food as Fuel: Food is considered fuel for the body. It is broken down in the body through a reaction with oxygen, producing heat, similar to how fuels burn to release energy.

Activity 4.1 : Testing for Combustibility

Objective: To identify materials as combustible or non-combustible through burning.

Materials to Collect:

• Straw, Matchsticks, Kerosene oil, Paper, Iron nails, Stone pieces, Glass, Charcoal

1. Under the supervision of your teacher, attempt to burn each of the materials one by one.
2. Observe whether combustion occurs.
3. Record your findings in the table below:

3.1 - Coal

Overview

• Description: Coal is a hard, black fuel used for cooking, electricity generation, and in various industries.
  
  

• Origin: Formed approximately 300 million years ago from dense forests in wetland areas.
• Process:
  • Forests were buried under soil due to natural events (e.g., flooding).
  • Over time, increased soil deposition led to compression and rising temperatures.
  • Dead vegetation was converted to coal through a process called carbonisation.
    
    

• Type: Classified as a fossil fuel due to its formation from organic matter.
  
  

1. Coke:
   • Tough, porous, black substance; almost pure carbon.
   • Uses: Manufacture of steel and extraction of metals.
2. Coal Tar:
   • Black, thick liquid with an unpleasant smell; a mixture of about 200 substances.
   • Uses:
     • Starting material for synthetic dyes, drugs, explosives, perfumes, plastics, paints, and roofing materials.
     • Naphthalene balls for repelling moths are also derived from coal tar.
   • Note: Bitumen, a petroleum product, is now often used instead of coal tar for road metalling.
3. Coal Gas:
   • Produced during coal processing for coke.
   • Uses: Fuel in industries near processing plants.
   • Historical Significance: First used for street lighting in London (1810) and New York (1820). Now primarily used for heat.

Activity 4.3: Observing Combustion and Ignition Temperature

Objective: To understand how combustion can be influenced by the availability of air and to explore the concept of ignition temperature.

Materials Needed:

• Burning wood or charcoal
• Iron plate or Tawa
• Glass jar, tumbler, or transparent plastic jar
  
  

1. Place a piece of burning wood or charcoal on an iron plate or Tawa.
2. Cover it with a glass jar or tumbler, or a transparent plastic jar.
3. Observation: Watch what happens to the burning material. Does it stop burning after some time?
   
   

• Why does the charcoal stop burning?
• When a person's clothes catch fire, why is it recommended to cover them with a blanket?
  
  

• Self-Ignition of Matchsticks:
  • Does a matchstick burn by itself? How does it burn?
  • Recall your experience with burning a piece of paper. Does it ignite when a burning matchstick is brought near it?
  • Can you light a piece of wood with a matchstick? Why do you need paper or kerosene oil to start a fire in wood or coal?
    
    
• Forest Fires:
  • In summer, dry grass can catch fire, spreading quickly to trees and leading to forest fires. Why is it challenging to control such fires?
    
    

• Different substances catch fire at different temperatures. 
• The lowest temperature at which a substance ignites is known as its ignition temperature.
• Why doesn't a matchstick ignite on its own at room temperature?
• A matchstick starts burning when rubbed against the side of the matchbox due to the friction generating enough heat to reach its ignition temperature.
  
  

The History of the Matchstick

1. Ancient Origins:
   • Over 5,000 years ago, ancient Egyptians used small pieces of pinewood dipped in sulfur as matches.
2. Development of Modern Safety Matches:
   • The modern safety match was developed about 200 years ago.
   • The head of the match was coated with a mixture of:
     • Antimony trisulphide
     • Potassium chlorate
     • White phosphorus
     • Glue and starch
3. Ignition Mechanism:
   • When the match was struck against a rough surface, the heat from friction ignited the white phosphorus, initiating combustion.
4. Safety Concerns:
   • White phosphorus posed dangers:
     • Risk to workers during manufacturing.
     • Health hazards for users.
5. Modern Composition:
   • Today, safety match heads contain:
     • Antimony trisulphide
     • Potassium chlorate
   • The rubbing surface includes:
     • Powdered glass
     • A small amount of red phosphorus (less dangerous than white phosphorus).
6. Ignition Process Today:
   • When struck against the rubbing surface:
     • Red phosphorus converts to white phosphorus.
     • This white phosphorus reacts with potassium chlorate, generating enough heat to ignite antimony trisulphide and start combustion.

Ignition Temperature and Combustion

1. Ignition Temperature:
   • A combustible substance will not catch fire or burn if its temperature is below its ignition temperature.
2. Examples of Ignition Temperature:
   • Cooking Oil:
     • Can catch fire if a frying pan is left on a burning stove for too long.
   • Kerosene Oil:
     • Does not catch fire at room temperature but can ignite if heated slightly.
   • Wood:
     • Requires more heat than kerosene oil to ignite and will not catch fire even if heated slightly.
3. Comparison of Ignition Temperatures:
   • Kerosene Oil has a lower ignition temperature than wood, meaning it is easier to ignite.
   • Storage Precautions:
     • Special care is necessary when storing kerosene oil due to its lower ignition temperature.

Refining of Petroleum 🛢️

Materials Needed:

• Two paper cups
• 50 mL of water
• A candle for heating
  
  

1. Preparation:
   • Fold a sheet of paper to make two paper cups.
   • Fill one cup with about 50 mL of water.
2. Heating:
   • Heat both cups separately with a candle.
     
     

1. Empty Paper Cup:
   • The empty paper cup will burn when heated directly over the flame.
2. Paper Cup with Water:
   • The paper cup with water does not catch fire.
   • The water in this cup may become hot, and it can even boil if heating continues.
     
     

• The heat supplied to the paper cup is transferred to the water by conduction.
• Since the water absorbs heat, the ignition temperature of the paper is not reached, preventing it from burning.
  
  

• Substances with very low ignition temperatures that can easily catch fire are called inflammable substances. Examples include:
  • Petrol
  • Alcohol
  • Liquefied Petroleum Gas (LPG)
    
    

• Students can add other examples such as:
  • Acetone
  • Ether
  • Gasoline
  • Turpentine
    
    

4.2 - How Do We Control Fire?

1. Fire Incidents:
   • Recall any fire accidents you've seen in homes, shops, or factories.
   • Write a short description in your notebook and share your experience with classmates.
2. Emergency Contact:
   • Find the telephone number for the fire service in your area. Knowing this number is crucial in case of a fire emergency.
3. Fire Brigade Response:
   • Determine if your city or town has a fire brigade station.
   • When a fire brigade arrives, their primary action is to pour water on the fire (see Fig. 4.6).
4. How Water Extinguishes Fire:
   • Cooling: Water cools the combustible material, reducing its temperature below the ignition temperature.
   • Oxygen Supply: Water vapor surrounds the combustible material, cutting off its supply of air, which is necessary for combustion.
5. Three Essential Requirements for Fire:
   • Fuel: The material that burns.
   • Air: Oxygen that supports combustion.
   • Heat: The temperature needed to ignite the fuel.
6. Controlling Fire:
   • Fire can be controlled by removing one or more of the three essential requirements.
   • Fire Extinguishers: These devices work by:
     • Cutting off the supply of air
     • Bringing down the temperature of the fuel
     • Often, the fuel cannot be completely eliminated (e.g., in a building fire, the structure itself serves as fuel).

Fire Extinguishers and Their Applications

1. Water as a Fire Extinguisher:
   • Commonly used for extinguishing fires involving materials like wood and paper.
   • Not suitable for:
     • Electrical Fires: Water can conduct electricity, posing a risk to those trying to douse the fire.
     • Oil and Petrol Fires: Water is heavier than oil, sinking below it and allowing the oil to continue burning on top.
2. Carbon Dioxide (CO₂) as a Fire Extinguisher:
   • Best for: Fires involving electrical equipment and flammable materials like petrol.
   • Mechanism:
     • CO₂ is heavier than oxygen, covering the fire like a blanket.
     • Cuts off contact between the fuel and oxygen, controlling the fire.
   • Advantages:
     • Typically does not harm electrical equipment.
     • Effective in cooling and suffocating the fire.
3. Supply of Carbon Dioxide:
   • Storage: CO₂ can be stored at high pressure as a liquid in cylinders.
   • Expansion: When released from the cylinder, CO₂ expands significantly in volume and cools down, creating an effective fire-suppressing blanket.
4. Alternative Sources of CO₂:
   • Dry powder chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate can be used near the fire.
   • When these chemicals are heated, they release CO₂, helping to extinguish the fire.

4.3 - Types of Combustion

1. Rapid Combustion:
   • Observation: When a burning matchstick or gas lighter is brought near a gas stove, the gas ignites quickly, producing heat and light.
   • Characteristics:
     • Fast process.
     • Releases a significant amount of energy in the form of heat and light.
     • Example: Burning of natural gas in a stove.
2. Spontaneous Combustion:
   • Definition: A type of combustion where a material ignites suddenly without an apparent external cause.
   • Examples:
     • Phosphorus: Burns in air at room temperature.
     • Coal Dust: Can spontaneously combust in coal mines, leading to disastrous fires.
     • Forest Fires: Often initiated by the heat of the sun or lightning strikes, but more frequently caused by human carelessness (e.g., unextinguished campfires).
3. Explosion:
   • Definition: A rapid combustion reaction that releases heat, light, and sound, often accompanied by a large volume of gas.
   • Characteristics:
     • Sudden and violent.
     • Can occur when pressure is applied to a combustible material.
   • Examples:
     • Fireworks during festivals, which explode when ignited due to a rapid reaction.

4.4 Flame

In this activity, we will observe whether various materials form a flame when they burn, and what the characteristics of the flame are. Let’s first observe the color of an LPG flame and a candle flame:

• LPG Flame: The color of the flame is usually blue, indicating complete combustion.
• Candle Flame: The flame is yellowish-orange, which suggests incomplete combustion (due to limited oxygen).
  
  

NCERT Science Notes - Class 8 | Science | Chapter 4 - Combustion and Flame

4.5 - Structure of a Flame

Activity 4.5

1. Steps:
   • Light a candle carefully.
   • Hold a 4-5 cm long thin glass tube with tongs and place one end in the dark zone of the candle flame.
   • Bring a lighted matchstick near the other end of the glass tube.
2. Observation:
   • After a while, you might notice a flame at the other end of the tube. This indicates that the vapors from the wax, which rise through the wick and vaporize during burning, produce the flame.
3. Explanation:
   • Substances that vaporize during combustion produce flames. For example, kerosene and molten wax vaporize and create flames, while charcoal does not vaporize, so it doesn’t form a flame.

• Introduce a clean glass plate into the luminous zone of a steady candle flame for about 10 seconds. Remove it and observe a black ring.
• This black ring is due to unburnt carbon particles present in the luminous zone.

• Hold a thin copper wire just inside the non-luminous zone of the flame for about 30 seconds.
• Notice that the part of the wire just outside the flame gets red hot, indicating that the non-luminous zone is the hottest part of the flame.

• Goldsmiths use the outermost zone of the flame to melt gold and silver because this part of the flame is the hottest and allows efficient heating.

4.6 - What is a Fuel?

• Definition of Fuel: Fuels are substances that provide heat energy when burned. Some common examples are wood, charcoal, petrol, and kerosene.
• Characteristics of a Good Fuel:
  • Readily available
  • Affordable
  • Burns easily in air at a moderate rate
  • Produces large amounts of heat
  • Leaves minimal undesirable byproducts
• While no fuel is considered "ideal," we choose the one that best meets the specific requirements for our needs.

Table 4.3: Types of Fuels with some common fuels:

4.7 - Fuel Efficiency

When choosing a fuel to boil a given quantity of water among cow dung, coal, and LPG, you would likely prefer LPG for the following reasons:

1. High Calorific Value: LPG has the highest calorific value (55,000 kJ/kg), which means it produces more heat energy per kilogram compared to cow dung (6000-8000 kJ/kg) and coal (25000-33000 kJ/kg).
2. Efficiency: LPG burns efficiently, producing a significant amount of heat quickly, making it ideal for boiling water.
3. Clean Combustion: LPG burns cleanly, producing fewer undesirable substances compared to cow dung and coal.
4. Convenience: LPG is easy to handle and store, making it a convenient option for domestic use.

Here’s Table 4.4: Calorific Values of Different Fuels filled out with the calorific values for each fuel:

Burning of Fuels Leads to Harmful Products

The increasing consumption of fuels has significant adverse effects on the environment. Below are some of the key harmful effects associated with fuel combustion:

1. Release of Unburnt Carbon Particles:
   • Carbon-based fuels such as wood, coal, and petroleum release unburnt carbon particles during combustion.
   • These fine particles are dangerous pollutants that can cause respiratory diseases, including asthma.
2. Carbon Monoxide Production:
   • Incomplete combustion of fuels results in the production of carbon monoxide (CO) gas, which is highly poisonous.
   • Burning coal in a closed room can lead to the accumulation of carbon monoxide, posing a serious danger, especially to sleeping individuals.
   • Boojho's Insight: It's advised never to sleep in a room with burning or smoldering coal fire due to the risk of carbon monoxide poisoning.
3. Carbon Dioxide Emission:
   • Combustion of most fuels also releases carbon dioxide (CO₂) into the atmosphere.
   • An increase in CO₂ concentration is linked to global warming, which refers to the rising temperature of Earth's atmosphere.
   • Consequences of global warming include the melting of polar glaciers, leading to a rise in sea levels and potential flooding of coastal areas. In severe cases, low-lying coastal regions may become permanently submerged.
4. Sulphur Dioxide and Nitrogen Oxides:
   • Burning of coal and diesel emits sulphur dioxide (SO₂), a suffocating and corrosive gas.
   • Petrol engines release gaseous oxides of nitrogen.
   • These oxides can dissolve in rainwater, forming acids that lead to acid rain, which is harmful to crops, buildings, and soil.

• Due to the harmful products generated by diesel and petrol in automobiles, there is a shift towards using Compressed Natural Gas (CNG).
• CNG is considered a cleaner fuel as it produces harmful by-products in significantly smaller amounts compared to traditional fuels like diesel and petrol.