Combustion and Flame - CBSE Class 8 Science Notes: Complete Study Guide
Introduction to Combustion and Flame
Combustion is a fundamental chemical process that we encounter daily, from cooking food on a gas stove to powering vehicles. Understanding combustion is essential for Class 8 students as it forms the foundation for advanced chemistry concepts in higher classes.
What is Combustion?
Combustion is the chemical process in which a substance reacts with oxygen to produce heat and light. When a material burns in the presence of air or oxygen, it undergoes combustion, releasing energy in the form of heat and often light as well.
The substance that undergoes combustion is called a combustible substance or fuel. Common examples include wood, coal, petrol, and natural gas. Air or oxygen acts as a supporter of combustion, making the burning process possible.
Scientific Definition: Combustion is an exothermic redox reaction where a fuel rapidly oxidizes, releasing energy in the form of heat and light.
Combustible vs Non-Combustible Substances
Understanding which substances can burn and which cannot is crucial for fire safety and practical applications.
Combustible Substances
Combustible substances are materials that can burn in air or oxygen, producing heat and light energy. These substances contain carbon and hydrogen atoms that react readily with oxygen.
Examples of Combustible Substances:
- Fuels: LPG (Liquefied Petroleum Gas), kerosene, petrol, diesel, coal, wood
- Gases: Hydrogen, methane, butane, propane
- Everyday materials: Paper, cloth, wax, alcohol, rubber
Key Characteristic: These substances have a molecular structure that allows them to react exothermically with oxygen.
Non-Combustible Substances
Non-combustible substances do not burn under normal conditions when exposed to air or oxygen. They either do not react with oxygen or require extreme conditions to undergo any reaction.
Examples of Non-Combustible Substances:
- Liquids: Water, salt solutions
- Solids: Sand, glass, cement, stones, metals (most)
- Gases: Carbon dioxide, nitrogen, noble gases (helium, neon, argon)
Important Note: Carbon dioxide is not only non-combustible but also acts as a fire extinguisher by cutting off the oxygen supply to burning materials.
Products of Combustion
The products formed during combustion depend on the type of fuel and the availability of oxygen during the burning process.
Hydrocarbons: The Common Fuels
Most fuels we use daily are hydrocarbons - compounds containing only hydrogen and carbon atoms. Examples include:
- Methane (CH₄): Main component of natural gas
- Butane (C₄H₁₀): Main component of LPG
- Octane (C₈H₁₈): Component of petrol
- Propane (C₃H₈): Used in portable gas cylinders
Primary Products
When hydrocarbons burn completely in sufficient oxygen:
Methane + Oxygen → Carbon Dioxide + Water + Heat + Light
CH₄ + 2O₂ → CO₂ + 2H₂O + Energy
General Products of Combustion:
- Carbon dioxide (CO₂): Produced when carbon burns completely
- Water vapor (H₂O): Formed from hydrogen in the fuel
- Heat energy: Released as temperature increases
- Light energy: Visible as flames or glow
Types of Combustion
Combustion can be classified based on the rate of burning and the amount of oxygen available.
Classification Based on Rate of Combustion
1. Rapid Combustion
Definition: Combustion that releases a large amount of heat and light in a very short time.
Characteristics:
- Very fast reaction rate
- Intense heat and bright light production
- Controlled burning process
Examples:
- Burning of LPG in a gas stove
- Lighting a matchstick
- Burning of candle wax
- Combustion of gasoline in engines
Practical Application: Rapid combustion is useful in cooking, heating, and power generation where quick energy release is needed.
2. Slow Combustion
Definition: Combustion that occurs very slowly at low temperatures without producing flames or intense light.
Characteristics:
- Gradual energy release
- Low temperature process
- No visible flames
- Continuous process over extended periods
Examples:
- Respiration in living organisms: The most important example where food molecules are slowly oxidized to release energy
- Rusting of iron (very slow oxidation)
- Decomposition of organic matter
- Burning of coal in traditional stoves
Biological Significance: Cellular respiration is controlled slow combustion that provides energy for all life processes without damaging cells.
3. Spontaneous Combustion
Definition: Combustion in which a substance suddenly bursts into flames on its own without any external heating source.
Characteristics:
- No external heat required
- Occurs at or near room temperature
- Automatic ignition
- Can be dangerous if uncontrolled
Examples:
- White phosphorus: Ignites at approximately 30°C, hence stored underwater
- Yellow phosphorus
- Sodium and potassium metals (when exposed to air)
- Coal dust in mines (under certain conditions)
- Oily rags in closed spaces
Safety Concern: Materials prone to spontaneous combustion must be stored carefully to prevent accidental fires.
Why Does It Happen? These substances have very low ignition temperatures, close to or below room temperature, causing them to ignite automatically when exposed to air.
4. Explosion
Definition: Very rapid combustion that produces heat, light, sound, and a large volume of gases almost instantaneously.
Characteristics:
- Extremely fast reaction
- Produces shock waves (sound)
- Generates enormous pressure
- Can be triggered by heat, spark, or pressure
Examples:
- Fireworks and crackers
- Combustion in internal combustion engines
- Dynamite and other explosives
- Gas cylinder explosions
- Dust explosions in grain silos
Mechanism: In an explosion, the fuel burns so rapidly that gases expand faster than they can escape, creating a pressure wave.
Complete vs Incomplete Combustion
The amount of oxygen available during combustion determines whether the process is complete or incomplete.
Complete Combustion
Definition: Combustion that occurs when a fuel burns in sufficient (excess) oxygen, producing the maximum amount of energy.
Conditions Required:
- Adequate oxygen supply
- Proper fuel-air mixture
- Sufficient temperature
Products of Complete Combustion:
- Carbon dioxide (CO₂)
- Water (H₂O)
- Maximum heat energy
- Blue flame (in case of gaseous fuels)
Chemical Equation Example:
Methane: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Characteristics:
- More efficient energy release
- Cleaner burning (minimal pollutants)
- Blue or pale blue flame
- No smoke or soot production
Examples:
- LPG burning in a properly adjusted gas stove (blue flame)
- Natural gas in modern furnaces
- Well-ventilated campfires
Incomplete Combustion
Definition: Combustion that occurs when a fuel burns in insufficient oxygen, producing less energy and harmful products.
Conditions:
- Limited oxygen supply
- Poor ventilation
- Lower temperature
Products of Incomplete Combustion:
- Carbon monoxide (CO) - toxic gas
- Carbon (soot/black particles)
- Water (H₂O)
- Less heat energy compared to complete combustion
- Yellow or orange flame
Chemical Equations Example:
Partial combustion of methane:
2CH₄ + 3O₂ → 2CO + 4H₂O + Heat (less energy)
CH₄ + O₂ → C + 2H₂O + Heat (produces soot)
Characteristics:
- Less efficient energy production
- Produces smoke and soot
- Yellow, orange, or sooty flame
- Releases harmful carbon monoxide
- Deposits black residue
Examples:
- Burning of coal in traditional stoves
- Vehicle engines with poor maintenance
- Candle burning (produces soot on glass)
- Gas stove with yellow flame (needs adjustment)
Comparison Table
| Aspect | Complete Combustion | Incomplete Combustion |
| Oxygen Supply | Sufficient/Excess | Insufficient/Limited |
| Main Products | CO₂ and H₂O | CO, C (soot), and H₂O |
| Flame Color | Blue or pale blue | Yellow, orange, or sooty |
| Energy Released | Maximum | Less than maximum |
| Efficiency | High | Low |
| Smoke Production | No smoke | Produces smoke |
| Environmental Impact | Less harmful | More harmful (toxic CO) |
| Examples | LPG with blue flame | Burning wood, coal |
Health and Environmental Concerns
Incomplete Combustion Dangers:
- Carbon monoxide poisoning: CO is a colorless, odorless, toxic gas that binds with hemoglobin, preventing oxygen transport in blood
- Air pollution: Soot particles cause respiratory problems
- Fuel wastage: Less energy extracted from the same amount of fuel
- Property damage: Soot deposits on surfaces
Prevention: Ensure adequate ventilation, maintain combustion equipment, and adjust air-fuel ratios properly.
Conditions Necessary for Combustion (The Fire Triangle)
For combustion to occur and continue, three essential conditions must be met simultaneously. This concept is known as the Fire Triangle.
The Three Essential Components
1. Fuel (Combustible Substance)
Definition: A combustible material that can undergo oxidation and release energy.
Types of Fuels:
- Solid fuels: Wood, coal, charcoal, paper
- Liquid fuels: Petrol, diesel, kerosene, alcohol
- Gaseous fuels: LPG, CNG, natural gas, hydrogen
Requirement: The substance must be capable of reacting with oxygen to release energy.
2. Oxygen (Supporter of Combustion)
Definition: A gas that supports burning by oxidizing the fuel.
Sources:
- Air (contains approximately 21% oxygen)
- Pure oxygen cylinders
- Chemical compounds that release oxygen
Minimum Requirement: Most combustion processes require at least 16% oxygen concentration in air.
Important Point: Combustion stops when oxygen concentration drops below the critical level, which is why fires are extinguished by cutting off air supply.
3. Ignition Temperature (Heat)
Definition: The minimum temperature at which a substance catches fire and starts burning in the presence of oxygen.
Also Called: Kindling temperature
Key Concept: Every combustible substance has a specific ignition temperature. Below this temperature, the substance will not catch fire even in the presence of oxygen.
Why Heating is Necessary: Initial heat raises the fuel's temperature to its ignition point, after which the combustion reaction becomes self-sustaining by releasing its own heat.
Understanding Ignition Temperature
Definition in Detail: Ignition temperature is the lowest temperature at which a substance spontaneously ignites in a normal atmosphere without an external source of ignition (like a spark or flame).
Factors Affecting Ignition Temperature:
- Chemical composition of the fuel
- Physical state (solid, liquid, or gas)
- Surface area exposed to air
- Presence of catalysts or inhibitors
Significance: Understanding ignition temperatures is crucial for:
- Safe storage of fuels
- Fire prevention
- Designing combustion systems
- Industrial safety protocols
Common Fuels and Their Ignition Temperatures
Different fuels have different ignition temperatures, which determines how easily they catch fire and how they should be stored.
Ignition Temperature Table
| Fuel | State | Ignition Temperature | Safety Classification |
| White Phosphorus | Solid | ~30°C | Extremely Dangerous (Spontaneous) |
| Carbon Disulfide | Liquid | -30°C | Extremely Inflammable |
| Petrol | Liquid | ~246°C | Highly Inflammable |
| Alcohol (Ethanol) | Liquid | ~363°C | Inflammable |
| Kerosene | Liquid | ~210-230°C | Inflammable |
| Diesel | Liquid | ~210°C | Inflammable |
| Paper | Solid | ~230-250°C | Inflammable |
| Wood | Solid | ~300-350°C | Combustible |
| Coal | Solid | ~400-600°C | Combustible |
| Charcoal | Solid | ~350°C | Combustible |
| Natural Gas (Methane) | Gas | ~580°C | Inflammable |
| LPG (Butane/Propane) | Gas | ~365-493°C | Highly Inflammable |
| CNG (Compressed Natural Gas) | Gas | ~540°C | Inflammable |
| Hydrogen | Gas | ~500-571°C | Highly Inflammable |
Inflammable vs Combustible Substances
Inflammable Substances (Low Ignition Temperature):
Definition: Substances that have very low ignition temperatures and can catch fire easily with a small flame or spark.
Characteristics:
- Ignition temperature typically below 100°C or near room temperature
- Catch fire quickly
- Require special storage conditions
- High fire hazard
Examples:
- Petrol (246°C)
- Alcohol (363°C)
- LPG (365-493°C)
- Acetone (~465°C)
- Ether (~160°C)
Safety Measures:
- Store in cool, well-ventilated areas
- Keep away from heat sources and flames
- Use explosion-proof electrical equipment nearby
- Proper labeling and warning signs
Combustible Substances (Higher Ignition Temperature):
Definition: Substances that can burn but require higher temperatures to ignite.
Characteristics:
- Higher ignition temperatures (typically above 300°C)
- Do not catch fire easily at room temperature
- Relatively safer to store
Examples:
- Wood (300-350°C)
- Coal (400-600°C)
- Rubber
- Most plastics
Practical Implications
Why Does Petrol Catch Fire More Easily Than Wood?
Petrol has a much lower ignition temperature (~246°C) compared to wood (~300-350°C). This means petrol requires less external heat to reach its ignition point, making it more dangerous and requiring careful handling.
Example: A small spark from static electricity can ignite petrol vapors, but wood needs prolonged heating (like from a match held for several seconds) before it catches fire.
Understanding Flame: Structure and Zones
A flame is the visible gaseous part of a fire that emits light and heat. When we light a candle or a gas stove, we see a flame with distinct characteristics.
What is a Flame?
Definition: A flame is a region of hot, luminous gases produced during combustion where fuel vapors react with oxygen, releasing heat and light.
Formation: Flames form only when:
- The fuel vaporizes (solid or liquid fuels must first convert to gaseous state)
- The vapor mixes with oxygen
- The temperature is at or above the ignition temperature
Note: Not all combustion produces flames. For example, coal burns with a glow but does not produce flames like a candle does.
The Three Zones of a Candle Flame
When you carefully observe a candle flame, you can distinguish three distinct zones, each with different characteristics, temperature, and chemical processes.
Zone 1: The Innermost Zone (Dark Zone)
Location: Center of the flame, closest to the wick
Color: Dark or black
Temperature: Approximately 600-800°C (lowest in the flame)
Characteristics:
- No combustion occurs in this zone
- Contains unburnt wax vapors (fuel in gaseous form)
- Oxygen has not yet reached this area
- Wax vapors rise from the wick due to heat from outer zones
- This zone is relatively cool compared to other zones
Why is it Dark?
- Absence of oxygen prevents combustion
- No light is produced without burning
- The fuel is present but not yet burning
Practical Application: If you quickly insert a glass tube into this zone and hold a burning matchstick near the other end, the unburnt gases will catch fire, proving that fuel vapors are present but not burning.
Chemical Process: None (no combustion) - just vaporization of fuel
Zone 2: The Middle Zone (Luminous Zone)
Location: Surrounds the dark zone, middle portion of the flame
Color: Bright yellow or orange (luminous/glowing)
Temperature: Approximately 1000-1200°C (moderate)
Characteristics:
- Partial or incomplete combustion occurs here
- Limited oxygen supply
- Contains carbon particles that get heated and glow (incandescence)
- Produces the bright yellow light we see
- Most luminous part of the flame
Why is it Yellow/Luminous?
- Tiny solid carbon particles are heated to high temperatures in this zone
- These hot carbon particles emit bright yellow-orange light (similar to how a metal glows when heated)
- This phenomenon is called incandescence
- Some carbon particles escape as smoke (soot)
Chemical Process:
- Partial oxidation of fuel vapors
- Incomplete combustion produces carbon particles and carbon monoxide
- CₓHᵧ → C + CO + H₂O + Heat + Light
Demonstration: If you hold a cool porcelain plate in this zone for a few seconds, black soot (carbon particles) will deposit on the plate.
Zone 3: The Outermost Zone (Non-Luminous/Blue Zone)
Location: Outer layer of the flame, in direct contact with air
Color: Pale blue or almost colorless (non-luminous)
Temperature: Approximately 1400-1650°C (hottest part of the flame)
Characteristics:
- Complete combustion occurs here
- Abundant oxygen supply from surrounding air
- No visible solid particles
- Hottest zone of the flame
- Produces very little visible light (appears almost transparent or pale blue)
Why is it the Hottest?
- Complete combustion releases maximum energy
- All fuel is completely oxidized to CO₂ and H₂O
- Efficient burning produces highest temperatures
Why is it Less Bright?
- No solid carbon particles to glow
- Complete combustion produces only gases (CO₂ and H₂O vapor)
- The blue color comes from excited gas molecules (C-H and C-C bonds breaking)
Chemical Process:
- Complete oxidation of fuel and carbon particles
- CH₄ + 2O₂ → CO₂ + 2H₂O + Maximum Heat
- 2CO + O₂ → 2CO₂ + Heat
- C + O₂ → CO₂ + Heat
Practical Application:
- Goldsmiths use this outermost zone for melting gold and silver because it is the hottest
- Welding torches are adjusted to maximize this zone for maximum heat
Flame Zone Temperature Summary
| Zone | Color | Temperature | Combustion Type | Oxygen Availability |
| Innermost (Dark) | Black/Dark | 600-800°C | No combustion | None |
| Middle (Luminous) | Yellow/Orange | 1000-1200°C | Incomplete | Limited |
| Outermost (Non-luminous) | Blue/Pale | 1400-1650°C | Complete | Abundant |
Why Understanding Flame Zones Matters
Scientific Significance:
- Helps understand combustion efficiency
- Demonstrates the relationship between oxygen and combustion
- Shows energy distribution in flames
Practical Applications:
- Adjusting gas stoves for optimal cooking (more blue = more heat)
- Industrial heating processes
- Metalworking and jewelry making
- Understanding fire behavior for safety
Experiment: You can observe these zones clearly by lighting a candle in a dark room and observing carefully. The distinct color differences show the different combustion processes occurring simultaneously.
Luminous vs Non-Luminous Flames
The appearance of a flame provides important information about the combustion process.
Luminous Flame
Definition: A flame that produces bright yellow or orange light due to the presence of glowing carbon particles.
Characteristics:
- Bright yellow or orange color
- Produces soot (carbon deposits)
- Incomplete combustion
- Lower temperature (relatively)
- Less efficient
Formation: Occurs when there is insufficient oxygen supply, causing incomplete combustion and formation of solid carbon particles that glow when heated.
Examples:
- Candle flame (yellow portion)
- Kerosene lamp
- Wood fire
- Gas stove with closed air holes (yellow flame)
- Matchstick flame
Advantages:
- Provides better visibility (brighter)
- Useful for lighting purposes
Disadvantages:
- Less heat production
- Produces soot and smoke
- Less efficient combustion
- Causes pollution
- Blackens cooking vessels
Non-Luminous Flame
Definition: A flame that produces little visible light, appearing blue or pale in color, due to complete combustion without carbon particles.
Characteristics:
- Blue or pale color (less bright)
- No soot production
- Complete combustion
- Higher temperature
- More efficient
Formation: Occurs when there is abundant oxygen supply, resulting in complete combustion where all fuel is converted to CO₂ and H₂O without forming solid carbon particles.
Examples:
- Bunsen burner with air hole open (blue flame)
- Gas stove with properly adjusted air holes (blue flame)
- Welding torch flame
- Modern gas heaters
Advantages:
- Maximum heat production
- Clean burning (no smoke or soot)
- More efficient combustion
- Does not blacken vessels
- Environmentally better
Disadvantages:
- Less visible (harder to see)
- Requires proper air-fuel ratio adjustment
Comparison Table
| Feature | Luminous Flame | Non-Luminous Flame |
| Color | Yellow/Orange | Blue/Pale |
| Brightness | Very bright | Dim/Less visible |
| Oxygen Supply | Insufficient | Sufficient |
| Combustion Type | Incomplete | Complete |
| Temperature | Lower (~1000-1200°C) | Higher (~1400-1650°C) |
| Soot Production | Yes | No |
| Smoke | Produces smoke | No smoke |
| Efficiency | Less efficient | More efficient |
| Heat Output | Lower | Higher |
| Carbon Particles | Present (glowing) | Absent |
Practical Application: Gas Stove Adjustment
Yellow Flame (Luminous):
- Air holes are closed or partially closed
- Insufficient oxygen
- Less efficient cooking
- Blackens vessels
- Solution: Open air holes
Blue Flame (Non-Luminous):
- Air holes are properly opened
- Sufficient oxygen
- Efficient cooking
- No vessel blackening
- Ideal for cooking
How to Adjust: Most gas stoves have air adjustment rings near the burner. Rotate them to open or close air holes until you get a steady blue flame.
Balancing Combustion Reactions and Energy Release
Understanding how combustion reactions are balanced helps us calculate fuel requirements and predict energy output.
Basic Principles of Balancing Combustion Reactions
Law of Conservation of Mass: Matter can neither be created nor destroyed during a chemical reaction. Therefore, the number of atoms of each element must be equal on both sides of the equation.
Balancing Steps for Hydrocarbon Combustion
General Formula for Complete Combustion of Hydrocarbons:
CₓHᵧ + O₂ → CO₂ + H₂O
Step-by-Step Process:
- Write the unbalanced equation
- Balance carbon atoms first (carbon from fuel becomes CO₂)
- Balance hydrogen atoms second (hydrogen from fuel becomes H₂O)
- Balance oxygen atoms last (oxygen comes from O₂)
- Check all atoms are balanced
- Convert fractions to whole numbers if necessary
Example 1: Combustion of Methane (CH₄)
Step 1: Write unbalanced equation
CH₄ + O₂ → CO₂ + H₂O
Step 2: Balance carbon (1 C on left, need 1 CO₂ on right) ✓
CH₄ + O₂ → CO₂ + H₂O
Step 3: Balance hydrogen (4 H on left, need 2 H₂O on right)
CH₄ + O₂ → CO₂ + 2H₂O
Step 4: Balance oxygen (need 4 O on right: 2 from CO₂ + 2 from 2H₂O = 4 O total) Need 2 O₂ molecules (2 × 2 = 4 O atoms)
CH₄ + 2O₂ → CO₂ + 2H₂O
Balanced Equation:
CH₄ + 2O₂ → CO₂ + 2H₂O + Energy
Check:
- Left: 1 C, 4 H, 4 O
- Right: 1 C, 4 H, 4 O ✓
Example 2: Combustion of Propane (C₃H₈)
Step 1: Write unbalanced equation
C₃H₈ + O₂ → CO₂ + H₂O
Step 2: Balance carbon (3 C on left, need 3 CO₂)
C₃H₈ + O₂ → 3CO₂ + H₂O
Step 3: Balance hydrogen (8 H on left, need 4 H₂O)
C₃H₈ + O₂ → 3CO₂ + 4H₂O
Step 4: Balance oxygen (need 10 O on right: 6 from 3CO₂ + 4 from 4H₂O = 10 O) Need 5 O₂ molecules
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Balanced Equation:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + Energy
Example 3: Incomplete Combustion of Methane
Producing Carbon Monoxide:
2CH₄ + 3O₂ → 2CO + 4H₂O + Less Energy
Producing Carbon (Soot):
CH₄ + O₂ → C + 2H₂O + Least Energy
Example 4: Combustion of Ethanol (C₂H₅OH)
Balanced Equation:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Energy
Energy Release in Combustion
Calorific Value: The amount of heat energy produced when 1 kg of a fuel is completely burned.
Unit: Kilojoules per kilogram (kJ/kg) or Kilocalories per kilogram (kcal/kg)
Why Energy is Released:
During combustion:
- Chemical bonds in fuel molecules break (requires energy - endothermic)
- New bonds form in products (CO₂ and H₂O) (releases energy - exothermic)
- Energy released in forming new bonds > Energy required to break old bonds
- Net result: Energy is released (exothermic reaction)
Energy Diagram:
Reactants (High Energy)
↓ Activation Energy needed
↓ Combustion occurs
Products (Lower Energy) + Released Energy (Heat + Light)
Calorific Values of Common Fuels
| Fuel | Calorific Value (kJ/kg) | Efficiency |
| Hydrogen | 150,000 | Highest |
| LPG | 55,000 | Very High |
| Natural Gas (Methane) | 50,000 | Very High |
| Petrol | 45,000 | High |
| Kerosene | 45,000 | High |
| Diesel | 45,000 | High |
| Coal | 25,000-33,000 | Moderate |
| Wood | 17,000 | Low |
| Cow Dung Cakes | 8,000 | Low |
Practical Significance:
- Higher calorific value = More energy per kg = More efficient fuel
- Hydrogen has the highest calorific value but is difficult to store
- LPG and natural gas are preferred for cooking due to high calorific value and clean burning
Fire Safety: The Fire Triangle and Prevention Methods
Understanding how fire starts and spreads is essential for fire prevention and firefighting.
The Fire Triangle Explained
The Fire Triangle is a simple model showing the three elements necessary for combustion. Remove any one element, and the fire cannot continue.
Three Sides of the Fire Triangle:
- Fuel (Combustible material)
- Oxygen (Supporter of combustion)
- Heat (Ignition temperature)
Modern Concept: Some experts add a fourth element creating a "Fire Tetrahedron":
- Fuel
- Oxygen
- Heat
- Chemical chain reaction (self-sustaining oxidation process)
Methods to Prevent or Control Combustion
To extinguish a fire, we must remove at least one element of the fire triangle. Different methods target different elements.
Method 1: Removing the Fuel
Principle: Take away or isolate the combustible material
Techniques:
- Creating firebreaks: In forest fires, clearing vegetation in the fire's path
- Shutting off gas supply: Closing valves during gas fires
- Removing flammable materials: Clearing area around fire
- Controlled burning: Burning fuel in a controlled manner before wildfire reaches it
Examples:
- Turning off gas cylinder during kitchen fire
- Creating firebreaks in forests
- Removing furniture from burning rooms
Method 2: Cutting Off Oxygen Supply
Principle: Prevent oxygen from reaching the burning material
Techniques:
a) Smothering:
- CO₂ fire extinguishers: CO₂ is heavier than air and forms a blanket over fire
- Fire blankets: Cover fire with non-flammable material
- Sand or soil: Covers fire and prevents oxygen access
- Foam: Creates barrier between fuel and oxygen
b) Chemical methods:
- Carbon dioxide extinguishers (ideal for electrical and oil fires)
- Foam extinguishers
- Dry powder extinguishers
Examples:
- Covering burning oil with a lid (never water!)
- Using sand on small fires
- CO₂ extinguisher on electrical equipment
- Fire blanket on person whose clothes are burning
Why CO₂ Works:
- CO₂ is heavier than air (density = 1.98 kg/m³ vs air = 1.29 kg/m³)
- Forms a layer over burning material
- Displaces oxygen
- Does not support combustion itself
- Leaves no residue
- Non-conductive (safe for electrical fires)
Method 3: Cooling Below Ignition Temperature
Principle: Reduce the temperature below the fuel's ignition point
Techniques:
a) Water:
- Most common and effective cooling agent
- Absorbs large amounts of heat
- Vaporizes and takes heat with it
- Best for: Wood, paper, cloth, rubber fires (Class A fires)
b) Water spray/mist:
- More effective than solid stream
- Larger surface area for heat absorption
- Less water wastage
How Water Works:
- Water has high specific heat capacity (4.18 kJ/kg°C)
- Absorbs heat from burning material
- Reduces temperature below ignition point
- Vaporization absorbs even more heat (latent heat of vaporization)
- Steam displaces some oxygen
Examples:
- Fire brigade using water hoses
- Sprinkler systems in buildings
- Water buckets for small fires
- Fire hydrants
When NOT to Use Water:
- Oil fires: Water sinks below oil, vaporizes rapidly, and spreads burning oil (use foam or CO₂)
- Electrical fires: Water conducts electricity (risk of electrocution) - use CO₂ extinguisher
- Metal fires: Some metals (sodium, potassium) react violently with water
- Chemical fires: Some chemicals react with water
Types of Fire Extinguishers and Their Uses
| Extinguisher Type | Suitable For | Not Suitable For | Principle |
| Water | Wood, paper, cloth (Class A) | Oil, electrical, metal fires | Cooling |
| CO₂ | Electrical, oil, gas fires (Class B, C, E) | Paper, wood (limited effectiveness) | Oxygen displacement |
| Foam | Flammable liquids (Class B) | Electrical (conducts) | Smothering + cooling |
| Dry Powder | All types (A, B, C, E) | Delicate equipment (residue) | Chemical interruption |
| Wet Chemical | Cooking oil/fat (Class F) | Electrical | Cooling + chemical reaction |
Fire Prevention Tips
At Home:
- Never leave cooking unattended
- Keep flammable materials away from heat sources
- Store fuels properly in ventilated areas
- Check electrical wiring regularly
- Install smoke detectors
- Keep fire extinguisher accessible
- Don't overload electrical outlets
- Never use water on oil or electrical fires
In Laboratories:
- Keep fire extinguishers accessible
- Know location of emergency exits
- Store chemicals properly
- Use fume hoods for volatile substances
- Never leave burners unattended
- Wear appropriate safety equipment
General Safety:
- Plan escape routes
- Practice fire drills
- Never lock fire exits
- Keep fire equipment maintained
- Educate family members about fire safety
Harmful Effects of Burning Fuels
While combustion provides energy for various purposes, it also has significant negative impacts on health and environment.
Environmental Effects
1. Global Warming
Cause: Excessive carbon dioxide (CO₂) release from burning fossil fuels
Mechanism:
- CO₂ is a greenhouse gas
- Traps heat in Earth's atmosphere (greenhouse effect)
- Increases average global temperature
Consequences:
- Melting of polar ice caps and glaciers
- Rising sea levels
- Flooding of coastal areas
- Changes in weather patterns
- Loss of habitat for polar animals
- Agricultural disruptions
Current Concern: Atmospheric CO₂ levels have increased from 280 ppm (pre-industrial) to over 420 ppm today.
2. Acid Rain
Cause: Burning of coal and petroleum products releases sulphur dioxide (SO₂) and nitrogen oxides (NOₓ)
Mechanism:
SO₂ + H₂O → H₂SO₃ (Sulphurous acid)
2SO₂ + O₂ → 2SO₃
SO₃ + H₂O → H₂SO₄ (Sulphuric acid)
2NO₂ + H₂O → HNO₃ + HNO₂ (Nitric acid)
Consequences:
- Damages buildings, monuments (e.g., Taj Mahal)
- Kills aquatic life in lakes and rivers
- Harms trees and crops
- Corrodes metals
- Damages stone structures
3. Air Pollution
Pollutants from Combustion:
- Particulate matter (PM2.5, PM10): Soot, ash particles
- Carbon monoxide (CO): Incomplete combustion
- Nitrogen oxides (NOₓ): High-temperature combustion
- Sulphur oxides (SOₓ): Burning coal and petrol
- Unburnt hydrocarbons: Incomplete combustion
- Volatile Organic Compounds (VOCs)
Sources:
- Vehicle exhaust
- Industrial chimneys
- Power plants
- Residential heating
- Biomass burning
Health Effects
1. Respiratory Problems
Caused by: Particulate matter, smoke, soot
Diseases:
- Asthma: Triggered or worsened by air pollutants
- Bronchitis: Inflammation of bronchial tubes
- Chronic Obstructive Pulmonary Disease (COPD)
- Reduced lung function
- Increased susceptibility to respiratory infections
Vulnerable Groups:
- Children (developing lungs)
- Elderly people
- People with pre-existing respiratory conditions
- Outdoor workers
2. Carbon Monoxide Poisoning
Why Dangerous: CO binds with hemoglobin 200 times more strongly than oxygen
Mechanism:
Hemoglobin + O₂ ⇌ Oxyhemoglobin (Normal: transports oxygen)
Hemoglobin + CO → Carboxyhemoglobin (Stable: prevents oxygen transport)
Symptoms:
- Headache, dizziness
- Nausea, vomiting
- Confusion, disorientation
- Loss of consciousness
- Death (in severe cases)
Sources:
- Incomplete combustion in poorly ventilated areas
- Vehicle exhaust in closed spaces
- Malfunctioning heaters
- Indoor charcoal burning
Prevention:
- Ensure proper ventilation
- Regular maintenance of combustion equipment
- Carbon monoxide detectors
- Never burn charcoal indoors
3. Cardiovascular Problems
Caused by: Long-term exposure to particulate matter and gases
Effects:
- Increased risk of heart attacks
- Stroke
- Irregular heartbeat
- Increased blood pressure
Solutions and Mitigation
Individual Level:
- Use cleaner fuels (LPG, CNG instead of coal/wood)
- Maintain vehicles properly
- Use public transport, carpool, or cycle
- Plant trees
- Reduce energy consumption
- Use energy-efficient appliances
Government/Policy Level:
- Promote renewable energy (solar, wind)
- Implement emission standards
- Encourage electric vehicles
- Industrial pollution control
- Afforestation programs
- Public awareness campaigns
Technological Solutions:
- Catalytic converters in vehicles
- Scrubbers in industrial chimneys
- Clean coal technologies
- Better combustion efficiency
- Alternative fuels (biofuels, hydrogen)
Key Formulas - Quick Reference Table
| Formula Name | Chemical Equation | Explanation |
| Complete Combustion of Methane | CH₄ + 2O₂ → CO₂ + 2H₂O | Methane burns completely in sufficient oxygen to produce carbon dioxide and water |
| Incomplete Combustion of Methane (CO) | 2CH₄ + 3O₂ → 2CO + 4H₂O | Limited oxygen produces toxic carbon monoxide instead of carbon dioxide |
| Incomplete Combustion of Methane (Soot) | CH₄ + O₂ → C + 2H₂O | Very limited oxygen produces solid carbon (soot) particles |
| Complete Combustion of Propane | C₃H₈ + 5O₂ → 3CO₂ + 4H₂O | LPG (propane) burns completely producing CO₂ and water |
| Complete Combustion of Butane | 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O | Another LPG component burns completely |
| Complete Combustion of Ethanol | C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O | Alcohol burns completely in sufficient oxygen |
| Formation of Nitrogen Dioxide | N₂ + 2O₂ → 2NO₂ | High-temperature combustion produces nitrogen oxides (air pollutant) |
| Formation of Nitric Oxide | N₂ + O₂ → 2NO | Another nitrogen oxide formed at high temperatures |
| Sulphur Dioxide Formation | S + O₂ → SO₂ | Burning sulphur-containing fuels produces SO₂ (acid rain cause) |
| Sulphur Trioxide Formation | 2SO₂ + O₂ → 2SO₃ | Further oxidation of SO₂ in atmosphere |
| Formation of Sulphuric Acid (Acid Rain) | SO₃ + H₂O → H₂SO₄ | SO₃ dissolves in rain to form sulphuric acid |
| Formation of Nitric Acid (Acid Rain) | 4NO₂ + 2H₂O + O₂ → 4HNO₃ | NO₂ dissolves in rain to form nitric acid |
| Combustion of Carbon (Complete) | C + O₂ → CO₂ | Carbon burns completely to form carbon dioxide |
| Combustion of Carbon (Incomplete) | 2C + O₂ → 2CO | Limited oxygen produces carbon monoxide |
| General Hydrocarbon Combustion | CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O | General formula for complete hydrocarbon combustion |
| Respiration (Cellular) | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy | Slow combustion of glucose in living cells |
| Photosynthesis (Reverse) | 6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂ | Reverse of combustion - energy storage process |
Understanding the Formulas
Key Points:
- Balancing: All equations must have equal numbers of each atom on both sides
- Complete combustion: Always produces CO₂ and H₂O with maximum energy
- Incomplete combustion: Produces CO or C (soot) with less energy
- Energy notation: "+Energy" or "+Heat" indicates exothermic reaction
- Oxygen requirement: Calculate oxygen needed based on carbon and hydrogen atoms
Practice Tip: To balance any hydrocarbon combustion:
- Balance C atoms first (form CO₂)
- Balance H atoms second (form H₂O)
- Balance O atoms last (from O₂)
- Multiply through to eliminate fractions
Conclusion
Understanding combustion and flame is crucial not just for academic success but also for practical life applications. The CBSE Class 8 Science chapter on Combustion and Flame provides foundational knowledge about:
- The chemical process of burning and energy release
- Different types of combustion and their characteristics
- The structure of flames and temperature distribution
- Safe handling of fuels and fire prevention
- Environmental impacts of burning fuels
- Importance of choosing appropriate fuels
Important Points:
- Combustion requires three elements: Fuel, oxygen, and ignition temperature (Fire Triangle)
- Complete combustion is more efficient and produces fewer pollutants than incomplete combustion
- Different flame zones have different temperatures and combustion characteristics
- Calorific value determines fuel efficiency and suitability for different applications
- Fire safety requires understanding how to remove one or more elements of the fire triangle
- Environmental consciousness is important when using fuels due to pollution and global warming
Practical Applications:
- Cooking safely and efficiently
- Understanding vehicle fuel choices
- Fire safety and prevention
- Environmental conservation
- Energy management
Study Tips:
- Practice balancing combustion equations
- Remember the three zones of flame and their characteristics
- Understand the difference between complete and incomplete combustion
- Learn ignition temperatures of common fuels
- Memorize key formulas and their applications
This comprehensive guide covers all aspects of the CBSE Class 8 Combustion and Flame chapter. Regular revision, solving numerical problems, and conducting safe experiments will help solidify these concepts.