Chapter 6 Combustion and Flame Class 8 Notes


Combustion and Flame Class 8 Notes

Combustion and Flame Class 8 Notes

What is Combustion?

Combustion is a chemical process in which a substance (Combustible->Fuel) reacts with oxygen to give energy in the form of heat and light.

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  • Combustion is an exothermic process.

Combustion Chemical Equation

Fuel + O2 → CO2 + H2O + Energy (Heat+Light)

Types of Combustion

1. Rapid Combustion
2. Spontaneous Combustion
3. Explosion

1. Rapid Combustion

It is a type in which substances burn rapidly i.e. in a very short span of time. Ex- LPG

2. Spontaneous Combustion

Combustion in which substances burst into flames without applying heat. Ex Forest fire, accidents in coal mines, burning of phosphorus at room temperature, reaction of Alkali metals with water.

Spontaneous combustion starts spontaneously at room temperature itself due to self-heating.

This kind of reaction mostly happens in those substances which have less ignition temperature.

Note: Spontaneous Combustion doesn’t require any external energy to start the combustion process whereas Rapid combustion requires external energy.

3. Explosion

When materials burst suddenly and produces enormous amount of heat, light and sound.

Examples- Crackers and Fireworks, Dynamite explosion

Incomplete Combustion

What is incomplete combustion or Define Incomplete Combustion?

When combustion process takes place in less or insufficient supply of oxygen, it is called Incomplete Combustion.

In this type of combustion poisonous gases like carbon mono oxide, carbon dioxide are released.

Carbon is usually released in the form of soot in this process.

Combustible Substances

What are Combustible Substances?

Combustible Substances- Substances that l in the presence of air (O2) to produce heat and light.

Example- Wood, Paper, Petrol, etc.

or, Combustible Substances are the substances that undergo combustion.

Non-Combustible Substances

What are Non-Combustible Substances?

Non-Combustible are the materials that don’t burn readily in the presence of air.

Or, Non-Combustible Substances are the substances that do not undergo combustion.

Some common examples of Non-Combustible Substances are – Glass, Sand, etc.

Ignition Temperature

Define Ignition temperature of a material.

Ignition Temperature– It is the minimum temperature at which a substance burn or catches fire easily.

Remeber that substances must reach its ignition temperature to burn or catch fire.

Example-

(i) The Ignition temperature of Phosphorus is 35 degree celsius

(ii) The Ignition temperature of petrol is 246 degree celsius

Inflammable Substances

What are Inflammable Substances?

Inflammable Substances- Substances that have low ignition temperature and catch fire easily. Ex- LPG, Petrol, CNG, etc.

  • Inflammable Substances burn with a flame.

Spontaneous Combustion vs Rapid Combustion

What is the difference between Spontaneous and Rapid Combustion

Spontaneous CombustionRapid Combustion
Spontaneous combustion takes place spontaneouslytakes place rapidly
No external energy is requiredRapid Combustion must be started by an external source
Depends on the ignition temperature of the fuel to start the combustion process.Depends on external heat energy to start the combustion process
Less amount of heat energy is releasedA large amount of heat energy is released
Less amount of light energy is releasedlarge amount of light energy is released

Necessary Conditions for Combustion

The three conditions must be fulfilled to carry out the process of combustion. If any of these conditions is not fulfilled, the combustion process will not take place.

  1. presence of air or oxygen
  2. Presence of a combustible substance, i.e., a fuel
  3. Heating the combustible substance to its ignition temperature and maintaining it.

What is Fuel?

Fuels are the substances that undergo combustion and releases energy (thermal) which is used for different work.

Some example of fuels are – cow dung, gasoline, coal, wood, etc.

Types of Fuel

What are the different types of Fuels?

Fuels can be divided into Primary Fuels and Secondary Fuels. 

Primary Fuels- Fuels are obtained directly from the nature such as – Crude oil, Natural gas, wood etc.

Secondary Fuels- The secondary fuels are also called the derived fuels, i.e., obtained from the primary fuels. Example- Petrol, Kerosene, LPG, Coke etc.

These Primary and Secondary Fuels can further be classified into Solid, Liquid and Gaseous Fuels.

  • Solid Fuels– Coal, Wood, Cow dung cakes
  • Liquid Fuels– Petrol, Kerosene, Diesel
  • Gaseous Fuels– LPG, CNG, Coal Gas

Calorific Value

Define Calorific Value?

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

The unit is kilojoule per kg (kJ/kg).

Efficiency of Fuel

Define Efficiency of Fuel?

Efficiency is the proportion of energy released by a combustion process that is converted into useful work.

It is directly proportional to its calorific value.

Characteristics of an Ideal Fuel

What are the Characteristics of an Ideal Fuel?

Characteristics of an Ideal Fuel are-

  • The ideal fuel should be economical or cheap.
  • It should be easily available
  • It should have a high calorific value.
  • Does not produce harmful gases or residues.
  • The combustion should not be explosive.
  • It is easy to handle and transport.

Flame

Define Flame?

Flame is the by-product of fire. Flame is the visible and gaseous part of the fire.

The substances which vaporize during burning, give flames.

For example, Kerosene oil and Molten wax rise through the wick and are vaporized during burning and so they form flames.

Charcoal does not produce a flame because it does not vaporize on burning.

Structure of flame

Outer Zone: The outermost zone is the hottest of all zones and is blue in colour. This is due to complete combustion. It is the non-luminous part of the flame.

Middle Zone: The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.

Innermost Zone: The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.

Controlling Fire

Fire can be controlled by removing any of the factors of combustion- fuel, oxygen (air) and ignition temperature.

  • removing the fuel
  • cutting off the air
  • not allowing its temperature to reach the ignition temperature

We can take the help of following to control the fire-

  1. Water- it lowers the Ignition temperature
  2. Blankets- it cut off the air supply
  3. Fire Extinguishers- it cut off the air supply

Fire Extinguishers

The fire extinguishers are used to cut off the supply of oxygen or bring down the temperature of the fuel or both. It contains highly compressed carbon di oxide. CO2 is heavier than oxygen so CO2 surrounds the burning materials and cut off the air from the surroundings.

Also Read: Chapter 16 Light Class 8 Notes

Types of Fire Extinguishers

  1. Water
  2. Foam Type fire Extinguisher
  3. Dry Powder
  4. Soda-Acid fire Extinguisher
  5. CCl4 fire Extinguisher
  6. CO2 fire Extinguisher

Water as Fire Extinguisher

When water is thrown on the burning material (fuel) it lowers the temperature below the Ignition temperature.

Limitations- cannot be used everywhere like in electrical fires, fires produced by oil, petrol.

Foam Type Fire Extinguisher

Principle- It cut-off the supply of oxygen.

It contains the following-

  • Aluminium Sulphate solution in a glass bottle container
  • Baking Soda (NaHCO3) or Sodium bi carbonate solution
  • Saponin for foam

Al2(SO4)3 + NaHCO3 → Al(OH)3 + CO2 + Na2SO4

This type is used in in fires produced by oil, petrol.

CCl4 Type Fire Extinguisher

Principle- It cut-off the supply of oxygen.

When thrown on the combustible material, it vaporizes and then moves down and surrounds the combustible material and thus cut-off the supply of oxygen.

It is used in places where fire is produced by electrical wires.

Limitation- It produces harmful gases like phosgene, HCl.

It is used in small scale.

CO2 Fire Extinguisher

In this type of fire extinguishers we use pure carbon di oxide.

Dry Powder Fire Extinguisher

Principle- It cut-off the supply of oxygen.

It uses Sodium bicarbonate (NaHCO3) and sand.

NaHCO3 → Na2CO3 + H2O + CO2

Soda-Acid Fire Extinguisher

Principle- It cut-off the supply of oxygen and cool the material below its ignition temperature.

It is a metallic cylinder that contains the following-

(i) contains aquesous solution of Sodium bi carbonate

(ii) It contains concentrated hydrochloric acid in a glass tube

When

NaHCO3 (aq) + H2SO4 (aq) → Na2SO4 (aq) +H2O + CO2 (g)

Harmful Impacts of Burning Fuels

  • Incomplete combustion produces harmful gases like Carbon monoxide which deplets ozone layer.
  • Harmul gases like carbon di oxide adds to global warming.
  • Oxides of sulphur, nitrogen causes acid rain.
  • Unburnt carbon particles causes respiratory diseases like asthama.

Extra Questions

Explain if Magnesium (Mg) burns with a flame or not?

Yes, Magnesium burns with a flame.

When Magnesium is burnt in the presence of air, a white dazzling flame is produced along with a white substance (Magnesium Oxide).

Mg + O2 → 2MgO

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