Combustion and Flame Notes for Class 8 Ideal for CBSE Board followed by NCERT

Important topics in Combustion and flame notes:

• What is Combustion?
• Ignition temperature
• Inflammable substance
• How Do We Control Fire?
• Types of Combustion
• Flame
• Structure of a Flame
• What is a Fuel?
• Fuel Efficiency
• Global warming
• Ideal Fuel

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What is combustion?

• Burning of a substance is said to be combustion.
• Or simply, the chemical process in which a substance reacts with oxygen to give off heat and light is known to be combustion.
• Oxygen is necessary for combustion.
• Combustible substance: Those substances which undergo combustion are said to be combustible substance.
• Non-combustible substances: Those substances which do not burn in air are said to be non-combustible substances.

Ignition temperature

• The lowest temperature at which a substance catches fire is said to be ignition temperature.
• If the temperature of the substances is below the ignition temperature then that substance will not burn.
• Have you ever seen the oil kept in pan catches fire when kept for long on a burning stove? You can conclude that the temperature of oil reaches the ignition temperature then catches fire.

Inflammable substances:

• Those substances which catch fire easily are said to be inflammable substances.
• These have a low ignition temperature.
• LPG, Petrol are some of its examples.

Essential condition for burning:

• Presence of combustible substances or fuel
• Presence of air containing oxygen
• A temperature above ignition temperature is required.

How Do We Control Fire?

• Generally, we use water to extinguish fire.
• For fires that are caused by petrol, diesel or highly inflammable substance we don’t use water.
• In these cases we use carbon dioxide, sand etc. to extinguish fire.
• When a person catches with fire he should use blankets to escape or save themselves.
• Forest fires are uncontrollable as they spread rapidly.

Types of combustion

Different types of combustion are:

Rapid combustion:

• The combustion which occurs rapidly yielding large amount of heat and light.
• External heat source is required for rapid combustion.
• Burning of gas stove when a burning matchstick is taken near it is an example of rapid combustion.

Spontaneous combustion:

• The combustion which occurs without any application of external source of heat and burst out into flames.
• No external heat sources are required.
• Forest fires are spontaneous combustion.

Explosion:

• The combustion in which sudden reaction results in a large amount of heat, light and sound.
• A heavy amount of gases are also gets released.
• Firecracker after combustion releases a large amount of heat and light.

Flame:

• Flames are the region of burning of gases.
• The combustible substances vaporize during burning produces flame.

Structure of flame:

• There are different zones of flame of candle.
• Blue zone: It is near the base of the flame. Carbon monoxide burns completely with a blue flame in this zone.
• Luminous zone: The region of the flame in which hydrogen burns with a bright yellow luminous flame. The soot of the flame rises due to some unburnt carbon particles which are left in it.
• Dark Zone which is the middle layer of flame.
• Non-luminous zone which is the outermost zone of the flame. As here complete oxidation takes place, it is the hottest part of the flame.

What is fuel?

• Fuel is another name for combustible substances.
• The fuel should be cost-effective, easily available, and less amount of it must be capable of producing a large amount of heat.
• They should produce less undesired material after burning.
• Petrol, kerosene, hydrogen etc. are some examples of fuel.

Global warming:

Most fuels on combustion release carbon dioxide in the environment. Increase in concentration of carbon dioxide in the air is believed to cause global warming.

The efficiency of fuel:

• The efficiency of fuel is the amount of energy it releases during burning.
• We can determine the efficiency of fuel by its calorific value.

Calorific value:

• The calorific value is the measure of efficiency of fuel.
• The unit of calorific value is expressed in Kilojoule per kg.

Ideal Fuel:

• Those fuel which can meet most of the requirements for a particular use can be said as ideal fuel.
• For instance, probably no fuel is there that could be considered as an ideal fuel.

Characteristics of a good fuel

• Cheap and readily available.
• Burn at a quiet slow rate and its combustion should be controllable.
• Easy to store.
• Low ignition temperature.
• Produce a very small amount of residues such as ash.
• Should possess high calorific value.
• Do not produce gases which pollute the air.
• Easily transportable.

Harmful impacts of burning fuel:

The increase in burning of fuel on this earth affects environment in many ways.

• The unburnt particle of carbon fuels are dangerous pollutants and causes respiratory diseases, such as asthma.
• Incomplete combustion some fuels gives carbon monoxide gas which is a very poisonous gas. It can kill persons sleeping in a room containing carbon dioxide.
• Must fuel on oxidation releases carbon dioxide which is the major cause of global warming.
• Burning of coal and diesel in vehicles releases sulphur dioxide gas which is responsible for acid rain.