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Class 8, Science, Chapter-4, Lecture-2, Flame (Notes)

Methane has Higher Calorific Value Amongst all Hydrocarbons.

Reason: 
Methane has highest percentage of hydrogen amongst all hydrocarbons.

Compounds containing Oxygen have Lower Calorific Value.

Reason:
The presence of oxygen, which is non-combustible, reduces the percentage of hydrogen atoms present in the compound.

Wood Has Lower Calorific Value.

Reason: 
The presence of oxygen in wood reduces the percentage of hydrogen in wood.

FLAME:

The hot luminous region where combustion of gases take place is called the Flame.

Two types of Flames:

  1. Luminous Flame
  2. Non-luminous Flame

Zones of a Flame:

  1. Blue Zone (outer zone) – Hottest due to complete combustion
  2. Yellow Zone (middle zone) – due to glowing of unburnt carbon particles.
  3. Black Zone (innermost zone) – due to presence of unburnt carbon particles.

Differences Between:

Luminous Flame

Non-Luminous Flame

1. Yellow in colour.

1. Blue in colour.

2. Incomplete combustion takes place.

2. Complete combustion.

3. Less hot.

3. Hotter.

The Flame of a Wick Stove is Blue, but the Flame of a Lamp is Yellow.

Reason:
In a wick stove kerosene burns with a blue flame due to sufficient supply of oxygen but in a lamp insufficient supply of air make the flame yellow.

CHARACTERISTICS OF AN IDEAL FUEL:

  1. High calorific value.
  2. Proper ignition temperature.
  3. Moderate rate of combustion.
  4. Absence of poisonous gases on combustion.
  5. Absence of residue on combustion.
  6. Cheap availability.
  7. Plentiful availability.
  8. Ease of storage and transport.

Pollution Due to Combustion of Fuels:

  1. Burning of Coal produces CO2, the excess of which may increase greenhouse effect.
  2. Burning of Petroleum Products produce CO2 and CO which is highly poisonous and may cause death.
  3. Burning of Petroleum Products also releases unburnt lead particles which are highly poisonous.