Class 8, Science, Chapter-5, Lecture-1, Combustion (Notes)
COMBUSTION:
The process of burning of a substance in air to release heat is called combustion.
FIRE TRIANGLE:
Conditions necessary for combustion
- Combustible substance
- Supporter of Combustion
- Ignition Temperature
Gaps are Left Between the Logs of Woods.
Reason:
Oxygen is necessary for combustion to take place.
Gaps between the logs of woods allow supply of oxygen to make combustion of woods possible.
Respiration is called Slow Combustion.
Reason:
During respiration, broken food particles oxidize (combine with oxygen) to produce energy and CO2, which is same as combustion, but it occurs very slowly and thus it is called slow combustion.
Differences between:
Respiration | Combustion |
---|---|
1. It is very slow process. | 1. It is a fast process. |
2. Takes place at body temperature. | 2. Takes place at ignition temperature. |
3. Takes place in the presence of catalyst. | 3. Takes place without catalyst. |
4. Takes place inside the body. | 4. Takes place outside the body. |
IGNITION TEMPERATURE (KINDLING TEMPERATURE):
The minimum temperature at which a substance catches fire is called its ignition temperature.
Water Can Be Boiled in a Paper Cup.
Reason:
The heat of the plane is quickly transferred to the water and it is used in converting water into steam which does not allow the temperature of the paper to reach its ignition temperature.
Matchstick Does Not Catch Fire at Room Temperature.
Reason:
The room temperature is lower than the ignition temperature of matchstick.
Matchstick Catches Fire on Rubbing
Reason:
The work done to overcome the friction between the surfaces produces heat which raises the temperature of the material at the tip of the stick to its ignition temperature.
CALORIFIC VALUE OF FUELS:
The amount of heat produced when one unit of fuel is completely burnt is called its calorific value.
${\rm{Calorific~value~of~a~fuel = }}{{{\rm{Amount~of~Heat~produced}}} \over {{\rm{Mass~of~fuel~ burnt}}}}$
Units of Calorific Value:
- I. Unit – J/kg
- Joule/gram – J/g or Jg-1
- Kilojoule/gram – kJ/g or kJg-1
- Kilowatt hour/kilogram – kWh/kg or kWh kg-1
Hydrogen Has Highest Calorific Value but it is Not Used as Domestic Fuel.
Reasons:
- Hydrogen is highly inflammable and thus difficult to be handled.
- Transportation of Hydrogen is difficult.
- Cost of production of Hydrogen is high.