CBSE Class 8 Chemistry Notes for Chapter 4 Combustion and Flame

                                     CHAPTER 4 COMBUSTION AND FLAME
 
COMBUSTION
When a substance reacts with oxygen to give off heat, the process is called combustion. The substance is called combustible substance or a fuel.
The lowest temperature required for a substance to catch fire is called its ignition temperature.
The substances which have low ignition point are called inflammable substances.
Controlling fire: To control fire, the oxygen supply to the fire has to be cut off.
Types of combustion:

• Rapid combustion: When a material burns rapidly and produces heat and light, it is called rapid combustion.
• Spontaneous combustion: When a material suddenly bursts into flames without any apparent cause, it is called spontaneous combustion.

 
EXPLOSION
When a substance burns and a sudden evolution of heat, light and sound takes place, it is called an explosion.
 
FLAME
The different zones of a candle flame are:

• Innermost zone: Unburnt wax vapours; least hot
• Middle zone: Partial combustion; yellow; moderately hot
• Outer zone: Complete combustion; blue; hottest

 
FUEL
A fuel is a substance which is a source of energy. The properties of a good fuel are:

• Readily available
• Cheap
• Burns easily at a moderate rate
• Produces a large amount of heat
• Does not leave behind any undesirable substances

 
FUEL EFFICIENCY
The amount of heat energy produced on complete combustion of 1 kg of a fuel is known as its calorific value. Unit of calorific value is kilojoule per kg (kJ/kg).
 
HARMFUL EFFECTS OF BURNING FUELS

• Carbon fuels release unburnt carbon in the form of fine particles which cause harmful effects to the respiratory system.
• Incomplete combustion of fuels gives out carbon monoxide which is a very poisonous gas.
• Combustion of fuels releases carbon dioxide into the atmosphere which leads to global warming.
• Burning of coal and diesel releases sulphur dioxide which is a highly corrosive and suffocating gas. Petrol engines release oxides of nitrogen. These oxides dissolve in rain water and which falls down as acid rain. Acid rain is harmful for the soil, the crops and the buildings.