Combustion And Flame

The conditions required for combustion to take place are:

1) The presence of oxygen.

2) Presence of an inflammable substance (to catch fire)

3) Ignition temperature.

(a) Burning of wood and coal causes pollution of air.

(b) A liquid fuel used in homes is Kerosene oil.

(c) Fuel must be heated to its ignition temperature before it starts burning.

(d) Fire produced by oil cannot be controlled by water.

The use of CNG in automobiles has reduced pollution in our cities as it is a cleaner fuel as compared to other petroleum containing fuels. The Combustion of petroleum leaves un-burnt carbon particles suspended in the air, along with carbon monoxide (CO) gas, which is harmful gas. Breathing CO causes a headache, dizziness, . But, Compressed Natural Gas (CNG) burns completely leaving no harmful residue . Therefore, the use of CNG has reduced pollution in our cities.

(a) Water is a conductor of electricity, so it can easily conduct electric current and cause danger of electric shocks or short-circuits. Therefore, water can not be used to control the fire involving electrical equipment.

(b) LPG is a better domestic fuel as it does not produce smoke and un-burnt carbon particles, which cause respiratory problems But wood produces a lot of unburnt carbon particles and also cause respiratory problems.

(c) A paper by itself catches fire easily because it has low ignition temperature but when wrapped around an aluminum pipe its temperature is lowered due to aluminum metal absorbing the heat supplied to paper. So it does not catch fire.

The labelled diagram of the candle frame is shown below:

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

^{Being heavier than oxygen, CO}₂^{acts as a barrier between the oxygen and the burning material. Since the contact between the fuel} and oxygen is cut off, the fire is controlled.

It is difficult to burn a heap of green leaves but dry leaves catch fire easily because green leaves have a lot of moisture in them. The heat provided to the heap is overcome in evaporating the water. But, dry leaves have no moisture in them. Therefore, they catch fire easily.

A goldsmith uses the outer part of the candle flame or the non-luminous zone of the flame for melting gold and silver. This is because in this zone the temperature is the highest which helps to melt these metals easily.

The calorific value of the fuel is the amount of heat produced by the complete combustion of 1 kg of fuel.

Now,

Heat produced by 4.5 kg of fuel is given to be = 180000 kJ

Therefore, heat produced by 1Kg of fuel = 180,000 kJ/4.5 Kg = 40,000 kJ

Hence, the calorific value of the fuel is 40,000 kJ/kg.

Combustion is a chemical process in which a substance reacts with oxygen and gives out energy. This energy produced in the process is in the form of either heat or light. Whereas Rusting of iron is an exothermic process in which energy is released during oxidation of iron. Hence, it can be regarded as slow combustion. Therefore, it is called slow combustion.

The water in Ramesh's beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.