Test: Combustion & Flame- 2 - UPSC

Those substances that start burning at ignition temperature in presence of oxygen is called combustible substance. Water, glass and sand are non combustible substance while wax is a combustible substance.

Water extinguishers work mainly by cooling (removing heat), but a dense spray of water droplets also helps to cut off oxygen.
Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air so that the fire is exhausted. But water cannot be used for fires involving electrical equipment and inflammable substances.

Fire can be extinguished by cutting the supply of oxygen and removing combustible substances from surrounding or lowering the temperature below the ignition temperature.

Non combustible substance does not burn in air or oxygen. Carbon is generally absent in non-combustible substance or present in very less quantity.

Oxygen gas supports the combustion of combustible substances including coal. Combustion of coal release carbon dioxide gas as well as solid residue.

If methane is burnt in sufficient oxygen, the products formed are carbon dioxide and water. CNG and biogas contain mainly methane gas.

Incomplete combustion of fuels release harmful gases like carbon monoxide. Carbon monoxide binds with haemoglobin and reduces the oxygen carrying capacity of blood.

Calorific value provides the amount of energy that is released when a fuel or material undergoes the process of combustion.

It is usually a measure of energy.

The SI unit (international system of units) of calorific value is Joule/Kg or Kilo Joule/Kg.

Those materials that combine with oxygen quickly burns faster. Oxygen help in combustion of substance by oxidising the carbon substance into carbon dioxide gas.

The middle zone of candle flame is known as zone of partial combustion. This zone is generally yellow in colour and has less heat.

Burning of a matchstick is an example of rapid combustion. Matchstick contains phosphorus that catches fire due to heat produce by friction.

Fire caused by burning of liquid fuels can be controlled by throwing soil and sand over it to avoid the contact of oxygen that supports the combustion.

Spontaneous combustion occurs itself with external heat. Spontaneous combustion occurs in those substances that have very low ignition temperature, lower than room temperature.

Burning of liquefied petroleum gas (LPG) is an example of rapid combustion. Rapid combustion starts very fast and lot of heat and light energy is produced.

Phosphorus is generally found as a waxy, yellow, transparent solid. When exposed to air, it spontaneously ignites and is oxidized rapidly to phosphorus pentoxide. Such heat is produced by this reaction that the element bursts into a yellow flame and produces a dense white smoke.

When a glass plate or slide is held into the luminous part of candle flame we see a circular blackish ring that represents unburnt carbon particles.

Different zone is formed in the flame having different colour. The hottest part of the flame is blue in colour present on the upper part of flame.

Combustible substance changes into vapour at its ignition temperature and start burning to give heat and light.

Sodium can catch fire without any external heat supply when come in contact with oxygen. Sodium is kept under kerosene oil to prevent accidental fire and protecting the sodium metal.

Fire caused by an electric short circuit should not be extinguished by using water because water is good conductor of electricity and may cause electric shock.