COMBUSTION AND FLAME
Ignition Temperature The lowest temperature at which a substance catches fire.
Inflammable Substances Substances having low ignition temperature and catch fire easily. e.g. petrol
Calorific Value The amount of heat produced in kilojoules when a gram of fuel is burnt completely.
Fuels Substances which produce heat and light energy on burning.
Combustion is a chemical process in which a substance reacts with oxygen to give off heat. During this process, sometimes light is also given off.
Substances that burn in air or oxygen to produce heat and light are called combustible substances. E.g. Paper, wood.
Substances which do not burn in air or oxygen to produce heat and light are called non-combustible substances. E.g. Water.
TYPES OF COMBUSTION
When a substance catches fire on its own, without application of heat, it is called spontaneous combustion. E.g. White phosphorous can catch fire on its own, without application of heat.
Rapid combustion occurs when a substance burns rapidly in a very short span of time and produces heat and light. E.g. when we bring a burning matchstick near a gas stove, the gas burns rapidly and produce heat and light.
The process of combustion during which large amount of gases is evolved along with heat, light and sound is called an explosion. Burning of firecrackers is an example of an explosion.
CONDITIONS NECESSARY FOR COMBUSTION
The following conditions are necessary for combustion to take place:
EXTINGUISHING A FIRE
A fire can be extinguished by cutting off the supply of air and bringing down the temperature of the burning fuel.
Cutting off the supply of air helps in extinguishing the fire. Throwing sand or water over the burning substance can cut off the supply of air.
Bringing down the temperature of the burning fuel helps in extinguishing fire. Water helps in bringing down the temperature of the fuel by cooling the combustible substance.
Water is commonly used when materials such as paper and wood are on fire. It cannot be used for fires involving electrical equipment, as water is a conductor of electricity. It can cause electrical shock to the person trying to douse the fire.
Carbon dioxide fire extinguisher consists of carbon dioxide filled under high pressure in cylinders. Fires involving electrical equipments and inflammable materials are extinguished by using carbon dioxide, as it is heavier than oxygen and covers the fire like a blanket. Hence, the contact between the fuel and the oxygen is cut off and the fire is controlled. Moreover, it does not harm the electrical equipments.
Soda-acid fire extinguisher contains concentrated solution of sodium hydrogen carbonate and sulphuric acid in separate compartments. When the extinguisher is used, the two substances come into contact and produce carbon dioxide. It can be used for extinguishing fire that involves electrical equipments.
Dry powder fire extinguisher contains sand and baking soda. When this mixture is thrown over fire, baking soda decomposes to release carbon dioxide which extinguishes the fire. It can be used for extinguishing fire that involves electrical equipments.
A flame is the region where combustion of fuel takes place.
A yellow flame is also known as a luminous flame because it emits a lot of light. A yellow flame is generally observed when the oxygen is not sufficient, which means that the combustion is incomplete.
A blue flame is also known as a non-luminous flame, as it emits very little light. It is generally observed when sufficient amount of oxygen is available, which means that the combustion is complete.
ZONES OF A CANDLE FLAME
A candle flame can be divided into three zones, depending on the amount of oxygen available.
The outer zone is the hottest part of a candle flame. It emits very little light. Because of adequate supply of oxygen in this zone, the wax vapours burn completely producing carbon dioxide and water vapour. The flame appears blue. It is also known as non-luminous zone.
The middle zone is the moderately hot part of a candle flame. This zone emits the most light. In this zone, the wax vapours do not burn completely as the supply of oxygen is not sufficient. The unburnt carbon particles give a yellow colour to the flame.
The innermost zone lies around the wick. This zone is completely dark and emits no light as no oxygen is available. So it is also known as the dark zone. Here, the wax vapours remain unburned as no oxygen is available. It is the least hot part of the flame.
Fuels are the substances that produce heat and light energy on burning. They are used for various purposes such as cooking, heating, running automobiles and for the generation of electricity.
TYPES OF FUELS
Fuels can be classified on the basis of their physical state as solid, liquid and gaseous.
Solid Fuels: Fuels that exist in solid state are called solid fuels. E.g. Charcoal, wood and coal.
Liquid Fuels: Fuels that exist in liquid state at room temperature are called liquid fuels. e.g. kerosene, petrol and diesel.
Gaseous Fuels: Fuels that exist in gaseous state at room temperature are called gaseous fuels. e.g. biogas and natural gas.
CHARACTERISTICS OF AN IDEAL FUEL
HARMFUL PRODUCTS FORMED BY COMBUSTION OF FUELS
Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of CO2 in the atmosphere can lead to global warming.
Carbon monoxide is produced by incomplete combustion of fossil fuels such as petrol and diesel. It is a toxic gas that reduces the oxygen-carrying capacity of the blood.
Sulphur dioxide is produced by coal based power plants. When coal is burned, sulphur dioxide is released. Sulphur dioxide reacts with oxygen and then dissolves in rainwater and causes acid rain, which damages crops, forests, soils, and acidifies lakes and streams.
Burning of fossil fuels such as coal and petroleum in thermal power plants and automobiles releases Nitrogen Dioxide. Oxides of sulphur and nitrogen dissolve in rainwater and cause acid rain.
Soot refers to the fine black particles, chiefly composed of carbon, produced by incomplete combustion of coal, oil, wood, or other fuels. Exposure to soot particles can cause respiratory problems and difficulty in breathing.