COMBUSTION IS A PROCESS IN WHICH A SUBSTANCE IS TO CATCH FIRE
Combustion is a chemical process in which a substance or fuel burns in the presence of air (oxygen) with the release of heat and light, the flame. On combustion the burning fuel gets oxidised (combines with oxygen) gets converted to form new substances. At times, combustion is accompanied with the production of hissing sound as well. The sound produced may be a hectic sound, crackling sound or an explosion.
Combustible and Non-combustible Substances
The substances which get ignited (catch fire) when brought near a flame are combustible substances and the substances which do not catch fire on bringing a flame near them are non combustible substances.
Types of Combustion
1. Explosive combustion: When a cracker is heated it explodes with a big-bang generate heat and light (sparkles). Same is also true with the firing of a gun. Big sound is produced along with combustion. Firewood burns with a crackling sound.
2. Spontaneous combustion: Strike the head of a matchstick side of the matchbox. The head on the matchstick catches flame instantaneously.
3. Rapid combustion : Cooking gas (in our homes), kerosene, petrol,diesel and camphor can catch flames even with a spark from a gas lighter.
These materials are highly inflammable. You might have noticed “Highly Inflammable” written on petrol tankers. This is a warning to keep a fire far away from these tankers.
4. Slow combustion: Burning of wood, coal, wax candle, pieces of paper or such other materials is a slow combustion.
Conditions Necessary for Combustion
1. Combustible material, the fuel: Try to set sand on fire. It will not catch fire. Sand is not combustible. It is not a fuel. Fire can catch easily with petrol, spirit, kerosene, wax, wood, coal, paper and straw due to the low ignition temperature of the substances. All these are combustible materals OR these are fuels.
2. Air (oxygen): Light a spirit lamp. It has a flame. Place a lid over the flame. The flame goes off. Why? The lid block the supply of oxygen to the flame and the flame goes off. Thus, oxygen is necessary for combustion. Oxygen supports combustion. Similarly, all fires need oxygen. Another way of cutting the supply of oxygen to the burning material is to fill the atmosphere around with carbon dioxide.
3. Ignition temperature: A fuel catches fire immediately if the fuel is in the
form of vapour (gas). Cooking gas catches fire immediately. Spirit and petrol vaporize at room temperature very rapidly hence they catch fire immediately. Similarly, camphor gets ignited fast. Cloth and paper get ignited at room temperature.
Kerosene vaporizes easily at a temperature above 150°C and hence it takes time to catch fire. When a slice of cloth, paper or straw is kept in kerosene and is contact on fire, the paper catches fire immediately and in turn heats up kerosene raising its surface temperature to 150°C and above and very early kerosene gets ignited. Same is also true of other vegetable oils. The flame on the tip of the wick vaporizes the oil below, which keeps the flame going.
The lowest temperature at which a substance or fuel catches fire easily is called its ignition temperature.
During the fire accident, fire-fighters rush to the accident site with a tanker full of plenty of water, a pumping device to pump water and other fire fighting tools like axe, rope and ladders.
First step is to spray the water on the burning material . This helps in two ways: (a) lowering the temperature of the burning substance, and (b) putting off flames by cutting off the contact of oxygen from the flames. This function is also done by soda-acid fire extinguisher or by keeping sand and soil over the burning substance.
Second step: Flattening of neighbouring huts, houses or stock of fuel to save spreading of fire by cutting off contact to the future supply of fuel.
Fireman fighting fire
Important precaution: In the case of oil fires, to extinguish the fire, the burning material is not sprayed with water. Water is heavier than c spraying water it sinks and oil floats, and the fire continues.During the soap-suds are sprayed which float on oil and cut off contact the supply of air burning material.
Fuel is a material used to make fire. Combustion without fuel is not possible. Fuel is also a source of energy. We eat food, food is a source of energy for our body OR food is a fuel which provides our body with energy for work and play.
Fuel is the substance or resource which is oxidised with the release of energy. Oxidation of a fuel may be catch fire in the presence of air (oxygen). This is combustion.
Kinds of Fuels
We make use of other kinds of fuels or substance in our everyday life. In the kitchen we use cooking gas to cook the food. Some public use kerosene in stoves or coal or even wood in hearths. Petrol and diesel are used in vehicles. We may group fuels as:
1. Solid fuels: Dung cakes, organic firewood, straw and other agricultural wastes, charcoal, coal (mined fossil fuel) like steam coke and soft coke, paraffin wax, tallow (animal fat) and may be camphor.
2. Liquid fuels: Fossil fuel (petroleum) or crude oil from which petrol (also called gasoline), kerosene, coal tar,diesel, benzene and toluene are created, vegetables oils, tallow in liquid form, alcohol (spirit and liquified hydrogen.
3. Gaseous fuels: LPG (Liquid petroleum gas) is used as cooking gas for the cooking purpose.CNG (Compressed) natural gas) made from natural oil wells. Natural gas is methane which is get out from putrefying organic matter, biogas (prepared in our villages from animals dung and farm organic waste) and also is collected and supplied for the fuel.
Acetylene produced is with a bit unpleasant smell and was at just once primarily used for welding purposes. Flame from acetylene is hot and is additionally used for cutting metals.
Fireman fighting fire
Important precaution: within the case of oil fires, to extinguish the hearth, the burning material isn’t sprayed with water. Water is heavier than c spraying water it sinks and oil floats, and also the fire continues. just in case of soap-suds are sprayed which float on oil and interrupt the availability of air burning material.
All fuels are carbon rich compounds. Carbon is that the base fuel. additionally to those compounds the fuels specially obtained from petroleum are rich in hydrogen. Hydrocarbons are the compounds of hydrogen and carbon and are superb as fuel. On burning, they furnish out water vapor (Hydrogen and oxygen) and dioxide gas. Wax is additionally a hydrocarbon and a product from petroelum.
Dung cakes, wood, straw, agricultural wastes, mined coal and vegetable oils are complex compounds which can include another elements also.
4. Some more fuels: Dry cells being employed in torches and toys generate current from chemical reactions. we’ve learnt about its structure within the previous class. Then there are storage batteries getting used in automobiles (cars, buses, trucks) store electricity in chemicals and wish recharging from time to time. Same is true about batteries in mobile phones.
Rockets (missiles and space vehicles) are powered by a mix of a fuel and oxygen. Oxygen or the other oxidising substance is required in rockets since oxygen isn’t available in space, where there’s no atmosphere. a number of the rocket fuels are: Solids: polyurethane or poybutadiene with ammonium perchlorate as oxidiser; OR nitroglycerine and nitrocellulose. Liquids: Liquified nitrogen tetroxide. liquified hydrogen (fuels) and liquid oxgen to assist burning. There are some more fuels about which we may learn in higher classes.
5. Nuclear fuel: When an atom, the littlest particle of part is broken its nucleus gives out huge amount of energy, which may be converted to power and used safely for homes and industry. it’s totally non-polluting but risky, if not handled properly since leakage may cause radiation, which ends in great loss to living
beings. energy is additionally available from fusion of atomic nucli at a awfully heat.
Nuclear fuel could be a big source of energy and is non-polluting. Of all the fuels we are mainly concerned with fuels getting used in our homes and little industries. For this purpose we’ve to pick out better of the fuels which provides more energy and doesn’t cause pollution. we’d first be learning about the characteristics of an honest fuel.
A Good Fuel Should be
1. readily available,
2. cheap, available at an affordable price, 3. should have a decent calorific value (energy giver),
4. might be easily ignited (low ignition temperature) 5. should burn at
a steady rate, 6. may well be ignited and defer easily,
7. easy to move from one place to a different,
8. Safe in handling.
9. mustn’t produce wastes like ash,
10. shouldn’t cause pollution, no smoke (soot) or other poisnous gases,
11.safe to store
Calorific value of a fuel
Calorific value of a fuel is that the amount of warmth energy given out on burning ONE UNIT of fuel within the presence of fully required amount of oxygen for its burnuing.
Calorific word has come from the word calorie. Calorie could be a unit of warmth. Calorie is defined because the amount of warmth required to lift the temperature of 1g (one gram) of water through 1°C at temperature of about 14°C. Thus, raising the temperature of 1 g of water from 14°C to 15°C. Calorific value of fuchs is mostly measured in kilocalories per kilogram of fuel OR cal. per g of a fuel.
NOTE: Generally, science books give the calorific value of a fuel in joules la unit of energy). Joules could also be a unit of energy to live work done’ and not heat. Here the scholars don’t seem to be required to travel into the little print of units except that one should be able to understand the comparitive calorific value of various fuels. Figures given below are within the units of calorie per gram of fuel (cal/g.). Otherwise, 1 calorie heat is capable 4. 1845 joules of energy.
Dung cake, wood, charcoal and coal are air polluting fuels. T calorific value is additionally low. Hence they’re not considered almost just about nearly as good Kerosene also gives out smoke unless utilized in pump stoves and tha carefully. Petrol, diesel and alcohol are utilized in driving cars and other vehicles Alcohol or ethanol is that the merchandise of fermenting waste sugars and are partly mixed with petrol and used as fuel. LPG, CNG and biogas are non-polluting fuels with high calorific value and are widely employed in homes and industry.
Hydrogen has the only calorific value among the fuels. it’s non polluting further. On burning it combines with oxygen to create vapor.
Since hydrogen is extremely explosive, specially when it gets mixed with air, its use is proscribed to rocketry. within the space, temperature being low, hydrogen remains in liquid form and there being no atmosphere there’s not any risk of it getting mixed with air. it’s a extremely expert fuel to use in rockets. In spaceships generally solid fuel is getting used.
A wax candle burns with a yellow flame. The household gas burners when lighted burn with a blue flame. what’s this flame ?
A region of burning gases is termed a flame.
Light a wax candle and watch its flame. Carefully note the various coloured zones within the flame. ranging from the underside of the flame, a flame has four zones.
1.Blue zone of the flame: it’s near the underside of the flame. Vaporised wax gets oxidised to monoxide gas gas and monoxide burs completely with a blue flame during this zone.
2. Dark inner zone of the flame : Surrounding the wick is that the dark zone. there’s no burning during this zone.
3.Luminous zone: during this region of the flame hydrogen burns with an impressive yellow luminous flame. Burning hydrogen combines with oxygen to create vapour. Carbon also burns during this zone giving some luminosity to the flame and producing gas. Some unburnt carbon particles are left which create to soot.
4.Outermost non-luminous zone: This zone is lightly visible and is slightly blue. it’s the foremost well liked a section of the flame where fuel goes under complete oxidation (burning).
Wicks within the Lamps
What can be a wick? Wick could also be a loosely twisted thread manufactured from absorbert fibre, which is often cotton or it’s a spongy material. One end of the wickin a lamp is kept dipped within the liquid fuel, the opposite end is kept free within the air. The free end of the wick is additionally soaked in oil or fuel and is lighted. The wick catches fire and bears the flame. The wick being fabricated from absorbert material absorbs the liquid fuel and then the fuel rises into the wick along the absorbent walls of the fibre. Then, there are fine capillaries along the fine fibres of the wick which help the fuel in rising into the wick and reach its free end lying within the air. This maintains an everyday supply of the liquid fuel to the flame.
Wick is placed Differently in several Fuels
1. The spirit and kerosene lamps are so made that the free end of the wick with flame totally remains discontinue (separated) from the fuel since the fuel is extremely inflammable.
2. Wick in an exceedingly fat lamp remains uncovered except that it’s partly under the oil (fuel). in an exceedingly very kerosine lamp wick soaked in oil remains exposed to the flame so on vaporise the oil and keep the flam going.
3. Wick during a wax candle is surrounded all round with wax. The flame melts the wax below. The molten wax rises into the wick and keep the flame going.
The Chemical History Of a Candle, by M.Faraday
Michael Faraday (1791 to 1867), an Englishman was an experimentalist functioning on electricity and field. He was keen on reading science books. He had joined a weekly club to debate elementary science in simple laymans’ language together with his friends and children. Once, within the year 1860 he gave an interview on Chemical History of a Candle’, a summarised version of which is as follows:
Wax is additionally a fuel, the storehouse of energy. It comes from beehives. Its constituents are hydrogen and carbon, the hydrocarbons. A wick, made of a fibre, is surrounded all around with wax within the kind of a candle. The wick is initially waxed and ignited. Heat from the fire on the wick helps the wax below to melt. Molten wax rises into the wick by natural phenomenon and gets heated to release hydrogen and carbon. Hydrogen burns with a golden yellow flame to form vapour. It also helps in oxidising (burning) carbon into acid gas. Both, forming the luminous a part of the flame. a component of the
carbon remains unbumt, which is then burnt within the outermost a component of the flame giving out enormous heat. this might be how a wax candle gives out light and warmth.