Combustion and Flame - PDF

# Combustion and Flame ## Learning Objectives - Combustion and its types. - Extinguishing fire. - Flame and history of candle. - Fuel, its classification and characteristics. - Impacts of burning fuels on environment. ## Combustion - When wood or coal is burnt, we observe that heat and light is...

# Combustion and Flame ## Learning Objectives - Combustion and its types. - Extinguishing fire. - Flame and history of candle. - Fuel, its classification and characteristics. - Impacts of burning fuels on environment. ## Combustion - When wood or coal is burnt, we observe that heat and light is evolved. - The process of burning of substances in the presence of oxygen to produce heat and light is known as combustion. - It is an exothermic reaction. - Materials which catch fire easily in presence of oxygen are called combustible materials like paper, wood, and petrol. - Substances that do not burn easily are called non-combustible materials like iron and glass. - The air or oxygen which helps in burning is called supporter of combustion. - The chemical reaction which takes place with the release of energy is called combustion. ## Conditions Necessary For Combustion - Presence of combustible substance - Supporter of combustion - Attainment of ignition temperature ## Activity 1 - **Aim:** To demonstrate the process of combustion - **Materials Required:** Pieces of magnesium ribbon, charcoal, and matchstick head, a pair of tongs, and a spirit lamp. - **Procedure:** Heat the magnesium ribbon, charcoal, and matchstick separately by holding with a pair of tongs on the flame of the spirit lamp. ## Observation - Substances burn and produce heat and light. - They combine with oxygen from the air and form respective oxides. ## Conclusion - Combustion is a process of oxidation and producing heat and light. ## Air - Air is necessary for combustion. If you light a candle and cover it with a glass jar, it stops burning after some time. This happens because no air is available for combustion. ## Activity 2 - **Aim:** To prove that a smaller piece of wood catches fire more easily than a larger piece. - **Materials Required:** A piece of wood, splinters of wood, a pair of tongs, and a spirit lamp. - **Procedure:** Take a piece of wood. Hold it with a pair of tongs over a flame for a few minutes. Repeat this experiment with a wood splinter. ## Observation - You will see that the splinter of wood catches fire more easily than the piece of wood. ## Conclusion - From this activity, we conclude that a smaller piece of wood catches fire more easily than a larger piece. ## Types of Combustion - **Slow combustion:** If combustion takes place very slowly and at low temperature, it is called slow combustion. Respiration, digestion of food, rusting of iron are few examples of slow combustion. ## Rapid Combustion - When a substance burns rapidly and produces large amounts of heat and light, it is called rapid combustion. Burning of matchsticks and LPG are few examples of rapid combustion. They have lower ignition temperature. ## Spontaneous Combustion - When a material suddenly bursts into flames without application of apparent cause, it is called spontaneous combustion. For example, sodium bursts in air. In this type of combustion, no external ignition is required. ## Explosion - When a mixture of combustible material and air completely burns in a very short period of time in a closed space to produce large amounts of sound, light, and heat energy, it is called explosion. For example, firecrackers, bombs, etc. ## Activity 3 - **Aim:** To study spontaneous combustion and slow combustion. - **Materials Required:** Small piece of yellow phosphorus, china dish, and a rusted iron nail. - **Procedure:** Place yellow phosphorus in the dry china dish. We will find that it catches fire without being heated. This is called spontaneous combustion. Observe rusted nail. The rust is formed on iron because iron gradually combines with oxygen from the air. This is slow oxidation process in which no light or sound are produced, and the heat generated in the process cannot be felt. Formation of rust is an example of slow combustion. ## Complete and Incomplete Combustion - When a combustible substance burns completely in the presence of oxygen, carbon dioxide is formed. This combustion is called complete combustion. For complete combustion to take place, sufficient amounts of oxygen are required during a chemical reaction. ## Disadvantages of Incomplete Combustion - Common fuels contain large percentages of carbon due to incomplete combustion. So, during incomplete combustion, the unburnt carbon passes into the atmosphere in the form of soot. This pollutes the atmosphere. - During incomplete combustion, carbon monoxide (CO) and sulfur dioxide (SO₂) are formed. The gases are highly poisonous. ## Extinguishing Fire - A fire should be controlled on time, otherwise it becomes fatal. In cities, we call a fire brigade to extinguish fire, but we should always have a fire extinguisher at home, office, and school. We have read that there are three conditions: - Removal of combustible substance - Cut off the supply of supporter of combustion, i.e., air - Cool the substance below its ignition temperature ## Fire Extinguisher - A fire extinguisher is a device which is used to put out fire. It cools the burning material and cuts off the supply of air and thus extinguishes fire. - The simplest fire extinguisher is a soda-acid fire extinguisher. Besides, carbon dioxide fire extinguisher, foam-type fire extinguisher, and dry powder fire extinguisher are also used to extinguish fire. ## Soda-Acid Fire Extinguisher - This type of fire extinguisher consists of three parts: container, bottle, and knob. The container is cylindrical containing sodium bicarbonate and the bottle inside the container contains concentrated sulphuric acid. When the knob is pressed, the bottle breaks and sulphuric acid reacts with sodium bicarbonate in the container to produce carbon dioxide gas with brisk effervescence that extinguishes fire. The fire is extinguished as the blanket of carbon dioxide gas blocks the air supply. - NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂ - Carbon Dioxide Fire Extinguisher - It has a cylinder that consists of carbon dioxide gas under high pressure. It is used to extinguish fire caused by flammable liquids, like oil and petrol. It is also effective to extinguish electrical fire. - Foam type fire extinguisher is used if the fire is caused by solid or liquid substances. - Dry powder fire extinguisher is effective in the case of liquids and electric wire. ## Flame - Everything that burns does not produce a flame. Substances which vaporize will only produce a flame. For example, wax, which is a hydrocarbon. ## History of Candle - The title of the series of lectures given by Michael Faraday in 1860-61 was the chemical history of a candle flame. Six lectures were printed as a book in 1861 which included the components, function of the candle wick, weight of the atmosphere, capillary attraction, the carbon content in oxygen and living bodies, the production of carbon dioxide from coal gas and sugar, the properties of carbonic acid, respiration and its analogy to the burning candle, and much more. 22 experiments were performed. ## According To Faraday - "There is no law under which any part of this universe is governed which does not come into play and is not touched upon during the time a candle burns.” - A candle is made of two parts: - Paraffin wax which is a product of petroleum. - Wick that is made of material which can absorb liquids fast. ## When We Light a Candle - Heat of the burning matchstick melts wax first and then changes it into wax vapour. The wax vapour mixes with air and then catches fire to produce a flame. Wax vapour takes the heat from the wick and cools it. That’s why the wick does not burn. Wax vapour combines with oxygen producing heat and light. - As a result of this oxidation, carbon changes into carbon dioxide. Candle flame is always in a spindleshape. During combustion of hydrocarbon, it may burn incompletely and form soot. This soot is carbon particles which settle on any object nearby. ## Zones of a Burning Candle - **Dark Zone:** (no combustion) Liquid (molten wax) Solid - **Non-luminous Zone:** (complete combustion) Luminous Zone (growing particles of soot) - **Blue Zone:** (carbon monoxide burns) ## Luminous Zone - The luminous zone of a candle flame refers to the visible portion of the flame that emits light. It is a yellow or light orange coloured region in the middle part of the flame. The luminous zone is the result of the incomplete combustion of the candle’s fuel. It produces soot particles that glow and give off light. ## Dark Zone - The wax vapours do not get oxygen at all from the air to burn. It is the innermost area just around the wick where no combustion takes place. ## Activity 4 - **Aim:** To prove that the luminous zone of the candle flame contains unburnt particles of carbon. - **Materials Required:** Candle, tongs, and glass slide. - **Procedure:** Fix a candle on a table. Light the candle. Let the flame be absolutely steady. Now introduce a clean glass slide into the luminous part of the flame, by holding it with a pair of tongs. Hold the glass slide in position for about a few seconds and then remove it. - **Observation:** You will observe that a circular grey (black) ring is formed on the glass slide, and deposition is there in the middle of ring. This black colour is due to deposition of unburnt carbon particles in the luminous zone of the flame. The centre of the ring does not have any unburnt carbon particles. The area is due to the fact that this part was over the dark inner zone, which does not have unburnt carbon particles at all. ## Blue Zone - In this area, carbon monoxide is produced due to incomplete combustion. The blue colour is due to burning of carbon monoxide. This area is at the base of the bottom of the flame. This area is at the base of the burning wick. ## In Sight - Candle wax and petrol both produce the same gases on heating. Why is petrol a better fuel than wax? ## Fuel - Fuel is the source of heat energy, and to obtain that heat energy, we burn the fuel. Burning is the process of rapid oxidation. Most of the substances are made up of carbon and hydrogen. When a substance burns, it produces heat and light. All combustible substances cannot be called fuels. ## Classification Of Fuels - **On the basis of their physical state**: They are generally classified as solid, liquid, and gaseous fuels. - **On the basis of their occurrence**: The fuels produced from the remains of plants and animals buried under the earth millions of years ago are known as fossil fuels. These fuels produce large amounts of energy on burning. Coal, petroleum, and natural gas are fossil fuels. ## Calorific Value Of Fuels - Calorific value of fuel is the amount of heat energy produced when one gram of a fuel is completely burnt in the presence of oxygen. It is usually expressed in units kilojoule per gram (kJ/g) or kilojoules per kilogram (kJ/kg). The calorific value of common solids, liquids, and gaseous fuels are given below: ## Properties of Smokeless Chulha in Comparison with Traditional Chulha - **Smokeless Chulha:** - In this, the combustion of fuel is complete. - It is provided with a tall chimney. - Its efficiency is better. - The products of combustion (like soot and smoke) do not come in contact with the person using the chulha as a chimney is provided. - **Traditional Chulha:** - In this, the combustion of fuel is incomplete. It utilises only 3-10% of fuel. - It does not have a chimney. - It is less efficient (6-8%). - The smoke (containing harmful gases) causes pollution and health problems like respiratory diseases. ## Ignition Temperature - The minimum temperature at which a fuel catches fire is termed as its ignition temperature. The substance having low ignition temperature catches fire easily. ## Characteristics of a Good Fuel - The fuel should be cheap and easily available. - The fuel should be easy to store and transport and should be safe in handling. - The fuel should have a high calorific value. - It should produce very small amounts of residues as ash. - It should not cause pollution on burning. ## Fuel Should Have: - Low ignition temperature well above the room temperature. - The fuel should burn at a slow rate – controlled combustion. ## Impacts of Burning Fuels on Environment - On burning of fossil fuels like coal and diesel, many harmful gases are produced. As a result of burning of these fuels, carbon monoxide (CO), sulphur dioxide (SO₂), oxides of nitrogen, etc. along with smoke are released. Oxides of sulphur and nitrogen dissolve in rainwater and form acids. Such rain is called acid rain. It is harmful for buildings, soil, plants, etc. - Carbon monoxide is a poisonous gas. When inhaled, it combines with haemoglobin present in the blood of a human body and forms carboxyhaemoglobin that decreases the oxygen carrying capacity in blood. As a results, it causes headache and several respiratory diseases. It can be fatal if inhaled in excess. - Carbon dioxide (CO₂) is produced as a result of combustion. If it is in excess, it causes global warming as it traps the sunrays and prevents them from reflecting back. Thus, the temperature of the earth's surface rises, which results in global warming. It leads to melting of polar ice caps and changes in weather causing floods. - Suspended Particulate Matter (SPM) is produced by incomplete combustion in the form of ash and smoke, reduces visibility and causes respiratory diseases such as asthma and bronchitis in humans. - Lead compounds produced by combustion of fuel in automobiles are poisonous. The prolonged inhalation may result in chronic diseases such as cancer. ## Science In The Vicinity - Fire has no shadow, as it itself is the source of light. - A fast-burning chimney fire consumes inside residue, which is highly flammable. The temperature of chimney fires can reach as much as 2000 °F. ## Quick Review - Combustible substances material burn completely in the presence of oxygen. Non-combustible substances do not burn completely in the presence of air or oxygen. - The three conditions must be fulfilled for combustion to take place. - There are four types of combustion: slow, rapid, spontaneous, and explosion. - Fire extinguishers are used to extinguish fire from fuel or electrical connection. - There are many types of fire extinguishers such as soda-acid and dry powder. - A candle flame consists of four zones: dark zone, blue zone, luminous zone, and non-luminous zone. - Substances which burn to produce heat and light at a reasonable cost are called fuels. - Calorific value of a fuel is the amount of heat produced on complete burning of one gram of fuel in pure oxygen. - Energy from fossil fuels creates atmospheric pollution as well as environmental problems. - Gaseous fuels are the best kind of fuels due to: - Easy transportation - Low ignition temperature - High calorific value - Complete combustion - The combustion of fossil fuels produces carbon dioxide, carbon monoxide, nitrogen dioxide, smoke, and dust particles.