Combustion and Flame - Science Tutorial PDF

Chapter 6 - Combustion and Flame Learning objectives Define combustion and Ignition temperature List the conditions necessary for combustion. Differentiate types of combustion Relate different ways to extinguish fire State different zones of candle flame List the criteria for an ideal fuel List the...

Chapter 6 - Combustion and Flame Learning objectives Define combustion and Ignition temperature List the conditions necessary for combustion. Differentiate types of combustion Relate different ways to extinguish fire State different zones of candle flame List the criteria for an ideal fuel List the pollutants created due to combustion COMBUSTION The process in which a substance undergoes a chemical reaction in the presence of air (oxygen) to produce heat and light is called combustion. The substance which undergoes combustion is called combustible. Conditions necessary for combustion 1. Presence of a combustible substance (a substance which can burn) 2. Presence of oxygen 3. Attainment of ignition temperature Ignition temperature: The lowest temperature at which a combustible substance catches fire under given pressure conditions is called ignition temperature. The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Eg- Petrol, LPG, Alcohol etc Types of combustion a) Rapid combustion: Combustion which occurs rapidly Eg: burning of matchstick or petrol b) Spontaneous combustion: Combustion which takes place on its own ( without any external help) Eg: Burning of Phosphorus, Sodium etc c) Explosion: Combustion which takes place with the evolution of heat, light & sound. Large amount of gases are liberated in the process. Eg: burning of firecrackers Ways of extinguishing fire Substances burn if they are heated to their ignition temperature and if a continuous supply of oxygen is there. Therefore fire can be extinguished if the temperature is lowered or supply of oxygen is blocked. Water is the most common fire extinguisher. It cools down the combustible substance below the ignition temperature. Water vapour also surrounds the combustible material , helping in cutting of the supply of oxygen. In this way fire can be controlled. Water cannot be used to control fires involving electrical equipment or oils. If an electrical equipment is on fire, water may conduct electricity and harm those who are trying to douse the fire. Water is also not suitable for fires involving oils. Water is heavier than oil, so it sinks below the oil & & oil keeps burning at the top. Carbon dioxide can be used to extinguish fire caused by petrol, oil etc.. It is heavier than oxygen and covers the combustible substance like a blanket, thus cutting of the contact between combustible substance and oxygen. When released from the cylinder, CO2 expands enormously in volume & cools down. So , it also brings down the temperature of the fuel. In that way fire is extinguished. Flame Some substances burns with a flame , whereas some burns without a flame Egs. of substances which burns with a flame :Candle, magnesium, camphor, kerosene….. Egs. of substances which burns without a flame : coal, charcoal … Different zones of a candle flame: Charactersistics of a good fuel: 1. Readily available 2. Cheap 3. Moderate rate of combustion 4. Moderate ignition temperature ( neither too low nor too high) 5. High calorific value( produces a large amount of heat) 6. Does not leave behind any undesirable substances ( after burning) Fuel efficiency is expressed in terms of its calorific value. Calorific value is defined as the amount of heat energy produced on complete combustion of 1 kilogram of a fuel. It is expressed in a unit called kilojoule per kg. The higher the calorific value of a fuel, the more is its efficiency. Calorific Value = Amount of heat produced Weight of fuel burnt Harmful effects of burning fuels The increasing fuel consumption has harmful effects on the environment 1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory disease, such as asthma. 2. Incomplete combustion forms carbon monoxide gas. It is very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room. 3. Combustion of most fuels releases carbon dioxide in the environment. Increased percentage of carbon dioxide in the air causes global warming. 4. Burning of coal and diesel release sulphur dioxide gas. Sulphur dioxide and nitrogen oxide dissolve in rain water to form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.