Combustion and Flame

Combustion is a chemical process in which a substance reacts with oxygen to give off heat, and often light. Substances that burn, such as wood, paper, kerosene and LPG, are combustible, while stone and glass are non-combustible. In this chapter you will learn that a substance catches fire only when it is heated to its ignition temperature, and that combustion needs three things together: fuel, air (oxygen) and heat. Removing any one of them puts out a fire, which is why water, sand and carbon dioxide extinguishers work, and why carbon dioxide is best for electrical and oil fires. You will meet rapid combustion, spontaneous combustion, as of phosphorus, and explosions, as in crackers. You will study the three zones of a candle flame, from the dark innermost zone to the hottest, blue outermost zone used by goldsmiths. You will compare fuels using their calorific value in kJ/kg, and learn how burning fuels releases carbon particles, carbon monoxide, carbon dioxide and gases that cause acid rain.

Detailed Worksheet: Combustion and Flame Section A - Definitions (10 marks) 1. Define combustion. Distinguish between combustible and non-combustible substances, giving two examples of each. (2 marks) 2. What is ignition temperature? Why does kerosene catch fire more easily than wood? (2 marks) 3. What are inflammable substances? Give two examples and state one precaution while storing them. (2 marks) 4. Define the calorific value of a fuel and write its unit. (2 marks) 5. Define rapid combustion, spontaneous combustion and explosion, with one example of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. In an experiment, 4.5 kg of a fuel was completely burnt and the heat produced was measured to be 1,80,000 kJ. Calculate the calorific value of the fuel. (3 marks) 7. The calorific value of wood is about 17,000 kJ/kg and that of LPG is about 55,000 kJ/kg. Find the heat produced by burning 2 kg of each fuel completely. How many kilograms of wood would give the same heat as 1 kg of LPG? (3 marks) 8. Hydrogen has a calorific value of about 1,50,000 kJ/kg. Calculate the heat produced by burning 0.5 kg of hydrogen completely. Why is hydrogen, despite its very high calorific value and clean burning, not yet used widely as a domestic fuel? (3 marks) 9. Using the idea that combustion needs fuel, air and heat, explain (i) how water puts out a fire in wood (ii) why water should not be used on a fire caused by oil or petrol (iii) why water should not be used on a fire in electrical equipment. (3 marks) 10. A paper cup filled with water is heated over a candle flame, and the water gets hot but the cup does not burn. Explain this observation. Also explain why it is difficult to light wet wood. (3 marks) Section C - Diagrams (10 marks) 11. Draw a neat labelled diagram of a candle flame showing the innermost zone, the middle zone and the outermost zone. Write the colour, the extent of combustion and the temperature (relative) of each zone. (4 marks) 12. Draw labelled diagrams of the activity in which a burning candle is placed (i) in an open glass chimney resting on wooden blocks (ii) in a chimney with no gap at the bottom (iii) in a chimney covered with a glass plate at the top. State what happens to the flame in each case. (3 marks) 13. Draw the fire triangle and label its three sides. Below each side, write one method of putting out a fire by removing that requirement. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. List four harmful effects of burning fuels such as wood, coal and petrol. Explain why it is dangerous to sleep in a closed room with a coal fire burning, and why the use of diesel and petrol in vehicles is being replaced by CNG. (5 marks) 15. Write five characteristics of an ideal fuel. Compare LPG with cow dung cakes as household fuels, using their calorific values (LPG about 55,000 kJ/kg, cow dung cake about 6,000 to 8,000 kJ/kg) and the pollution they cause. Why is biogas a better choice than cow dung cakes in villages? (5 marks) 16. A goldsmith blows air through a metallic blow-pipe into the outermost zone of a flame to melt gold and silver. Explain why. Why is the flame of a kerosene stove or LPG burner blue, while the flame of a candle is yellow? What do you observe when a glass plate is held in the luminous zone of a candle flame, and why? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil, and perform any burning activity only under the supervision of your teacher.