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8th Class Science Combustion and Flame Question Bank Combustion and Flame Formative Objective Problems

  • question_answer
    Sujit took four different beakers I, II, III, and IV each containing 1 litre water. Also, he took four fuels W, X, Y, and Z. Then, he started heating the water in beakers I, II, III, and IV using 1 kg of each W, X, Y, and Z respectively. He found out that the heat produced by complete combustion of 2 kg of W, 4 kg of X, 1.2 kg of Y, and 3.2 kg of Z is 110000 kJ, 100000 kJ, 72000 kJ, and 12800 kJ respectively. Assume that all the heat released by burning the fuel is absorbed by water. In which beaker does the maximum evaporation take place?

    A)  I                             

    B)  II      

    C)  III          

    D)  IV

    Correct Answer: C

    Solution :

      The higher the calorific value of fuel, the higher is the amount of heat energy released on its combustion. The calorific value of fuel is the amount of heat energy produced by complete combustion of 1 kg of fuel. For fuel W: 2 kg of W produces 110000 kJ of heat. 1 kg of W will produce\[\frac{110000}{2}\]kJ of heat. \[=55000\text{ }kJ\]of heat Hence, the calorific value of W is\[55000\text{ }kJ/kg.\] For fuel\[X:\] 4 kg of\[X\]produces 100000 kJ of heat. 1 kg of\[X\]will produce\[\frac{100000}{4}\]kJ of heat. \[=25000\text{ }kJ\] of heat Hence, the calorific value of\[X\]is 25000 kJ/kg. For fuel Y: 1.2 kg of Y produces 72000 kJ of heat. 1 kg of Y will produce \[\frac{72000}{1.2}\]kJ of heat.                 \[=60000\text{ }kJ\]of heat Hence, the calorific value of Y is 60000 kJ/ kg. For fuel Z: 3.2 kg of Z produces 12800 kJ of heat. 1 kg of Z will produce\[\frac{12800}{3.2}\]kJ of heat.                 \[=4000\text{ }kJ\]of heat Hence, the calorific value of Z is 4000 kJ/ kg. Since the calorific value of Y is the highest (60000 kJ/kg), the water in beaker III will evaporate quickly. Hence, maximum evaporation takes place in beaker III. The correct answer is C.   


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