Ans. The substances which vapourise during burning, give flames. For example, kerosene oil and molten wax rise through the wick and are vapourised during burning and form flames. Charcoal, on the other hand, does not vapourise and so does not produce a flame.
Ans. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
Heat produced from 4.5 kg of a fuel = 180,000 kJ
Heat produced from 1 kg of a fuel = 180,000 kJ ÷ 4.5 kg
= 40000 kJ/kg
Ans. There are three conditions which are necessary for combustion to take place. These are:
i. Presence of a combustible substance (fuel)
ii. Presence of air or oxygen
iii. Heating the combustible substance to its ignition temperature
Ans. When the clothes of a person catch fire the person is covered with a blanket to extinguish fire because when the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off and hence the burning stops.
Ans. It pours water on the fire. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.
Ans. The water in the Ramesh’s beaker will heat up in shorter time. This is because outermost part of the flame is the hottest part of the flame whereas the yellow zone of the flame (the middle zone of a flame or luminous zone) in which Abida kept the beaker produces moderate temperature.
Ans. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The rust is formed when iron slowly combines with the oxygen present in air (in the presence of moisture) to form iron oxide. The process of rusting of iron is a slow combustion and liberates very little heat but no light.
Ans. Characteristics of an ideal fuel
i. An ideal fuel is cheap, readily available, readily combustible and easy to transport.
ii. It has high calorific value.
iii. It does not produce gases or residues that pollute the environment.
Ans. Burning of coal and diesel release sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
Ans. A small fire can be extinguished by throwing sand or soil over it. When sand is thrown over burning kerosene oil, the sand covers it like a blanket. The sand cuts off the air supply to the burning kerosene oil due to which the fire gets extinguished. For heavy fires involving inflammable materials carbon dioxide (CO2) is the best extinguisher.
Ans. A very fast combustion reaction in which a large amount of heat, light and sound are produced is called explosive combustion. A large amount of gases is released quickly. It is the rapid expansion of these gases which causes a loud sound. Example: fireworks on festival days. When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound.
Download to practice offline.