Topic outline

• Combustion and Flame

Q51. Why some substances burn with a flame while as some burn without a flame?

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.

Q52. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

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

Q53. List conditions under which combustion can take place.OrWhat are the conditions necessary for combustion to take place?

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

Q54. When the clothes of a person catch fire the person is covered with a blanket to extinguish fire. Can you guess why?OrWhy is a person whose clothes have caught fire wrapped in a thick blanket?OrWhy do we cover a burning person in the blanket as a first aid?OrWhat should be done if the clothes of a person catch fire accidentally? Why?

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.

Q55. What does a fire brigade do when it arrives at a place where a building is on fire?OrWhen a fire brigade arrives at a place where a building is on fire, what does it do?

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.

Q56. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

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.

Q57. Can the process of rusting be called combustion? Discuss.

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.

Q58. State any three characteristics of an ideal fuel.OrWhat are the characteristics of an ideal fuel?OrWhat is an ideal fuel?

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.

Q59. What is acid rain? How is acid rain harmful?OrWhat are the causes and effects of acid rain?

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.

Q60. If a can full of kerosene catches fire, what is the simplest way to put off this fire?

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.

Q61. What is explosive combustion? Give one example.

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.