“Combustion and Flame” is Chapter 7 taken from the JKBOSE textbook of Science for students of Class 8th. In the previous post, I provided you with Questions and Answers for Chapter 6 Food Production and Management. This post is about the Combustion and Flame Class 8 Question Answers. Before we move on to solutions you must have your basics clear. So, let me begin by giving you a brief overview of what you have studied in this chapter.
Combustion and Flame Class 8 Question Answers
Overview of Chapter
- WHAT IS COMBUSTION?
- HOW DO WE CONTROL FIRE?
- TYPES OF COMBUSTION.
- STRUCTURE OF FLAME.
- WHAT IS FUEL?
- FUEL EFFICIENCY.
WHAT IS COMBUSTION: The process in which a substance reacts with oxygen to give heat and light is called combustion. The substance which undergoes combustion is termed a combustible substance or fuel.
HOW DO WE CONTROL FIRE: Fire needs three things to be present air, a combustible substance and ignition temperature (heat). Fire can be controlled by removing any one of them.
TYPES OF COMBUSTION: The various types of combustion are 1) Rapid Combustion 2) Spontaneous Combustion 3) Explosive Combustion.
FLAME: A flame is a region where the combustion of gaseous substances takes place. The substances which vaporise give flames like kerosene oil, molten wax, petrol, wood etc
STRUCTURE OF FLAME: The structure of flame has 3 parts: 1) Outer zone (Blue zone or hottest zone) 2) Middle zone (Yellow zone or luminous zone) 3) Innermost zone (Dark zone or Black zone)
WHAT IS FUEL AND FUEL EFFICIENCY: The substance which undergoes combustion to produce heat is called fuel. The amount of heat energy produced on the complete combustion of 1 kilogram of fuel is called its calorific value. The fuel with more calorific value is more efficient. Now let us move on to solutions.
Combustion and Flame Class 8 Questions
- List conditions under which combustion can take place.
Ans. There are three important conditions necessary for combustion. These are: –
- Presence of fuel or combustible material.
- Presence of air or oxygen.
- Heating of fuel to its ignition temperature.
- Fill in the blanks.
- Burning of wood and coal causes the pollution of air.
- A liquid fuel used in homes is kerosene.
- Fuel must be heated to its ignition temperature before it catches fire.
- The fire produced by oil cannot be controlled by water.
- Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. The use of petrol and diesel produces a lot of unburnt carbon particles in the air. It leads to air pollution. However, the combustion of CNG produces a very less amount of unburnt carbon particles in the air. In this manner using CNG as fuel in place of petrol and diesel reduces the pollution of air in our cities.
- Compare LPG and wood as fuel.
Ans. LPG is better fuel than wood owing to its high calorific value and less pollution. Here are some differences between LPG and wood used as fuel.
|LPG (Liquified Petroleum Gas)
|1. It is present in nature as a gaseous fuel.
|1. It is present in solid form in nature.
|2. It does not cause pollution.
|2. It causes pollution of air.
|3. It does not produce smoke on burning.
|3. It produces smoke on burning.
|4. It can be stored easily in cylinders.
|4. It requires more space and labour to store.
|5. It has a high calorific value. (55000kJ/kg)
|5. It has a low calorific value. (17000-22000kJ/kg)
- Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Ans. Water is a good conductor of electricity so it is not used to control a fire involving electric equipment because it may result in an electric shock to firefighters.
(b) LPG is better domestic fuel than wood.
Ans. LPG is better domestic fuel than wood because it has more calorific value, causes less pollution and is easy to store.
(c) Paper by itself catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not.
Ans. Paper has a very low ignition temperature so it catches fire easily but when a piece of paper is wrapped around an aluminium pipe it does not catch fire because heat is absorbed by the pipe and paper does not reach its ignition temperature.
- Make a well-labelled diagram of a candle flame.
- Name the unit in which the calorific value of a fuel is expressed.
Ans. The calorific value of a fuel is expressed in Kilo joule per kilogram (kJ/kg).
- Explain how CO2 is able to control fires.
Ans. Carbon dioxide is heavy and so forms a blanket around the fire thereby cutting the supply of oxygen to the combustible material which extinguishes the fire.
- It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. Green leaves have some amount of water present in them due to this they have ignition temperature as compared to dry leaves which do not have any water content in them. So dry leaves catch fire easily.
- Which zone of the flame does a goldsmith use for melting gold and silver and why?
Ans. The goldsmiths use the outermost non-luminous zone of the flame for melting gold and silver because it is the hottest zone of the flame and provide the temperature needed for melting gold and silver.
- In an experiment, 4.5 kg of fuel was completely burnt. The heat produced was measured to be 180000kJ. Calculate the calorific value of the fuel.
- Can the process of rusting be called combustion? Discuss.
Ans. No, the process of rusting can’t be called combustion because the process of combustion involves the production of heat and light which does not take place in rusting.
- 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. Ramesh’s water will be heated in a shorter period of time due to higher temperature in the outermost (non-luminous) zone of the flame. As outermost zone is the hottest zone of the flame.
That’s all about Combustion and Flame Class 8 Question Answers. Hope it has helped you. Do share your views about this post in the comment section below.