Thursday, 3 March 2016

TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS VIII SCIENCE Chapter 6 COMBUSTION AND FAME




Exercise


1) List conditions under which combustion can take place.

ANSWER: - There are certain conditions required for combustion to take place. They are:
(i) Presence of a fuel
(ii) Air (or oxygen)
(iii) Ignition temperature (minimum temperature at which a substance catches fire)

2) Fill in the blanks:

a) Burning of wood and coal causes __pollution__ of air.
(b) A liquid fuel used in homes is __liquefied petroleum gas (LPG) _.
(c) Fuel must be heated to its __ignition temperature__ before it starts burning.
(d) Fire produced by oil cannot be controlled by __water__.

3) Explain how the use of CNG in automobiles has reduced pollution in our cities.

ANSWER:-Combustion of fuels like petroleum causes formation of un-burnt carbon particles along with carbon monoxide gas. These harmful pollutants enter the air and cause respiratory diseases.
Compressed Natural Gas (CNG) produces these harmful products in very less quantity. It is a comparatively cleaner fuel. Therefore, the use of CNG has reduced pollution in our cities.

4) Compare LPG and wood as fuels.

ANSWER:-

LPG:-1. LPG is a costly fuel but readily available, combustible and easy to transport in cylinders and tanker.
2. LPG is more energy efficient
3. Its calorific value is 55000 kJ/ kg
4. It causes less air pollution and prevents deforestation by supplementing the fuel need in place of wood as fuel
5. It is an exhaustible natural resource

Wood as fuel-:1. Wood is a cheap fuel, not readily available in cities and not readily combustible.
2. Wood as Fuel in not much energy efficient
3. Its calorific value is 17000-22000 kJ/ kg
4. It causes air pollution and deforestation.
5. It is not an exhaustible natural resource as trees can be grown in 5-10 yrs.

5) Give reasons:
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not

ANSWER :-(a) Water is a good conductor of electricity. If it is used for controlling a fire involving electrical equipments, then the person dousing the fire might get an electric shock. Also, water can damage electrical equipments.

(b) LPG is a better domestic fuel as it does not produce smoke and un-burnt carbon particles, which cause respiratory problems.

(c) A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good conductor of heat. Therefore, heat is transferred from the paper to the metal and the paper does not attain its ignition temperature.

6) Make a labelled diagram of a candle flame.



7) Name the unit in which the calorific value of a fuel is expressed.

ANSWER:-The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

8) Explain how CO2 is able to control fires.

ANSWER:-CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. The added advantage of CO2 is that in most cases it does not harm the electrical equipment. It not only forms a blanket around the fuel fire and cut off oxygen supply, it also brings down the temperature of the fuel. That is why it is an excellent fire extinguisher.

9) It is difficult to burn a heap of green leaves but dry leaves catch fire easily.

ANSWER:-Green leaves have a lot of moisture in them. This moisture does not allow them to catch fire easily. However, dry leaves have no moisture in them. Therefore, they catch fire easily.

10) Which zone of a flame does a goldsmith use for melting gold and silver and why?

ANSWER:-A goldsmith use outermost zone of the flame with a metallic blow-pipe for melting gold and silver as it is the hottest part of flame.

11) 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.

ANSWER:-The calorific value of fuel is the amount of heat produced by the complete combustion of 1 kg of fuel.
Now, Heat produced by 4.5 kg of fuel = 180000 kJ
Calorific value of the fuel = Amount of heat energy produced / Weight of Fuel burnt
= 180,000 /4.5 kJ/kg
= 40,000 kJ/J

12) Can the process of rusting be called combustion? Discuss.


ANSWER:-Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting of iron is an exothermic process as heat is released during rusting. Hence, it is a kind of slow combustion.




13)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?

ANSWER: The water in the Ramesh’s beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.


Written By Badal Paul
The author is a passionate blogger, writer and educationalist. This blog is created to ease troubles of the Cbse students. The author holds multiple blogs and the writer of the Novel KALCHAKRA, OOM AND THE CHOSEN FIVE.