NCERT VIII- COMBUSTION AND FLAME

CBSE Class VIII (8th) Science

 Chapter 6. COMBUSTION AND FLAME

 Lesson Exercises

 

Question 1. List conditions under which combustion can take place.

Answer :  Conditions under which combustion take places :

1. Some Fuel is  is required for combustion. During combustion a fuel substance reacts with oxygen to give off heat.The fuel may be solid, liquid or gas.
2. Oxygen in air  is essential for combustion. Combustion can not take place in the absence of air (oxygen).
3. For a combustion,  an inflamabe substance must be heated to its ignition tempreture. Ignition temperature is the lowest temperature at which a combustible substance catches fire.

Question 2. Fill in the blanks:

(a) Burning of wood and coal causes  Pollution of air.

(b) A liquid fuel, used in homes is  kerosene .

(c) Fuel must be heated to its Ignition temperature before it starts burning.

(d) Fire produced by oil cannot be controlled by water .

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

Answer : Our cities generally have high numbers of automobiles vehicals. The use of diesel and petrol as fuels in automobiles is a major cause of air pollution to day. During combustion, these fuel release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas.Combustion of most fuels releases carbon dioxide in the environment. Increased emission of carbon dioxide in the air is one of the major cause for global warming.

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel. It has high fuel efficiency. Hence use of CNG in automobiles has reduced pollution in our cities to a noticable extent .

Question 4. Compare LPG and wood as fuels.

Answer :

Comparasion LPG and Wood as Fuel
LPG	Wood as Fuel
LPG is a costly fuel but readily available, combustible and easy to transport in cylinders and tanker. LPG is more energy efficient Its calorific value is 55000 kJ/kg It causes less air pollution and prevent deforestation by supplementing the fuel need in place of wood as fuel It is an exhaustible natural resource.	Wood is a cheep fuel, not readily available in cities and not readily combustible. Wood as Fuel in not much energy efficient Its calorific value is 17000-22000 kJ/kg It causes air pollution and deforestation. It is not an exhaustible natural resource as trees can be grown in 5-10 yrs.

Question 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 not used to control fires involving electrical equipment : As water may conduct electricity and harm those trying to douse the fire.

(b) LPG is a better domestic fuel than wood : As LPG is a affordable, readily available, combustible  and easy to transport in cyllinders and tanker.

LPG is more energy efficient. Its calorific value is  55000 kJ/kg where as that of wood is just 17000-22000 kJ/kg. It causes less air  pollution and prevent deforestation by supplimenting the fuel need in place of wood as fuel

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not : Because for burning or combustion, a substance must me heated to its ignition temperature. Paper   wrapped around an aluminium pipe does not catch fire as on heating, it is unable to attain the ignition temperature due to transfer of heat to aluminium pipe which is good conductor of heat.

Question 6. Make a labelled diagram of a candle flame.

Answer :

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

Answer : The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question 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.

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

Explain.

Answer : A heap of green leaves contains a lot of water and has very high ignition tempreture. Water is a natural fire extinguisher and do not let leaves catch fire easily where as dry leaves contains no water and have low ignition temprature and catch fire easily.

Question 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.

Question 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 : Calorific value of the fuel   = Amount of heat energy produced / Weight of Fuel burnt

                                                                              = 180,000 /4.5 kJ/kg 

                                                                              = 40,000 kJ/J

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

Answer :Yes,  to some extent, the process of rusting can   called slow combustion .The Rusting of iron is a oxidation process. In this iron using oxygen and water, gets oxidized and is rusted out.It is a slow process which does produce heat at a very slow rate. The combustion process is also a oxidation process and  a chemical reaction by which fuel and an oxidizer react and produce  heat or light. Combustion needs  heat, an oxidizer, and fuel. Combustion is much faster than rusting.

Question 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 : A candle flame is less hot  near the wick as this is Innermost zone of unburnt wax vapours. where as outer most part of flame is the zone of complete combustion and is the hottest. There for, in case of Ramesh,  water will get heated in a shorter time than that of Abida

river basin draft bill, 2013

Centre unveils river basin draft bill, 2013

The Centre has released a Draft River Basin Management bill which aims to set up 12 river basin authorities in the country to settle inter-state water disputes, prevent flood and pollution.

The Draft River Basin Management Bill, which seeks to amend the River Boards Act, 1956, proposed to create a mechanism for integrated planning, development and management of water resources of a river basin. The current river boards do not have the provision.

The bill proposes a two-tier structure for each of the 12 river basin authorities-Brahamani-Baitarini basin, Cauvery basin, Ganga basin, Godavari basin, Indus basin, Krishna basin, Mahanadi basin, Mahi basin, Narmada basin, Pennar basin, Subarnreakha basin and Tapi basin.

Launch of National Free Drug Service and National Free Diagnostics Service

Launch of National Free Drug Service and National Free Diagnostics Service:

The initiatives, called 'Free Drug Service' and 'Free Diagnostics Service' under the National Health Mission, are aimed at sprucing up health care facilities in the country. Public can avail free drugs and free diagnostics services at the government health centres and hospitals if the states initiate the scheme. The states can access funds from the central government under the scheme by following certain guidelines laid down by it. The guidelines include:

a) It is mandatory for a state to have a clearly notified policy of providing free essential drugs and diagnostics to all patients coming to public sector facilities at least up to district hospital level. 
b) The state should have a robust procurement, logistics and supply chain system that is backed by Information technology. 
c) It must have differential facility-wise Essential Drug list/diagnostics list depending on the nature of the facility and a sound drug regulatory and quality assurance system.
d) The state should have adopted Standard Treatment Guidelines and provide for prescription audits to ensure rational use of drugs and it should name the initiative as 'National Health Mission-Free Drug Service' or 'National Health Mission-Free Diagnostic Service' or its translation in Hindi or regional language.

JULY - Current Affairs

APTITUDE MATERIALS - Percentage solutions

Percentage solutions:

Practice 1:

1. Number of runs made by running = 110 - (3 x 4 + 8 x 6)

= 110 - (60)

= 50.

Required percentage =		50	x 100	% = 45	5	%
		110			11

2. Let their marks be (x + 9) and x.

Then, x + 9 =	56	(x + 9 + x)
	100

 25(x + 9) = 14(2x + 9)

 3x = 99

 x = 33

So, their marks are 42 and 33.

3. Suppose originally he had x apples.

Then, (100 - 40)% of x = 420.

	60	x x = 420
	100

x =		420 x 100	= 700.
		60

4. Clearly, the numbers which have 1 or 9 in the unit's digit, have squares that end in the digit 1. Such numbers from 1 to 70 are 1, 9, 11, 19, 21, 29, 31, 39, 41, 49, 51, 59, 61, 69.

Number of such number =14

Required percentage =		14	x 100	% = 20%.
		70

5. Answer: Option E

x% of y =		x	x y		=		y	x x		= y% of x
		100					100

 A = B.

6. Answer: Option A

20% of a = b	20	a = b.
	100

b% of 20 =		b	x 20		=		20	a x	1	x 20		=	4	a = 4% of a.
		100					100		100				100

7. Answer: Option E

Let the number of students be x. Then,

Number of students above 8 years of age = (100 - 20)% of x = 80% of x.

80% of x = 48 +	2	of 48
	3

	80	x = 80
	100

 x = 100.

8. Answer: Option D

Let the number be x.

Then, error =	5	x -	3	x =	16	x.
	3		5		15

Error% =		16x	x	3	x 100	% = 64%.
		15		5x

9. Answer: Option A

Number of valid votes = 80% of 7500 = 6000.

 Valid votes polled by other candidate = 45% of 6000

=		45	x 6000		= 2700.
		100

10. Answer: Option A

Total number of votes polled = (1136 + 7636 + 11628) = 20400.

Required percentage =		11628	x 100	% = 57%.
		20400

11. Answer: Option C

Let the amount taxable purchases be Rs. x.

Then, 6% of x =	30
	100

x =		30	x	100	= 5.
		100		6

 Cost of tax free items = Rs. [25 - (5 + 0.30)] = Rs. 19.70

12. Answer: Option A

Rebate = 6% of Rs. 6650 = Rs.		6	x 6650		= Rs. 399.
		100

Sales tax = 10% of Rs. (6650 - 399) = Rs.		10	x 6251		= Rs. 625.10
		100

 Final amount = Rs. (6251 + 625.10) = Rs. 6876.10

13. Answer: Option B

Increase in 10 years = (262500 - 175000) = 87500.

Increase% =		87500	x 100	% = 50%.
		175000

Required average =		50	% = 5%.
		10

Practice 2

1. 1/5 X 2/5 X a = ¼ X a X b  =>  b = 8/25

2. % difference = (200-50)/50 X 100 = 300 %

3. % increase = (80-30)/30 X 100 = 166.66 %

4. 1.3 x = 78000  =>  x = 60000.

5. Net effect = 1.2 X 1.1 X 0.7

0.924  =>  7.6% decrease.

6. Let I be the income.

Expenditure = 0.8I                

Savings = 0.2I => 20%

New income = 1.2I  (since 20% rise)

New expenditure = (0.8I) X 1.3  (Since 30% rise)

= 1.04I

So, new savings = 1.2I – 1.04I = 0.16I

=> 16%

(So income decreased form 20% to 16%)

% decrease = (20-16)/20 X 100 = 20%.

7. It is equivalent to 1.25 decreased to 1.

% decrease = (1.25-1)/1.25 X 100 = 20%

8. % change in area = 1.2 X 1.2 (since area = side X side)

= 1.44 => 44%.

9. It is equivalent to 1.25 decreased to 1. So 20% decrease.

9.        Valid Votes:

A got 56%  =>  B got 44%

Difference = 12% = 48000

So, 100% = 400000. These are valid votes.

But valid votes are only 80% of total votes.

So, 80% of total votes  = 400000  =>  total votes = 500000

11. Total money = Rs. 300.

Saving of the lady = 10% of 300 = 30/-

With 30/- she bought 5 kg sugar => each kg costs Rs. 6/-

12. In 20lt, salt = 20% => 4 lt.

New volume = 18 lt (2 lt evaporated)

So, new % = 4/18 X 100 = 22.22%

13. % correction = (58-46.4)/58 X 100 = 20%

14. Three successive decreases of 20%, 20% and 10%

=> 0.8 X 0.8 X 0.9 = 0.576

Again 10% decrease =>

0.576 – 0.1 = 0.476.

So, 0.476 x = 9520 => x = 20000.

15. Total discount = 0.9 X 0.8 X 0.7 = 0.504 of actual price.

So, price = 0.504 X 10000 = 5040.

16. In 10 lt, alcohol is 10% = 1 lt.

Let x lt alcohol is added.

So, (1+x)/(10+x) = 20% = 1/5  =>  x = 1.25 lt.

17. A = 2B and B = 3C (ince 200% more)

A = 6C  =>  500 % more.

18. 50% of max marks – 40% of max marks = 25

max marks = 250

Pass marks = 40% of max + 10 => 100 + 10 = 110.

19. A = 1.2 B => B = A/1.2 => 0.8333A => 16.66%.

(OR) Decrease from 1.2 to 1 => 16.66%.

20. 1.1 X 1.1 X x = 12100 => x = 10000.

20. Salt = 15% of x = 0.15x (x = volume of solution)

Now, 0.15x/(x-30) = 20% = 1/5  (since 30 lt evaporated)

x = 120

22. 20% empty => 80 % full = 240 lt  => 20% = 60

22. A => 1.4 X 1.4 = 1.96

B => 1.9      => 6% difference.

24. From 1960 to 1975, in 15 years population doubled every 5 yrs => three times

So, 2 X 2 X 2 = 8 times => 700% more.

25. [(75% X 60) + (x% X 20)] / 80 = 80%  =>  x = 95.   (since required is 80%)

(OR)  60 out of 80 is 3/4. So,  (3/4 X 75)  +  (1/4 X x)  = 80  =>  x =95.