Lesson: Combustion and Flame

Exemplar:

# Question: 1

A substance which reacts with oxygen giving heat is called a combustible substance.

Which, one of the following is a combustible substance?

(a) Iron nail

(b) Glass

(c) Stone piece

(d) Wood

d

# Question: 2

Which one of the following has the highest calorific value?

(a) Kerosene

(b) Biogas

(c) LPG

(d) Petrol

c

# Question: 3

Magnesium ribbon on burning in air produces:

(a) Magnesium oxide, water and light

(b) Magnesium oxide and heat

(c) Magnesium oxide, heat and light

(d) Magnesium oxide, water and heat

c

# Question: 4

Which of the following is not a combustible substance?

(a) Camphor

(b) Glass

(c) Straw

(d) Alcohol

b

# Question: 5

The substance that does not burn with flame is:

(a) LPG

(b) Camphor

(c) Dry grass

(d) Charcoal

d

# Question: 6

On placing an inverted tumbler over a burning candle, the flame extinguishes after some time.

This is because of non-availability of:

(a) Oxygen

(b) Water vapours

(c) Carbon dioxide

(d) Wax

a

# Question: 7

If a person’s clothes catch fire, the best way to extinguish the fire is to:

(a) Throw water on the clothes.

(b) Use fire extinguisher.

(c) Cover the person with a woollen blanket.

(d) Cover the person with a polythene sheet.

c

# Question: 8

The substance expected to have the highest ignition temperature out of the following is:

(a) Kerosene

(b) Petrol

(c) Coal

(d) Alcohol

c

# Question: 9

Choose the correct statement about inflammable substances from the following.

They have:

(a) Low ignition temperature and cannot catch fire easily.

(b) High ignition temperature and can catch fire easily.

(c) Low ignition temperature and can catch fire easily.

(d) High ignition temperature and cannot catch fire easily.

c

# Question: 10

Choose the incorrect statement from the following.

Forest fires are usually due to:

(a) Carelessness of humans

(b) Heat of sun

(c) Cutting of trees

(d) Lightning strike

c

# Question: 11

The calorific value of a fuel is expressed in a unit called:

(a) Kilojoule per litre

(b) Kilogram per mililitre

(c) Kilojoule per gram

(d) Kilojoule per kilogram

d

# Question: 12

In villages, people use wood as fuel because:

(a) It is considered to be an ideal fuel.

(b) Of its easy availability and low cost.

(c) It is environment friendly.

(d) It catches fire easily.

b

# Question: 13

Which among the following is considered as the cleanest fuel?

(a) Cow dung cake

(b) Petrol

(c) Kerosene

(d) Hydrogen gas

d

# Question: 14

Choose the incorrect statement from the following.

A good fuel is one which:

(b) Produces a large amount of heat.

(c) Leaves behind many undesirable substances.

(d) Burns easily in air at a moderate rate.

c

# Question: 15

Shyam was cooking potato curry on a chulha.

To his surprise he observed that the copper vessel was getting blackened from outside.

It may be due to:

(a) Proper combustion of fuel.

(b) Improper cooking of potato curry.

(c) Improper combustion of the fuel.

(d) Burning of copper vessel.

c

# Question: 16

Fill in the blanks in the following sentences.

a)      A ____________ process in which a substance reacts with__________ to give off heat is called combustion.

b)      When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.

c)      The __________ temperature at which a substance catches fire is called its

__________ temperature.

d)      The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called__________ substances.

e)      The substances which vaporise during __________, give flame.

## Solution:

(a) Chemical, Oxygen;

(b) Fire, Blanket

(c) Lowest, Ignition;

(d) Low, Inflammable

(e) Burning.

# Question: 17

Some words (underlined) in the following sentences are jumbled up.

Write them in their correct form.

(a)   Seldie is a combustible substance.

(b)   Slags is a non-combustible material.

(c)   Chittsmack does not burn by itself.

(d)   Some substances on combustion produce thea and mafel.

(e)   The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.

(a) Diesel

(b) Glass

(c) Matchstick

(d) Heat, flame

(e) Calorific

# Question: 18

Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously.

In which jar will the candle remain lighted for a longer time and why?

## Solution:

The candle will remain lighted for a longer time in jar B, because oxygen gas is a supporter of combustion.

# Question: 19

Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

## Solution:

In order to boil water quickly in a test tube, Anu should keep her test tube in the outermost zone of the flame, which has the highest temperature, and hence is the hottest zone.

# Question: 20

Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities?

## Solution:

It is because CNG produces much smaller amount of harmful products and is a cleaner fuel, compared to diesel and petrol.

# Question: 21

Boojho wants to separate the following materials as combustible and non-combustible. Can you help him?

Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, candle, wood.

## Solution

(a) Combustible $—$ charcoal, straw, card board, paper, candlewood,

(b) Non-combustible $—$ chalk, stone, iron rod, copper coin, glass.

# Question: 22

Indicate whether the following statements are True or False.

Also write the false statements in their correct form.

(a) Air is necessary for combustion.

(b) Magnesium is a non-combustible metal.

(c) Carbon dioxide is an excellent fire extinguisher.

(d) Calorific value of wood is higher than that of coal.

## Solution:

Both (a) and (c) are true statements.

(b) Magnesium is a combustible metal.

(d)  Calorific value of coal is higher than that of wood.

# Question: 23

Match the items of Column A with the items of Column B.

Column A                                                       Column B

(a) Oxides of sulphur and nitrogen     (i) fire extinguisher

(b) CNG                                              (ii) incomplete combustion of coal

(c) Oxygen                                          (iii) very low ignition temperature

(d) inflammable substance                  (iv) acid rain

(e) carbon dioxide                               (v) necessary for combustion

(f) carbon monoxide                           (vi) fuel for automobiles.

(a) (iv)

(b) (vi)

(c) (v)

(d) (iii)

(e) (i)

(f) (ii)

# Question: 24

Match the following for the flame of a candle:

 Column A Column B (zone) Column C (colour) (a)Hottest part (i) Innermost zone of unburnt wax vapours (x) Blue (b)Moderately hot (ii) Middle zone of partial combustion (y) Black (c) Least hot (iii) Outer zone of complete combustion (z) Yellow

## Solution:

(a)$–$(iii)$–$(x)

(b)$–$(ii)$–$(z)

(c)$–$(i)$–$(y)

# Question: 25

If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe?

Will it produce a flame?

## Solution:

Iron wire will not produce a flame, but it will become red hot and will glow.

# Question: 26

Fill in the blanks using the words given in the box:

ignition, petrol, combustion, calorific value, combustible, inflammable

(a)   A chemical process in which a substance reacts with oxygen to give off heat is called __________.

(b)   Wood, paper, CNG are __________ substances.

(c)   The lowest temperature at which a substance catches fire is called its

__________ temperature.

(d)   Ignition temperature of __________ is lower than that of wood.

(e)   The substances which have very low __________ temperature and can easily catch fire with a flame are called __________substances.

(f)    The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________.

## Solution:

(a) Combustion,

(b) Combustible,

(c) Ignition,

(d) Petrol,

(e) Ignition, inflammable,

(f) Calorific value.

# Question: 27

People usually keep Angethi/burning coal in their closed rooms during winter season.

Why is it advised to keep the door open?

## Solution:

Ina closed room, due to the burning of angethi or coal, there will be insufficiency of oxygen, due to the consumption of the gas in the process of burning. Carbon monoxide gas is produced as a result of the burning process in the closed room and the person sleeping in that room can die by inhaling the poisonous carbon monoxide gas. So, the door should be left open to let the fresh air in.

# Question: 28

Write True/False against the following statements and also correct the false statement.

(a) A physical process in which a substance reacts with oxygen to give off heat is called combustion.

(b) Water is the best extinguisher for fires involving electrical equipment.

(c) Alcohol, CNG and LPG are inflammable substances.

(d) Increased concentration of nitrogen in air is believed to cause global warming.

(e) Greater the calorific value, better is the fuel.

(f) Middle zone is the hottest zone of a flame.

(g) The substances which vapourises during burning, give flame.

## Solution:

(a)   False. A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

(b)   False. Water cannot be used for controlling fires from electrical appliances, as water is a good conductor of electricity.

(c)   True

(d)   False. Increased concentration of carbon dioxide in air is believed to cause global warming.

(e)   True

(f)    False. The outermost zone is the hottest zone of a flame.

(g)   True

# Question: 29

Cracker on ignition produces sound. Why?

## Solution:

The burning of a cracker is a chemical reaction. When a cracker is ignited, it produces a sound because of the sudden release of the gases formed due to the chemical reaction.

# Question: 30

What do you understand by fuel efficiency?

## Solution

Fuels are burnt to convert its chemical energy into other forms of energy like heat energy, light energy, chemical energy, etc. A fuel is considered to be efficient if it has high calorific value. It means it should produce high amount of energy when a specified amount of it is completely burnt.

# Question: 31

You are provided with three watch glasses containing milk, petrol and mustard oil, respectively.

Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?

## Solution:

Amongst the provided material(s), petrol will catch fire instantly, as it is highly inflammable.

# Question: 32

Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable, If yes, why? If not, why not? In such a condition what should Manu have done?

## Solution:

Manu’s action was not suitable as water is not appropriate to extinguish the fire when cooking oil goes up in flame.

He should have switched off the gas flame and put a lid on the frying pan. The lid would have cut off the contact between the fuel (i.e., the cooking oil) and oxygen, and the fire would have extinguished soon after.

# Question: 33

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.

## Solution:

The three essential requirements to produce fire are fuel, air and heat.

Fire extinguisher either cuts off the supply of air, or brings down the temperature of the fuel, or does both. This helps in extinguishing the fire.

# Question: 34

Give two examples each for a solid, liquid and gaseous fuel along with some important uses.

## Solution:

Examples of solid fuel $–$ Coal, wood, etc.

·         Coal is used as a source of energy for heating or burning.

·         Wood is used in making furniture. Wood is burnt to produce heat.

Examples of liquid fuel $–$ Kerosene oil, petrol, etc.

·         Kerosene oil is used for lighting lamps and burning woods.

·         Petrol is used in automobiles as fuel.

Examples of gaseous fuel $–$ CNG, LPG, etc

·         CNG is a used as a fuel in automobiles.

·         LPG is used as a fuel in cooking.

# Question: 35

The calorific values of petrol and CNG are 45000 kJ/kg and50,000 kJ/kg, respectively.

If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

## Solution:

If I have a vehicle which can run on petrol as well as CNG, I will prefer CNG.

Reasons for preferring CNG:

The calorific value of CNG is comparatively higher than that of petrol. Therefore, CNG will be more economical. CNG causes lesser pollution.

# Question: 36

Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

## Solution:

Though wood has a very high calorific value, we will still discourage its use as a fuel.

Reasons:

(a)   Burning wood results in very high air pollution.

(b)   Wood, as a fuel, is obtained by cutting down trees. Extensive use of wood will lead to deforestation and environmental pollution.

# Question: 37

Forest fire produces a lot of air pollution.

Write in brief about the reasons of forest fires.

## Solution:

Forest fire causes air pollution and should be prevented. Some of the causes of forest fire are given below:

High temperature: When the temperature is very high, there are high possibilities of dry grasses and leaves catching fire. This fire spreads throughout the forest.

Human negligence: Human negligence, like discarded cigarettes, camp fire left unattended, etc., results in large number of forest fire.

Natural phenomenon: Natural phenomenon, like lighting, causes forest fire.

# Question: 38

Complete the crossword with the help of the clues:

Across

1. Non-metal which catches fire if exposed to air (10)

3. The lowest temperature at which a substance catches fire is called its __________ temperature. (8)

5. The most common fire extinguisher. (5)

Down

2. A chemical process in which a substance reacts with oxygen to give off heat. (10)

4. Petrol is used as a __________ in automobiles. (4)

6. It is as hard as stone and black in colour. (4)