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Page No 79:

Question 1:

Fuel Calorific Value (kj/g)
charcoal 33
coal 25.33
wood 17
diesel 45
kerosene 48
petrol 50
biogas 35 to 40
butane (LPG) 50
methane 55
hydrogen 150

Based on the table above, which of the following is the correct sequence in which the fuels that give out most heat to the least heat energy will be arranged?
(a) hydrogen, wood, coal, biogas, methane
(b) methane, kerosene, diesel, charcoal
(c) coal, kerosene, petrol, hydrogen
(d) hydrogen, butane, diesel, kerosene

Answer:

(b) methane, kerosene, diesel, charcoal

Calorific value is nothing but the amount of heat energy evolved when 1 g of a fuel is burnt completely in the presence of oxygen. Based on the calorific value, methane gives the highest heat energy with 55 kJ/g, which is followed by kerosene with 48 kJ/g and diesel with 45 kJ /g. Charcoal gives the least amount of heat energy, i.e. 33 kJ/g. 



Page No 80:

Question 2:

Which of the following is not a criterion for a good fuel?
(a) The burning of the fuel should release heat in a controlled manner.
(b) The fuel should be easy to store and transport.
(c) The fuel should not pollute the air by emanating harmful gases.
(d) The fuel should have low calorific value.

Answer:

(d) The fuel should have low calorific value.

A good fuel should always have high calorific value. In other words, a good fuel should produce a great amount of heat.

Page No 80:

Question 3:

Which of the following has been placed in the wrong column?

Spontaneous
combustion
Rapid
combustion

(a) forest fires
(b) petrol in a fuel tank catching fire
(c) bomb explosions
(d) firecrackers

Answer:

(b) petrol in a fuel tank catching fire

It is an example of rapid combustion and not of spontaneous combustion. This is because petrol catches fire only when heat is supplied externally. It cannot catch fire on its own.

Page No 80:

Question 4:

Fuel can be classified as
(a) combustible and non combustible fuel.
(b) efficient and non efficient fuel.
(c) solid, liquid and gas.
(d) all the above.

Answer:

(c) solid, liquid and gas

Fuels are classified as solid (e.g. wood, charcoal, etc.), liquid (e.g. kerosene, petrol, etc.) and gaseous (e.g. LPG, biogas, etc.).

Combustibility is a character of every fuel. A non-combustible substance cannot be a fuel. Similarly, a substance has to be efficient from the point of view of energy output. A non-efficient substance cannot be called a fuel.

Page No 80:

Question 5:

When a hydrocarbon is burnt, the products obtained are _________ and ________.

(a) CO + H2
(b) CO2 + H2
(c) CO2 + H2O
(d) CO + H2O

Answer:

(c) CO2 and H2O

Hydrocarbons burn and react with the oxygen present in the air to form carbon dioxide and water along with the generation of a huge amount of energy.

Page No 80:

Question 6:

Why is biogas considered a good fuel?

(a) It burns without smoke.
(b) It is environment friendly as the residue can be used as manure.
(c) It uses animal and plant waste as raw material instead of wood.
(d) all the above

Answer:

(d) all the above

When plant and animal materials decay, they produce biogas. Methane is a major constituent of biogas. It is considered to be a good fuel.

Page No 80:

Question 7:

What is common to the following?

petrol      diesel      kerosene       fuel oil

(a) All are by-products of petroleum.
(b) All are obtained by the fractional distillation of petroleum.
(c) All of them are hydrocarbons.
(d) all the above

Answer:

(d) all the above

Petrol, diesel, kerosene and fuel oil are all hydrocarbons and by-products of petroleum, which are obtained by fractional distillation of petroleum.

Page No 80:

Question 8:

The table below gives the difference between respiration and combustion. One of the entries in the table is wrong. Which one is it?

  Respiration Combustion
a. Respiration is a slow process. Combustion is relatively a fast process.
b. Respiration takes place at the body temperature. Combustion takes place at the ignition temperature of the fuel.
c. Respiration produces CO2 + H2O and energy. Combustion of hydrocarbons always produces only CO2.
d. Respiration takes place inside a cell. Combustion takes place outside a cell.
e. Respiration takes place at all times. Combustion only takes place when a fuel or a combustible substance burns.

Answer:

The wrong entry is:

  Combustion
c.  Combustion of hydrocarbons always produces only CO2.

This is because combustion of hydrocarbon always produces carbon dioxide and water along with the generation of a huge amount of energy.

Page No 80:

Question 9:

Which of the following pollutants is produced by the burning of fuels like coal?
(a) sulphur dioxide
(b) carbon monoxide
(c) soot
(d) all the above

Answer:

(d) all of the above

When coal is burnt in inadequate supply of oxygen, carbon monoxide gas, which is highly toxic in nature, is produced. Sulphur impurities in coal lead to a release of sulphur dioxide during the burning of coal. Burning of coal also produces soot. When we heat a substance in a container over coal, it's bottom is blackened due to the release of soot from coal.



Page No 81:

Question 10:

If an electrical gadget catches fire in your home, which of these should you follow?
A. Pour water on the flames.
B. Switch off the electricity from the mains.
C. Smother the flames using a thick blanket
D. Pull the plug out of the socket.

(a) all the above
(b) only B, C and D
(c) A and B
(d) A and D

Answer:

(b) only B, C and D

If an electrical gadget catches fire, we should not throw water on the flames. This is because water is a good conductor of electricity and pouring water on the flames may result in electrical short circuits. 

Page No 81:

Question 11:

State whether the following statements are 'true' or 'false'.
(a) The matter contained in the body of plants and animals is called biomass.
(b) Biogas is produced by the decay of biomass.
(c) Fossil fuels should be conserved because they give out a lot of heat energy.
(d) Petroleum is formed under the sea from the remains of dead marine animals.
(e) Charcoal is a better fuel than kerosene for use in cooking at home.
(f) Combustion and oxidation are similar reactions.
(g) CNG is a more polluting fuel than petrol.

Answer:

(a) True
(b) True
(c) False. Fossil fuels should be conserved as they are non-renewable sources of energy. It takes millions of years for them to form.
(d) True
(e) False. Kerosene is a better fuel than charcoal for use in cooking. It has a lower ignition temperature than charcoal  and catches fire without any difficulty.
(f) True
(g) False. CNG causes less pollution than petrol. 

Page No 81:

Question 12:

Arrange the names of these liquids in order of their boiling points starting from the highest to the lowest:
naphthalene, fuel oil, kerosene, petrol

Answer:

The boiling points of the given liquids are as follows:
 

  Liquid Fuel Boiling Point
1.  Naptha 70°C
2.  Fuel oil 600°C
3.  Kerosene 170°C
4.  Petrol 120°C

Now, we can arrange the given liquids in order of their boiling points starting from the highest to the lowest as shown below:
  
Liquid Fuels:      Fuel oil > Kerosene > Petrol > Naptha
Boiling Points:    600°C > 170°C > 120°C > 70°C

Page No 81:

Question 13:

What are fossil fuels? Give a few examples. Is wood a fossil fuel? How can animal waste be used as a source of energy?

Answer:

Fuels that are generated very deep under the earth from the debris of plants and animals, which existed millions of years ago, are called fossil fuels. Coal, oil and natural gases are fossil fuels. These resources are non-renewable as they cannot be regenerated easily.

Wood is not a fossil fuel. It is a common fuel that is obtained from trees that have grown in the recent times or that are still living. Trees obtain energy from the Sun and store it in the wood. When wood is burnt, this energy is released in the form of heat. Wood is a renewable resource.

Animal waste is a kind of biomass. Animal dung is shaped into cakes and dried under the Sun. These dried cakes are used directly as fuel. When these dried cakes are burnt, they produce heat. It is a common source of energy in villages.

Page No 81:

Question 14:

When a vessel containing cold water is kept above a gas burner, water droplets quickly form on the outside. After a short time, these disappear and the outside of the vessel gets dry again. Explain these changes.

Answer:

Whenever cold water is poured into a vessel, the temperature of the air around it is reduced and, consequently, the air is condensed (since air has moisture in it). As a result, water droplets form on the outside of the vessel. As the vessel gets heated up, these water droplets also get heated and they evaporate, leaving the outside of the vessel dry again.

Page No 81:

Question 15:

Why are piped natural gas and LPG considered good fuels for domestic use?

Answer:

Natural gas (methane) and LPG (butane) are lighter than air and are easily transported through large distances. Natural gas can be passed through the pipelines to the storage places, while LPG can be compressed into liquid and stored in cylinders. When burnt, these gaseous fuels produce large amount of energy in the form of heat and light. They leave behind very small amount of waste as compared to solid fuels that leave behind large amount of ash after burning.

These gaseous fuels, which are colourless and odourless, are mixed with chemicals to produce smell so that any leakage can be easily detected. Thus, ​natural gas (methane) and LPG (butane) are considered good fuels for domestic use.

Page No 81:

Question 16:

The use of fossil fuels is harmful to us. Explain this statement and also mention some of the alternate fuels being used by man.

Answer:

The use of fossil fuels such as petroleum oil and coal causes environmental pollution from which many serious health problems arise. Thus, the use of fossil fuels is harmful to us.

In order to overcome this, alternate fuels can be used by man. These alternate fuels are generated by renewable resources like the Sun, wind, water and soil. They are inexhaustible, easily found and can be used again and again. They do not cause environmental pollution. Some of the renewable alternate fuels are solar energy, wind energy and water energy. Solar energy is used to produce electricity through solar panels. It is also used for cooking purposes. Wind energy is used to generate electricity with the help of wind mills. Kinetic energy of flowing water is also used to generate electricity.

Page No 81:

Question 17:

Name the compounds formed during the combustion of a hydrocarbon. Write an equation as an example.

Answer:

The compounds formed during the combustion of a hydrocarbon are carbon dioxide and water. Hydrocarbons burn and react with the oxygen present in the air to form carbon dioxide and water along with the generation of a huge amount of energy. Thus, these hydrocarbons are used as fuels. Let us consider an example of the hydrocarbon methane and write the chemical equation involved in its combustion reaction.

CH4 + 2O2  CO2 + H2O + Heat

Page No 81:

Question 18:

(a) Draw a fully-labelled diagram showing the different parts of a candle flame.
(b) Explain why the yellow part of the flame is luminous.
(c) Why does complete combustion takes place in the non-luminous zone?

Answer:

(a) A fully-labelled diagram showing different parts of a candle flame is given below.


(b) In the yellow part of the flame, the vapours of the wax (hydrocarbons) are unable to get sufficient quantity of oxygen from the air. Thus, they do not burn completely. A few carbon particles are left unburnt in this zone. These unburnt carbon particles become hot and show radiance of yellow light. Thus, the yellow part of the flame is luminous and is called the luminous zone.
 
(c) The exterior part of the flame, which covers the whole candle flame, is called the non-luminous zone. This zone gets sufficient quantity of oxygen from the air surrounding it. Therefore, complete combustion takes place in this zone. The non-luminous zone is considered as the hottest part of the candle flame.

Page No 81:

Question 19:

Write balanced chemical equations for the following:
(a) burning of coal (carbon) in a sufficient supply of air
(b) When coal burns in insufficient amount of oxygen, carbon monoxide is formed.
(c) Name the type of reaction in each of the above cases.
(d) How is carbon monoxide harmful?

Answer:

(a) Coal (carbon) burns with adequate amount of oxygen in the air to form carbon dioxide. The balanced chemical equation for this reaction is given below:
             C      +     O2       Burns        CO2Coal (Carbon)      Oxygen            Carbon dioxide                           (sufficient)

(b) When coal (carbon) burns with inadequate amount of oxygen in the air, it produces carbon monoxide gas, which is toxic in nature. The balanced chemical equation for this reaction is given below​:
        2C      +     O2       Burns        2COCoal (Carbon)   Oxygen            Carbon monoxide                         (insufficient)

(c) The type of reaction in each of the above cases is oxidation reaction. In both the cases, coal (carbon) reacts with oxygen and gets oxidised to form carbon dioxide in one case (a) and carbon monoxide in the other (b).

(d) Carbon monoxide is a toxic gas. It decreases the capability of blood to carry oxygen, which is necessary for the living beings to survive.  Hence, ​carbon monoxide is a pollutant to the atmosphere.



Page No 82:

Question 20:

Fill in the blanks with appropriate words selected from those given below.

heights, condense, lower, bitumen, hydrocarbons,
high, fractional distillation, cool, fractionating column,
diesel oil, petrol, refinery, higher up, lower down

(a) Crude oil is made up of many different ___________. In an oil ___________, the hydrocarbons are separated into different products by a process called _____________ ___________. The crude oil is heated to a high temperature and passed into the____________ __________. As the vapours rise towards the top the different hydrocarbons ___________ at different ___________ and are withdrawn separately.
(b) Substances with _____________ boiling points condense first, in the column. Substances with ___________ boiling points condense ______________. Light products like ___________ are withdrawn from near the top of the column. Heavier products like ___________ and fuel oil are taken out lower down. The heaviest hydrocarbons, such as __________ are left at the bottom.

Answer:

(a) Crude oil is made up of many different hydrocarbons. In an oil refinery, the hydrocarbons are separated into different products by a process called fractional distillation. The crude oil is heated to a high temperature and passed into the fractionating column. As the vapours rise towards the top, the different hydrocarbons condense at different heights and are withdrawn separately.
(b) Substances with lower boiling points condense first in the column. Substances with high boiling points condense lower down. Light products like petrol are withdrawn from near the top of the column. Heavier products like diesel oil and fuel oil are taken out lower down. The heaviest hydrocarbons, such as
bitumen, are left at the bottom.



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