Solutions of Back Exercise, Chapter 6 Combustion and Flame (CLASS 8)

 1. List conditions under which combustion can take place.

Soln:

The burning of a substance in the presence of oxygen is defined as combustion.

The conditions under which combustion can take place are

• The presence of air or oxygen.
• The presence of fuel plays a significant role.
• Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)

2. Fill in the blanks.

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

(b) A liquid fuel, used in homes is__________ .

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

(d) The fire produced by oil cannot be controlled by___________ .

Soln:

(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) The fire produced by oil cannot be controlled by Water.

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

Soln:

CNG played an important role in reducing pollution among automobiles for the following reasons

• CNG is comparatively a cleaner fuel.
• The CNG can be an alternative for diesel, petrol and propane/LPG.
• It usually contains a few undesirable gases than the other fuels mentioned above.
• The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide, which leads to respiratory diseases.

4. Compare LPG and wood as fuels

Soln:

Wood

• It is considered as a traditional fuel used for both domestic and industrial purposes.
• Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
• The usage of wood to a large extent causes deforestation.
• The calorific value of wood ranges between 17000 to 22000 kJ/kg
• However, wood may be used as a furnace, stove or fireplace indoors while it is used for a campfire, furnace outdoors.

      LPG

• The usage LPG (Liquefied petroleum gas) has replaced wood.
• It doesn’t release smoke and other pollutants.
• It is a cleaner fuel.
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg.
• Hence, LPG is the most preferred choice.

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.

Soln:

a)

• Water is a good conductor of electricity.
• If added to an electrical fire, the water would just spread the electricity further.
• The person dousing the fire might get an electric shock

b)

• LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
• Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
• Hence, LPG is a better domestic fuel than wood.

c)

• The paper by itself catches fire easily because of its low ignition temperature.
• The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
• While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So, there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.

6. Make a labelled diagram of a candle flame.

Soln:

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

Soln:

Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg

kJ=kilo joules and kg=kilogram

8. Explain how CO2 is able to control fires.

Soln:

CO2 is a non-combustible gas and extinguishes fire in two ways:

(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.

(ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.

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

Soln:

A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore, it does not catch fire easily.

But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore, it catches fire easily.

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

Soln:

The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the 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.

Soln:

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

Soln:

No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.

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?

Soln:

The water placed in the outermost part of the flame will be heated in a short time since it is a non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame) is comparatively less hot.

SOLUTIONS OF BACK EXERCISE, Chapter 6 Changes Around us

 1. To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?

Solution:

Yes, the process can be reversed.

2. You accidentally dropped your favorite toy and broke it. This is a change you did not want. Can this change be reversed?

Solution:

No, this change cannot be reversed

3. Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not.

Sl No	Change	Can be reversed Yes/No
1	The sawing of a piece of wood
2	The melting of ice candy
3	Dissolving sugar in water
4	The cooking of food
5	The ripening of a mango
6	Souring of milk

Solution :

Sl No	Change	Can be reversed Yes/No
1	The sawing of a piece of wood	No
2	The melting of ice candy	Yes
3	Dissolving sugar in water	Yes
4	The cooking of food	No
5	The ripening of a mango	No
6	Souring of milk	No

4. A drawing sheet changes when you draw a picture on it. Can you reverse this change?

Solution:

This change can be reversed if a pencil is used to draw the picture. If a pen, paint, oil /water colours are used to draw the picture, change cannot be reversed.

5. Give examples to explain the difference between changes that can or cannot be reversed.

Solution:

Changes that can be reversed

i) Opening and closing the door

ii) Filling glass with water

Changes that cannot be reversed

i) Conversion of milk into curd

ii) Ripening of fruit

6. A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilized. Can the change in POP be reversed?

Solution:

No, the change cannot be reversed

7. A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?

Solution: No, the change cannot be reversed

Solutions of Coal and Petroleum, Class 8 Chapter 5

 1. What are the advantages of using CNG and LPG as fuels?

Soln:

The advantages of using CNG and LPG as fuels are as follows

• CNG and LPG burn easily.
• CNG and LPG give a lot of heat energy when burnt.
• CNG and LPG can be transported easily through pipelines.
• CNG and LPG are clean fuels and they do not release smoke when burnt.

2. Name the petroleum product used for surfacing of roads.

Soln:

Bitumen is used for surfacing of roads.

3. Describe how coal is formed from dead vegetation. What is this process called?

Soln:

Dense forests got buried under the soil due to natural processes, millions of years ago. More and more soil got deposited over them and they got compressed more. This led them to get exposed to very high temperature and pressure. They slowly got converted into coal under these extreme conditions. The whole process of formation of coal from dead vegetation is known as carbonization.

 4. Fill in the blanks.

(a) Fossil fuels are  ____________,  ____________, and ____________ .

(b) Process of separation of different constituents from petroleum is called _____________ .

(c) Least polluting fuel for a vehicle is ______________ .

Soln:

(a) Fossil fuels Coal, Petroleum, and natural gas.

(b) Process of separation of different constituents from petroleum is called refining.

(c) Least polluting fuel for a vehicle is Compressed Natural Gas (CNG).

5. Tick True/False against the following statements.

(a) Fossil fuels can be made in the laboratory. (T/F)

(b) CNG is more polluting fuel than petrol. (T/F)

(c) Coke is the almost pure form of carbon. (T/F)

(d) Coal tar is a mixture of various substances. (T/F)

(e) Kerosene is not a fossil fuel. (T/F)

Soln:

a) False

b) False

c) True

d) True

e) False

6. Explain why fossil fuels are exhaustible natural resources.

Soln:

The process of formation of fossil fuels requires millions of years. Dead vegetation and animals that get buried deep inside the earth require high temperature and pressure for the formation of fossil fuels, which cannot be done in the laboratory. Thus, fossils are limited. Therefore, the use of fossil fuels at a rapid rate will lead to their exhaustion.

7. Describe the characteristics and uses of coke.

Soln:

Characteristics:

i) Coke is tough.

ii) Coke is porous.

iii) Coke is black in colour.

Uses:

i) In the manufacture of steel.

ii) In the extraction of metals (as a reducing agent).

8. Explain the process of the formation of petroleum.

Soln:

Dead organisms that got buried in the sea millions of years ago got covered with layers of sand and clay. Due to lack of air, high temperature and high pressure, these dead organisms got transformed into petroleum and natural gas.
9. The following table shows the total power shortage in India from 1993–1999. Show the data in the form of a graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.

S.No.	Year	Shortage (%)
1.	1993	7.7
2.	1994	7.5
3.	1995	8.2
4.	1996	7.1
5.	1997	7.7
6.	1998	9.1
7.	1999	11.2

Solutions of Separation of Substances, Chapter 5, Class 6

 1. Why do we need to separate different components of a mixture? Give two examples.

Solution:

When two or more substances are mixed together they form a mixture. Components of a mixture should be separated because some components may not be useful or may spoil the useful component of the mixture.

Examples:

Tea leaves are separated from the liquid with a strainer, while preparing tea

Removal of stone pieces from wheat, rice or pulses by hand

2. What is winnowing? Where is it used?

Solution:

The method of separating the components from a mixture is known as winnowing. In this method, heavier and lighter components of a mixture are separated by wind or by blowing air. This method is used by farmers to separate lighter husk particles from heavier seeds of  grain.

3. How will you separate husk or dirt particles from a given sample of pulses before cooking?

Solution:

Husk and dirt particles are separated from pulses by winnowing.

4. What is sieving? Where is it used?

Solution:

Sieving is a method in which fine particles are sieved through holes of the sieve while the bigger impurities remain on the sieve. Sieving is used in a flour mill to separate impurities like husk and stones from wheat before grinding it. It is also used at construction sites to separate pebbles and stones from sand.

5. How will you separate sand and water from their mixture?

Solution:

Sand and water are separated from their mixture by the following steps:

a) The mixture is allowed to stand without any disturbances

b) Now sand settles down

c) Slowly pour the water into another container to obtain sand in the bottom

6. Is it possible to separate sugar mixed with wheat flour? If yes, how will you do it?

Solution:

Yes it is possible to separate sugar mixed with wheat flour by the following method

a) Mix sugar and wheat flour in water

b) Stir the solution to allow sugar to dissolve

c) Now filter the mixture

d) Filtrate contains sugar solution and residue will be wheat flour.

7. How would you obtain clear water from a sample of muddy water?

Solution:

The following process should be carried out to obtain clear water from muddy water

i) Allow muddy water to stand

ii) Mud gets settled down in the water

ii) Slowly pour water to another container

8. Fill up the blanks

(a) The method of separating seeds of paddy from its stalks is called ___________.

(b) When milk, cooled after boiling, is poured onto a piece of cloth the cream (malai) is left behind on it. This process of separating cream from milk is an example of ___________.

(c) Salt is obtained from seawater by the process of ___________.

(d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this example is called ___________.

Solution:

(a) The method of separating seeds of paddy from its stalks is called threshing.

(b) When milk cooled after boiling is poured onto a piece of cloth, the cream (malai) is left behind on it. This process of separating cream from milk is an example of filtration.

(c) Salt is obtained from seawater by the process of evaporation.

(d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this example is called decantation.

9. True or false?

(a) A mixture of milk and water can be separated by filtration.

(b) A mixture of powdered salt and sugar can be separated by the process of winnowing

(c) Separation of sugar from tea can be done with filtration.

(d) Grain and husk can be separated with the process of decantation.

Solution:

a) False

b) False

c) False

d) False

10. Lemonade is prepared by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar? In which case would it be possible to dissolve more sugar?

Solution:

Ice should be added to lemonade after dissolving sugar. It is possible to add more sugar before adding ice.