CBSE Class 8

NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame.

NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame

NCERT TEXTBOOK EXERCISES

Question 1.
List conditions under which combustion can take place.
Answer.
Conditions under which combustion can take place are as follows :

1. The presence of air (oxygen).
2. The ignition temperature should be reached.
3. A source of inflammable substance.

Question 2.
Fill in the blanks:

1. Burning of wood and coal causes………….of air.
2. A liquid fuel, used in homes is…………..
3. Fuel must be heated to its………… before it starts burning.
4. Fire produced by oil cannot be controlled by……………..

Answer.

1. pollution
2. Kerosene
3. ignition temperature
4. water.

Question 3.
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer.
CNG is a clean fuel. It produces harmful products in a very small amount. That is why pollution in our cities has reduced.

Question 4.
Compare LPG and wood as fuels.
Answer.

Question 5.
Give reasons:

1. Water is not used to control fires involving electrical equipment.
2. LPG is a better domestic fuel than wood.
3. Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer.

1. Water is not used to control fires produced by .electrical equipment because water is a conductor of electricity and may cause electrocution or electric shock.
2. LPG is a substance which is readily available. It is cheaper and bums easily in the air at a moderate rate. It produces a large amount of heat and does not leave behind any undesirable substance.
3. Paper catches fire easily, but when it is wrapped around an aluminium pipe and heat is supplied then in the presence of aluminium pipe, the ignition temperature of the paper does not reach, because heat given to it is passed to the aluminium pipe which lowers the temperature of the paper. That is why the paper does not catch fire.

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

Fig. 6.1. Different zones of a candle flame

Question 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 kg (kJ/kg).

Question 8.
Explain how CO₂ is able to control fires.
Answer.
CO₂, being heavier than oxygen, covers the fire like blanket and also brings down the temperature of fuel. Since the contact between the fuel and oxygen is cut off, the fire comes under control.

Question 9.
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer.
Green leaves contain a lot of water. So, when we try to burn green leaves, water present in the leaves cools the combustible materials (leaves), so that their temperature is brought below their ignition temperature. This prevents the burning of green leaves.

On the other hand, dry leaves do not contain any water. So when a burning process starts, their temperature is raised drastically above their ignition temperature and the leaves catch fire easily.

Question 10.
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer.
A goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame (temperature ≈ 800°C) and is non-luminous in nature.

Question 11.
In an experiment, 4.5 kg of 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 the fuel:
Calorific value = heat produced/amount of fuel = 1,80,000 kJ/4.5 kg = 40,000 kJ/kg

Question 12.
Can the process of rusting be called combustion? Discuss.
Answer.
Yes, the process of rusting can be called combustion, in fact, slow combustion because rusting also takes place in the air (O₂) in the presence of humidity.

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.
The water of Ramesh’s beaker will get heated in a shorter time because the outermost part of the flame is the hottest.

We hope the NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame, drop a comment below and we will get back to you at the earliest

NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum.

NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum

NCERT TEXTBOOK EXERCISES

Question 1.
What are the advantages of using CNG and LPG as fuels?
Answer.
The advantages of using CNG and LPG as fuels are as follows:

1. These are clean fuels.
2. These are low-cost fuels.
3. These are available easily.
4. These can be used directly for burning in homes and factories where it can be supplied through pipes.

Question 2.
Name the petroleum product used for surfacing roads.
Answer.
A petroleum product ‘Bitumen’ is used for surfacing of roads.

Question 3.
Describe how coal is formed from dead vegetation. What is this process called?
Answer.
About 300 million years ago, the earth was having dense forests in low-lying wetland areas. Due to earthquakes and volcanic eruptions, these forests got buried under the soil. As more soil deposited over them, they were compressed.

The temperature also raised as they sank deeper and deeper. Due to high temperature and lack of oxygen, dead plants inside the earth got slowly converted to coal. As coal contains mainly carbon, the slow process of conversion of dead vegetation into coal is called carbonization.

Question 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……………
Answer.
(a) coal, petroleum, natural gas
(b) refining
(c) CNG.

Question 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 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)
Answer.
(a) False
(b) False
(c) True
(d) True
(e) False.

Question 6.
Explain why fossil fuels are exhaustible natural resources.
Answer.
Fossil fuels are present in nature in limited quantity and are being exhausted abundantly by us. That is why fossil fuels are called exhaustible natural resources.

Question 7.
Describe characteristics and uses of coke.
Answer.
Coke is a tough, porous, and black substance. It is an almost a pure form of carbon. “Coke is obtained by heating soft coal in the absence or little supply of air”.
It is used in the manufacture of steel and in the extraction of many metals.

Question 8.
Explain the process of the formation of petroleum.
Answer.
Petroleum is formed from organisms living in the sea. When these organisms died, their bodies settled at the bottom of the sea and got covered with layers of sand and clay. Over millions of years, they were transformed into petroleum and natural gas under the absence of air and the presence of high temperature and pressure.

Question 9.
The following table shows the total power shortage in India from 1991-1997.
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.
Answer.

We hope the NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 5 Coal and Petroleum, drop a comment below and we will get back to you at the earliest

NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals.

NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals

NCERT TEXTBOOK EXERCISES

Question 1.
Which of the following can be beaten into thin sheets?
(a) Zinc
(b) Phosphorus
(c) Sulphur
(d) Oxygen
Answer.
(a) Zinc.

Question 2.
Which of the following statements is correct?
(a) All metals are ductile.
(b) All non-metals are ductile.
(c) Generally, metals are ductile.
(d) Some non-metals are ductile.
Answer.
(c) Generally, metals are ductile.

Question 3.
Fill in the blanks.

1. Phosphorus is a very ……… non-metal.
2. Metals are ……….. conductors of heat and
3. Iron is …….. reactive than copper.
4. Metals react with acids to produce ………………. gas.

Answer.

1. reactive
2. good, electricity
3. more
4. hydrogen.

Question 4.
Mark T’ if the statement is true and ‘F’ if it is false.
(a ) Generally, non-metals react with acids. [   ]
(b) Sodium is a very reactive metal. [    ]
(c) Copper displaces zinc from zinc sulphate solution. [   ]
(d) Coal can be drawn into wires. [    ]
Answer.
(a) F
(b) T
(c) F
(d) F.

Question 5.
Some properties are listed in the following Table. Distinguish between metals and non-metals on the basis of these properties.

Answer.

Question 6.
Give reasons for the following.

1. Aluminium foils are used to wrap food items.
2. Immersion rods for heating liquids are made up of metallic substances.
3. Copper cannot displace zinc from its salt solution,
4. Sodium and potassium are stored in kerosene.

Answer.

1. It maintains the temperature of food items intact as it is a good conductor of heat.
2. Metals are good conductors of electricity. That is why immersion rods are made up of metals.
3. It is because copper is less reactive than zinc.
4. It is because sodium and potassium react vigorously when coming in contact with air or water.

Question 7.
Can you store the lemon pickle in an aluminium utensil? Explain.
Answer.
Aluminium is a metal. Metals are more reactive to acids. That is why acidic foodstuffs like lemon pickles must not be stored in aluminium utensils.

Question 8.
Match the substances given in Column A with their uses given in Column B.

Answer.

Question 9.
What happens when
(1) Dilute sulphuric acid is poured on a copper plate?
(2) Iron nails are placed in copper sulphate solution?
Write word equations of the reactions involved.
Answer.

1. Copper sulphate is formed and hydrogen gas is released.
   Copper + dil. sulphuric acid → Copper sulphate + Hydrogen (gas)
2. Brown coating is deposited on the iron nails. This is because of the displacement of copper from copper sulphate solution by iron.
   Iron + Copper sulphate (solution) → Iron sulphate (solution) + Copper

Question 10.
Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.
(1) How will she find the nature of the gas?
(2) Write down word equations of all the reactions taking place in this process.
Answer.

1. She will bring a wet litmus paper in contact with the gas. If the gas turns wet blue litmus paper into red, the gas will be acidic.
   1. Carbon + Oxygen → Carbon dioxide.
   2. Carbon dioxide + Water → Carbonic acid.

Question 11.
One day Reeta went to a jeweller’s shop with her mother. Her mother gave old gold jewellery to the goldsmith to polish. The next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?
Answer.
In polishing, the jeweller put the gold jewellery in a mild acidic solution, which in reaction with acid goes into the solution. So, in this process, a certain amount of gold is lost to the acidic solution.

We hope the NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 4 Materials: Metals and Non-Metals, drop a comment below and we will get back to you at the earliest

NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics.

NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics

NCERT TEXTBOOK EXERCISES

Question 1.
Explain why some fibres are called synthetic.
Answer.
Since man-made fibres are synthesised from petrochemicals, they are called synthetic fibres.

Question 2.
Mark the correct answer.
Rayon is different from synthetic fibres because
(a) it has a silk-like appearance
(b) it is obtained from wood pulp
(c) its fibres can also be woven like those of natural fibres
Answer.
(b) It is obtained from wood pulp.

Question 3.
Fill in the blanks with appropriate words:

1. Synthetic fibres are also called…………. or……………fibres.
2. Synthetic fibres are synthesised from raw material called………..
3. Like synthetic fibres, plastic is also a …………..

Answer.

1. artificial, man-made.
2. petrochemicals.
3. polymer.

Question 4.
Give examples which indicate that nylon fibres are very strong.
Answer.
They are used to make parachutes, and ropes for rock climbing.

Question 5.
Explain why plastic containers are favoured for storing food.
Answer.
Three main advantages of using plastic containers for storing food are:

1. They do not react with food items.
2. They do not get rusted.
3. They are light, strong and durable.

Question 6.
Explain the difference between thermoplastic and thermosetting plastics.
Answer.
Thermoplastics can be softened on heating and can be bent easily whereas thermosetting plastics cannot be softened on heating and breaks when forced to bend.

Question 7.
Explain why the following are made of thermosetting plastics.
(1) Saucepan handles
(2) Electric plugs Iswitches /plugboards.

Answer.
The above articles are made up of bakelite (a thermosetting plastic) because it is—

1. the bad conductor of heat.
2. poor conductor of electricity.

Question 8.
Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’:
Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.
Answer.

Question 9.
Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.
Answer.
I would advise Rana to buy cotton shirts as cotton shirts absorb the sweat and thus endorse cooling. Besides, they also provide aeration.

Question 10.
Give examples to show that plastics are noncorrosive in nature.
Answer.

1. It does not react with the chemical or other items stored in the containers made of it.
2. It does not get rusted when exposed to moisture and air.
3. It does not decompose when left in open for a long period.

Question 11.
Should the handle and bristles of a toothbrush be made of the same material? Explain your answer.
Answer.
The handles and bristles of a toothbrush should be made of a material which has lightweight good strength and is hygiene. “But the bristles should be soft enough so that ” they do not harm gums while the handle should be quite stiff so that it may not get bent while brushing.

Question 12.
‘Avoid plastics as far as possible.’ Comment on this advice.
Answer.
Since plastic takes several years to decompose, it is not environment friendly. It causes environmental pollution. Besides, when the synthetic material is burnt it takes a long time to get completely burnt. In the process, it releases a lot of poisonous fumes into the atmosphere causing air pollution.

Question 13.
Match the terms of Column A correctly with the phrases given in Column B.

Answer.

Question 14.
‘Manufacturing synthetic fibres are actually helping conservation of forests. ’ Comment.
Answer.
The synthetic fibres are made up of petrochemicals. So, manufacturing synthetic fibres does not depend upon plants. These synthetic fibres cater to the need of people up to great extent. Thus, the forests are not destroyed to manufacture clothes and other items. So, indirectly, we can come to the conclusion that manufacturing synthetic fibres is actually helping the conservation of forests.

Question 15.
Describe an activity to show that thermoplastic is a poor conductor of electricity.
Answer.
A thermoplastic (or plastic) is a poor conductor of electricity as shown in Fig. 3.3.
If we place a piece of copper in the gap touching points A and B (in Fig. 3.3), we observe that bulb glow. If we place plastic in the gap between A and B, we observe that the bulb will not glow. It means plastic is a poor conductor.

We hope the NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fibres and Plastics, drop a comment below and we will get back to you at the earliest

NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe.

NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe

NCERT TEXTBOOK EXERCISES

Question 1.
Fill in the blanks.

1. Microorganisms can be seen with the help of a………………
2. Blue-green algae fix………….. directly from the air and enhance the fertility of the soil.
3. Alcohol is produced with the help of……………….
4. Cholera is caused by………………….

Answer.

1. microscope
2. nitrogen
3. yeast
4. bacteria.

Question 2.
Tick the correct answer.
(a) Yeast is used in the production of
(i) sugar
(ii) alcohol
(iii) hydrochloric acid
(iv) oxygen
Answer.
(ii) alcohol

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast
Answer.
(ii) Streptomycin

(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito
(ii) cockroach
(iii) housefly
(iv) butterfly
Answer.
(i) female Anopheles mosquito

(d) The most common carrier of communicable diseases is
(i) ant
(ii) housefly
(iii) dragonfly
(iv) spider
Answer.
(ii) housefly

(e) The bread or idli dough rises because of
(i) heat
(ii) grinding
(iii) growth of yeast cells
(iv) kneading
Answer.
(iii) growth of yeast cells

(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation
(ii) moulding
(iii) fermentation
(iv) infection
Answer.
(iii) fermentation.

Question 3.
Match the organisms in Column A with their action in Column B.

Answer.

Question 4.
Can microorganisms be seen with the naked eye? If not, how can they be seen?
Answer.
No, we cannot see microorganisms with unaided eyes. They can be seen with the help of a microscope.

Question 5.
What are the major groups of microorganisms?
Answer.
Microorganisms are classified into four major groups based on their size. These are:

1. Bacteria
2. Fungi
3. Protozoa
4. Some algae.

Viruses are another type of microorganisms.

Question 6.
Name the microorganisms which can fix atmospheric nitrogen in the soil.
Answer.
They are

• Rhizobium
• Azobhcter
• blue-green algae (such as Anabaena and Nostoc), etc.

Question 7.
Write 10 lines on the usefulness of microorganisms in our lives.
Answer.
Microorganisms are useful for us in many ways e.g.,

1. the bacterium, lactobacillus converts milk into curd.
2. bacteria are also involved in the making of cheese.
3. Acetobacter acetic is used for the production of acetic acid from alcohol.
4. Yeast is used for the commercial production of alcohol and wine.
5. Antibiotics are manufactured by growing specific microorganisms.
6. Some bacteria fix atmospheric nitrogen and increase soil fertility.
7. Bacteria are used in the preparation of medicines like antibiotics and vaccines.
8. Bacteria are used in the preservation of pickles and many other food items.
9. Yeast is also used in the baking industry for making bread, pastries, and cakes.
10. They act as cleansing agents and decompose the waste products into manure.

Question 8.
Write a short paragraph on the harmful effects of microorganisms.
Answer.
The harms caused by microorganisms are as follows:

1. Many communicable diseases, such as cholera, common cold, chickenpox, tuberculosis, etc., are caused by microorganisms.
2. Malaria is caused by a microorganism called Plasmodium carried by the female Anopheles mosquito.
3. The Female Aedes mosquito acts as a carrier of the dengue virus.
4. Anthrax is a dangerous human and cattle disease caused by a bacterium called Bacillus anthracis.
5. Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple, etc., and reduce the yield of the crops.
6. Food poisoning is also caused by microorganisms. They make food poisonous by producing toxic substances in the food.

Question 9.
What are antibiotics? What precautions must be taken while taking antibiotics?
Answer.
The medicines, that kill or stop the growth of the disease-causing microorganisms, are called antibiotics. Streptomycin, tetracycline, erythromycin, etc. are some of the commonly known antibiotics which are made from fungi and bacteria.
The precautions to be taken while taking antibiotics are as follows :

1. These medicines should be taken only on the advice of a qualified doctor.
2. One must complete the course prescribed by the doctor.
3. If anybody takes antibiotics when not needed, his/her body may develop resistance against that antibiotic.

We hope the NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe, drop a comment below and we will get back to you at the earliest