Chemistry cbse class 8

 

why are some fibres called synthetic?

Some fibres are called synthetic fibers because these are manmade fibers prepared by using chemicals. These are made of small units that join together to form long chains. Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester, etc.

Which indicate that nylon fibres are very strong?

Following are the examples that indicate nylon fibres are very strong

• Parachutes and ropes of rock climbing are made of nylon
• Nylon is used in making seat belts, fishing nets and tyre cords.
• Nylon is used in making sports accessories like rackets.

Why are plastic containers favored for storing food?

Because plastic is non-reactive and will not react with the ingredients of the food items.

How to show that plastics are non-corrosive in nature?

Plastics are non-corrosive in nature, they do not react even if they come in contact with strong chemicals. For example, the cleaning chemicals that we use at home are stored in plastic bottles, instead of metal containers

Chemistry cbse class 8

 Exercise Questions

1. Explain why some fibres are called synthetic.

Soln:

Some fibres are called as synthetic fibers because these are manmade fibers prepared by using chemicals. These are made of small units that join together to form long chains. Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

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.

Soln:

Answer is (b) it is obtained from wood pulp.

3. Fill in the blanks with appropriate words.

(a) Synthetic fibres are also called __________ or __________ fibres.

(b) Synthetic fibres are synthesised from raw material called __________.

(c) Like synthetic fibres, plastic is also a __________

Soln:

(a) Synthetic fibres are also called artificial or manmade fibres.

(b) Synthetic fibres are synthesised from raw material called petrochemicals.

(c) Like synthetic fibres, plastic is also a polymer.

4. Give examples which indicate that nylon fibres are very strong.

Soln:

Following are the examples that indicate nylon fibres are very strong

• Parachutes and ropes of rock climbing are made of nylon
• Nylon is used in making seat belts, fishing nets and tyre cords.
• Nylon is used in making sports accessories like rackets.

5. Explain why plastic containers are favored for storing food.

Soln:

Because plastic is non-reactive and will not react with the ingredients of the food items.

6. Explain the difference between thermoplastic and thermosetting plastics.

Soln:

The difference between the thermosetting plastics and thermoplastics are

Thermosetting plastic	Thermoplastic
Cannot be bent, it will break if we attempt to bend thermo setting plastic	Thermoplastic can be bent easily
On heating thermosetting plastics, it cannot be softened. This is the reason it cannot be reshaped once it is molded.	On heating the thermoplastics, it becomes softened and can be molded and reshaped easily.

7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Soln:

a) Thermosetting plastics are used to make saucepan handles because they are bad conductors of heat and also on heating, these plastics do not get softened.

b) Bakelite is a kind of thermosetting plastic which is a bad conductor of heat and electricity. Because of this property it is used for making electric plugs, switches, plug boards, etc.

8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.

Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Soln:

Can be Recycled	Cannot be recycled
Plastic chairs	Cooker handles
Plastic bowls	Electrical switches
Plastic toys	telephone
The plastic covering on electrical wires
Carry bags
Ballpoint pens

9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Soln:

Rana should buy a cotton shirt for the summer and not a synthetic shirt. Cotton is a good absorber of water. So it can soak the sweat coming out of the body and expose it to the environment. Thus, it helps in evaporating the sweat and helps in cooling our body.

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

Soln:

Plastics are non-corrosive in nature, they do not react even if they come in contact with strong chemicals. For example, the cleaning chemicals that we use at home are stored in plastic bottles, instead of metal containers.

11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer

Soln:

The handle and bristle of a toothbrush should not be made of the same material as the handle of the toothbrush should be hard and strong while the bristle should be soft and flexible.

12. ‘Avoid plastics as far as possible’. Comment on this advice.

Soln:

We should avoid plastics as far as possible because plastics are non-biodegradable in nature. Once introduced into the environment they cause pollution. If burnt they release poisonous gases and the the plastic bags thrown in the garbage dumb are swallowed by animals choke their respiratory system and causes fatalities in animals.

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

A	B
(i) Polyester	(a) Prepared by using wood pulp
(ii) Teflon	(b) Used for making parachutes and stockings
(iii) Rayon	(c) Used to make non-stick cookwares
(iv) Nylon	(d) Fabrics do not wrinkle easily

Soln:

A	B
(i) Polyester	(d) Fabrics do not wrinkle easily
(ii) Teflon	(c) Used to make non-stick cookwares
(iii) Rayon	(a) Prepared by using wood pulp
(iv) Nylon	(b) Used for making parachutes and stockings

14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.

Soln:

The manufacturing of synthetic fibers is helpful in the conservation of forests because if we use natural fibers, the raw materials for them have to be derived from the plants, which require cutting off lots of trees. This proves to be helpful in the conservation of forests.

15. Describe an activity to show that thermoplastic is a poor conductor of electricity

Soln:

In order to show that thermoplastic is a poor conductor of electricity, we will design a circuit. For that, we need a bulb, some wires, a battery, a piece of metal and a plastic pipe (as shown in the figure below). After setting the experiment switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow. Hence a plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.

Lets assess

 Exercise Questions

1. Give two examples each of situations in which you push or pull to change the state of motion of objects

Soln:

Examples of situations in which you push or pull to change the state of motion of objects.

1. Pull situations

a) In order to open a drawer, we have to pull it. This action changes the state of motion of the drawer.

b) To draw water from the well, the rope is pulled. This action changes the state of motion of the bucket.

2. Push Situations

a) A football is pushed by the foot of a player. This action changes the state of motion of the ball.

b) In order to change the place of the heavy box from one room to another, we have to push it. This action changes the motion of the box.

2. Give two examples of situations in which applied force causes a change in the shape of an object.

Soln:

The forces which change the shape of an object are as follows:

i) By pressing the clay between the hands, it deforms.

ii) The shape of the plastic bottle changes by squeezing it.

3. Fill in the blanks in the following statements.

(a) To draw water from a well we have to __________ at the rope.

(b) A charged body __________ an uncharged body towards it.

(c) To move a loaded trolley we have to __________ it.

(d) The north pole of a magnet __________the north pole of another magnet.

Soln:

1. To draw water from a well we have to pull at the rope.
2. A charged body attracts an uncharged body towards it.
3. To move a loaded trolley we have to pull or push it.
4. The north pole of a magnet repels the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms.

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its __________.

(b) The force applied by the archer to stretch the bow is an example of __________ force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force.

(d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air

Soln:

(a) To stretch the bow, the archer applies a force that causes a change in its shape.

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height

Soln:

a) We make a muscular force to extract the juice of the lemon by squeezing it. As a result, the shape of the lemon gets changed.

b) To take out paste from the toothpaste tube we use our muscular force. This muscular force acting on the toothpaste tube leads to a change of its shape.

c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

d) The feet of the athlete exert the muscular force on the ground, which pushes the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.

6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Soln:

A blacksmith uses his muscular force while hammering a hot piece of iron. The muscular force changes the shape of the iron so that it can be given the desired shape.

7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Soln:

When an inflated balloon rubbed with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body. When this charged balloon is pressed against a wall, it sticks to the wall. Thus, the electrostatic force acts between the charged balloon and the wall.

8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Soln:

For holding the bucket of water above the ground, we use muscular force. This muscular force acts against the force of gravity that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in the state of motion.

9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Soln:

The force of gravity is the one which acts on the rocket to pull it towards the ground and the other one is the force of friction due to earth’s atmosphere, which opposes its motion.

10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water.

(b) gravity of the earth.

(c) shape of rubber bulb.

(d) atmospheric pressure

Soln:

a) Due to the atmospheric pressure, there is a rise of water in the dropper.

When all the air escapes from the nozzle, the atmospheric pressure which is acting on the water forces the water to fill the nozzle of the dropper.

Force nd pressure notes

 

Push

A push is a force exerted away from the body, e.g: Hitting a snooker ball, kicking a football.

Magnetic force

The force of attraction or repulsion between two magnetic bodies due to their poles is known as a magnetic force.

Net force

• The resultant of all the forces acting on a body is known as net force.
• The acceleration of the body is along the direction of the net force.

Frictional force

• The force that opposes the relative motion between two surfaces.
• Acts between the surface of the two bodies in contact.
• Type of contact force

Pull

A pull is a force exerted towards the body, e.g: drawing a bucket of water from a well, playing tug of war.

Force

• A force is a push or a pull.
• The interaction between objects that can change the state of the objects.

Vector

– Vector quantities are expressed in magnitude as well as the direction of the object. E.g: Velocity, displacement, weight, momentum, force, acceleration etc.
– Vectors are used to find the resultant component acting on a body.
– When multiple forces act on a body, they can be resolved into one component known as the net force acting on the object.

Application of Force

– A force is an effort that changes the state of an object at rest or in motion.
– It can change an object’s direction and velocity.
– Force can also change the shape of an object.

State of Motion

The state of motion of an object is defined by its velocity – the speed with a direction. Thus, inertia could be redefined as follows:
Inertia = tendency of an object to resist changes in its velocity.
An object at rest has zero velocity – and (in the absence of an unbalanced force) will remain with a zero velocity; it will not change its state of motion (i.e., velocity). An object in motion with a velocity of 2 m/s, East; will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East; it will not change its state of motion (i.e., velocity). Objects resist changes in their velocity.

Types of Forces

Contact

A touch or contact is required to do most of our everyday activities. E.g Lifting, pulling, kicking, pushing etc.

Contact forces

Forces that require a touch or contact to be applied are known as contact forces. E.g: Muscular forces, frictional forces

Muscular force

The force applied by the effort of our muscles e.g lifting a heavy box, pulling a bucket of water, pedalling a cycle.

Non-contact forces

• Forces that do not need a contact, or that have their influence without a touch.
• Example: magnetic force, electrostatic force, gravitational force.

Gravitational force

• The attractive force that a body experience towards the centre of the earth is called the force of gravity due to earth.
• Property of the universe, every object attracts or exerts a force on every other object.

Electrostatic force

The force of attraction or repulsion experienced by a charged body from another charged body in the same vicinity is known as Electrostatic Force.

Nuclear forces

• The nuclear force acts between all the particles in the nucleus. i.e., between two neutrons, between two protons and between a neutron and a proton.
• It is an attractive force in all cases.
• It is the force that keeps the nucleus intact by overcoming the enormous repulsive force between positive protons.

Thrust and Pressure

Pressure

• Force acting per unit area is known as pressure.
• P=forcearea
• SI unit is Pascal.

Distribution of pressure

• Force acting on a smaller area applies more pressure than the same force acting on a larger area.
• Examples: Porters place a round cloth on their heads to increase surface area and reduce pressure.
• A sharp knife cuts better as more pressure is exerted over a smaller area.

Pressure in fluids

The pressure exerted by a fluid(gases or liquids) in a container is transmitted undiminished in all direction on the walls of the container.

Upthrust

The upward force exerted by a fluid on an object is known as upthrust or buoyant force.

Atmospheric Pressure

Gaseous pressure

Gases exert the same pressure on the walls of the container in all directions.

Atmospheric pressure

• Our atmosphere extends to several kilometres above sea level. The weight of the air acts as pressure known as atmospheric pressure.
• The pressure in our body balances the atmospheric pressure and that is why we do not feel it.

English

 1.Character sketch 

Sachin was an eleven year old boy. He often played cricket with his colony friends at Bandra in his hometown. He often tried to imitate the great cricket players of that time. He was studying in the New English School, Mumbai. At the age of eleven, his kind and loving brother `Ajith took him to Achrekar, a famed cricket 

coach. His brother identified his natural skill and he found a great future in him.He got an opportunity to get the training due to the insistence of Ajith. He was studying in the New English school,Mumbai. Sachin changed his school to Shardashram where Achrekar sir was coaching. Sachin recalled his admirable coach for his kind approach. It was necessary to be a professional player like him. Sachin was sincere in his practice sessions in the morning and evening. He received the blessing of coach. . He would bat from morning to evening with hundreds of fielders. This type of harsh coaching molded him to be a world renowned cricketer. It influenced Sachin’s life immensely.

2.Profile

Write a short profile of Sachin Tendulkar using the details given below

Year of birth : 1974

Place of birth : Mumbai

Occupation : Former Indian cricketer and captain

Famous : One of the greatest cricketers of all time.

Awards and honours : Arjuna Award, Rajiv Gandhi Khel Ratna award, 

Padma Sree, Padma Vibhushan and Bharath Ratna. India’s highest civilian award 

in 2015, he was nominated to the Rajya Sabha

3.NOTICE 

The sports club of your school has decided to organize a function to congratulate Mr.Ramakanth Achrekar, Sachin’s exceptional coach . Draft a notice including all details.

4.conversation writing 

Imagine you got an opportunity to interview Sachin, write the conversation 

5.FORMAL LETTER

write a letter to Sachin Tendulakr to be the chief guest for your annual sports meet of your school

Crop production and management

 Crop

 Plants of the same kind that argrown and cultivated as a source of food in a large cultivable land is called a crop.

Types of Crops

Crops, which are grown in the winter season (from October to March) are called Rabi crops.

The crops, which are sown in the rainy season (from July to October) are called Kharif crops.

Rice, groundnut, maize, cotton , pulses are some common kharif crps.
Wheat, barley, mustard, potato and peas are some common rabi crops.

Preparation of Soil

• Preparation of soil is the first step in cultivating crop for food production.
• The soil is prepared for sowing the seeds of the crop.
• This is carried out using various processes and tools.

The Process of farming :

1.Tilling or Ploughing 

The process of loosening and turning of the soil is called tilling or ploughing and is done by using a plough.

Plough

• A plough is a device that is used by farmers for different purposes such as adding of fertilizers, tilling and loosening of the soil.
• It is also used for adding fertilizers to the soil, removing weeds, scraping of soil, etc.
• The ploughshare is the triangular iron strip.
• A ploughshaft is the main part of the plough, which is made using a log of wood.
• The other end of the shaft has a handle.
• The other end is attached to a beam which is placed on the bull’s necks.
• A wooden, traditional plough can be operated by a pair of an ox and a man.
• Nowadays these wooden ploughs are being replaced by the iron ploughs.

Hoe

A hoe is a tool that used to dig up soil to remove weeds and also loosen up the soil before planting a sapling.

Cultivator

• A cultivator is attached to the tractor and helps in loosening soil.
• Cultivators are used instead of ploughs since they are faster

2.Sowing

• Sowing the process of planting the seeds in the soil.
• The seeds are sowed in the soil that is loosened by a cultivator or plough.

Quality of the Seeds

• Quality of the seed is an important factor that determines the crop yield.
• Selection of good seeds is done by putting the seeds in water.
• The dead and damaged seeds become hollow and float on water whereas the good seeds sink.

Traditional Tools

• Before the advent of modern agricultural machinery, traditional tools were used by farmers.
• These include ploughs, shovels, scythes and pickaxes.
• The traditional tool used to sow the seeds was like a funnel.
• Once seeds were put into this funnel, they would go to 2-3 tubes having sharp ends.
• The ends will pierce into the soil and place the seeds there.

Seed Drill

• Seed drills are used for sowing with the help of tractors.
• It ensures that seeds are sown uniformly, at a particular depth and are covered by soil after sowing.

Nursery

• A nursery is a place where young plants and trees are grown for planting elsewhere.
• Nursery acts as a repository of saplings.

Germination of Seeds

• Germination of the seed happens when the seed is sown in the land and watered.
• A plant starts to emerge from the seed and starts to grow.

3.Adding Manure and Fertilisers

Manure/Fertilizers

• Manures and fertilizers are the substances that are added to the soil to increase their fertility.
• While manures are made by decomposition of organic substances, fertilizer is made of inorganic chemicals.

Differences between Manure and Fertilizers

Fertilizer	Manure
Fertilizer is an inorganic salt	Manure is prepared from organic matter such as human waste, cow dung and farm waste
Fertilizers are manufactured in factories	Manures can be prepared in farms
Fertilizers are added in comparatively smaller quantities	Manures need to be added in large quantity as the nutrient content is less
Fertilizers do not provide any humus to the soil	Manures provide a lot of humus to soil

Disadvantages of Using Fertilizers

• Excessive use of fertiliser can cause pollution.
• It can also change pH of the soil in certain rare cases.

Leaving the Land Fallow

• When land is left fallow for a certain period of time, the land replenishes its nutrients by itself.
• This land can be used for agriculture again.

Crop Rotation

• Crop rotation ensures that the same crop will not grow continuously and lead to the erosion of the soil fertility.
• By growing crops that require different sets of nutrient, we can ensure the soil fertility is restored.

Irrigation:

Irrigation is the artificial supply of water to farms when needed.
Some of the modern irrigation methods are as follows:

• Sprinkler system
• Drip irrigation

Implements used for traditional ways of Irrigation:

     ▸(a) Moat (b) Dhekli

     (c) Rahat

     (d) Chain pump and

     (e) Motor pumps

4.Protecting from Weeds

Weeds

 

Weeds are undesirable plants that may grow naturally along with the crop.

• Weeds compete with the crops by absorbing all the water, nutrients, space and light.

Tilling

• Tilling is a process done before sowing of crops that helps in uprooting and killing of weeds.

Manual Removal

• Manual removal includes physical removal of weeds by uprooting them from the soil or chopping them off to ground level periodically.

Weedicides

• Chemicals used to kill the weeds are known as weedicides.
• They usually don’t damage the crop.

5.Harvesting

• Harvesting is the process of cutting the crop after it is mature.

Methods of Harvesting

• Harvesting is done by 2 methods.
• First is the manual method where a sickle is used.
• Second is the mechanical method where a huge machine called harvester is used.

Threshing

• Threshing is the process of loosening the grains from the chaff.
• While it can be done manually, these days a machine is used that separates all the grain seeds.

.Winnowing

• Winnowing is the process of separation of grain seeds from the chaff using the help of the wind.
• Due to the wind, the lighter chaff flies away and the heavier grains fall down.

6.Storage

• Storage of the grains is an important step in agriculture.
• After harvesting steps, the ready grains are stored in granaries or silos.
• The grains have to be stored in a dry place that does not have a rodent or fungal infestation.
• Fumigation of storage places is carried out to make it free from microbes.

Granaries

• Granaries are the place where the freshly obtained food grains are stored.

Silo: Harvested grains are usually dried before being stored because moisture encourages the growth of microorganism. They are then stored in metal or earthen container, gunny bags. Such stores are also called silo

Hybridization is a technique used for plant breeding in which new varieties with desired characteristics of high yield and resistance to disease, are developed.

Nitrogen cycle:

Air contains about 78% nitrogen.  The cyclic process of nitrogen being fixed, used by plants and animals and later returned to the atmosphere is referred to as the nitrogen cycle.

Nitrogen cycle involves the following steps:

• Nitrogen fixation: fixing free nitrogen gas of the atmosphere into inorganic compounds by organism such as Rhizobium.
• Nitrogen assimilation: converting inorganic nitrogen into usable organic compounds in organisms.
• Ammonification: Conversion of organic nitrogen into ammonia.
• Nitrification: Ammonia is converted into nitrates in the soil with the help of bacteria.
• Denitrification: Conversion of nitrates into nitrogen gas by denitrifying bacteria.

.

Animal Husbandry

• Animal husbandry is the management and care of farm animals for milk, egg or meat.
• Meat or egg yielding animals such as goat, poultry animals (e.g. chicken, duck and trkey), fish, sheep.
• Milch or (milk yielding) animals such as cow, buffalo, goat and camel.
• Large scale rearing of fish for food is known as pisciculture.
• Large scale rearing of honeybee is known as apiculture.