CBSE Textbook

Science (Class VIII)


Kharif Crops: The crops which are sown in the rainy season are called kharif crops. The rainy season in India is generally from June to September. Paddy, maize, soyabean, groundnut, cotton, etc., are kharif crops.

Rabi Crops: The crops grown in the winter season are called rabi crops. Their time period is generally from October to March. Examples of rabi crops are wheat, gram, pea, mustard and linseed.

The preparation of soil is the first step before growing a crop. The loose soil allows the roots to breathe easily even when they go deep and helps in the growth of earthworms and microbes present in the soil. Since only a few centimetres of the top layer of soil supports plant growth, turning and loosening of soil brings the nutrient-rich soil to the top so that plants can use these nutrients. The process of loosening and turning of the soil is called tilling or ploughing. This is done by using a plough.

Before sowing the seeds, it is necessary to break soil to the size of grains to get better yield. The main tools used for this purpose are the plough, hoe and cultivator. Sowing by using a seed drill saves time and labour.

Continuous growing of crops makes the soil poorer in certain nutrients. The substances which are added to the soil in the form of nutrients for the healthy growth of plants are called manure and fertilisers.

Manure is an organic substance obtained from the decomposition of plant or animal wastes. Manure provides a lot of humus to the soil. Manure is relatively less rich in plant nutrients.

Fertilisers are chemical substances which are rich in a particular nutrient. A fertiliser does not provide any humus to the soil. Fertilisers are very rich in plant nutrients like nitrogen, phosphorus and potassium.

But excessive use of fertilisers can make the soil less fertile. They are also a source of water pollution.

Therefore, in order to maintain the fertility of the soil, we have to substitute fertilisers by organic manure or leave the field uncultivated (fallow) in between two crops.

The use of manure improves soil texture as well as its water retaining capacity. It replenishes the soil with all the nutrients. Another method of replenishing the soil with nutrients is through crop rotation.

The supply of water to crops at different intervals is called irrigation. Sprinkler system and drip system are the two kinds of modern methods of irrigation.

In a field many other undesirable plants may grow naturally along with the crop. These undesirable plants are called weeds.

Tilling before sowing of crops helps in uprooting and killing of weeds, which may then dry up and get mixed with the soil. Khurpi is used to manually remove them. The best time for the removal of weeds is before they produce flowers and seeds.

The cutting of crop after it is mature is called harvesting. It usually takes 3 to 4 months for a cereal crop to mature. Harvesting in our country is either done manually by sickle or by a machine called harvester.

In the harvested crop, the grain seeds need to be separated from the chaff. This process is called threshing. Farmers with small holdings of land do the separation of grain and chaff by winnowing.

Special festivals associated with the harvest season are Pongal, Baisakhi, Holi, Diwali, Nabanya and Bihu.

Large scale storage of grains is done in silos and granaries.

Food is also obtained from animals for which animals are reared. This is called animal husbandry.

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae. Viruses are also microscopic. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.

Louis Pasteur discovered fermentation in 1857. Alexander Fleming discovered penicillin.

Disease-causing microorganisms are called pathogens.

Female Anopheles mosquito carries the parasite of malaria and female Aedes mosquito acts as carrier of dengue virus.

TB, cholera, typhoid- bacteria

Measles, chicken-pox, polio, hepatitis-B- virus

Malaria –protozoa

Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.

Citrus canker- Bacteria- Air

Rust of wheat- Fungi- Air, seeds

Yellow vein mosaic of bhindi(Okra)- Virus –Insect

Food poisoning occurs due to the consumption of food spoilt by some microorganisms.

Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore they are called preservatives.

Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.

Although rayon or artificial silk is obtained from a natural source, wood pulp, yet it is a man-made fibre.

Plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics. Polythene and PVC are some of the examples of thermoplastics.

On the other hand, there are some plastics which when moulded once, cannot be softened by heating. These are called thermosetting plastics. Two examples are bakelite and melamine. Bakelite is a poor conductor of heat and electricity. It is used for making electrical switches, handles of various utensils, etc. Melamine is a versatile material. It resists fire and can tolerate heat better than other plastics. It is used for making floor tiles, kitchenware and fabrics which resist fire.

Plastics are poor conductors of heat and electricity.

CNG pipelines exist in Vadodara (Gujarat), some parts of Delhi and other places.

A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called a fuel. The fuel may be solid, liquid or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow.

The lowest temperature at which a substance catches fire is called its ignition temperature. The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

These days the head of the safety match contains antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus.

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg. It refers to fuel efficiency.

An ideal fuel is cheap, readily available, readily combustible and easy to transport. It has high calorific value. It does not produce gases or residues that pollute the environment.

Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain.

There are three different zones of a flame -dark zone, luminous zone and non-luminous zone. Non-luminous zone of flame is the hottest part.

Unburnt carbon particles in air are dangerous pollutants causing respiratory problems. Incomplete combustion of a fuel gives poisonous carbon monoxide gas.

Biosphere is that part of the earth in which living organisms exist or which supports life. Biological diversity or biodiversity, refers to the variety of organisms existing on the earth, their interrelationships and their relationship with the environment.

To protect our flora and fauna and their habitats, protected areas called sanctuaries, national parks and biosphere reserves have been earmarked. Plantation, cultivation, grazing, felling trees, hunting and poaching are prohibited there.

Sanctuary: Areas where animals are protected from any disturbance to  them and their habitat.

National Park: Areas reserved for  wild life where they can freely use the habitats and natural resources.

Biosphere Reserve: Large areas of protected land for conservation of wild life, plant and animal resources and traditional life of the tribals living in the area.

The Pachmarhi Biosphere Reserve consists of one national park named Satpura and two wildlife sanctuaries named Bori and Pachmarhi. Satpura National Park is the first Reserve Forest of India. The finest Indian teak is found in this forest.

Endemic species are those species of plants and animals which are found exclusively in a particular area. They are not naturally found anywhere else. A particular type of animal or plant may be endemic to a zone, a state or a country.

An ecosystem is made of all the plants, animals and microorganisms in an area along with non-living components such as climate, soil, river deltas, etc.

Red Data Book is the source book which keeps a record of all the endangered animals and plants.

Migration is the phenomenon of movement of a species from its own habitat to some other habitat for a particular time period every year for a specific purpose like breeding.

Reforestation is the restocking of destroyed forests by planting new trees.

Robert Hooke discovered cells.

The cells having nuclear material without nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes (pro: primitive; karyon: nucleus). Examples are bacteria and blue green algae.

The cells, like onion cells and cheek cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes. (eu: true; karyon: nucleus).

Green coloured plastids are called chloroplasts.

Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.

The animals which give birth to young ones are called viviparous animals. Those animals which lay eggs are called oviparous animals.

The transformation of the larva into an adult through drastic changes is called metamorphosis.

This type of reproduction in which only a single parent is involved is called asexual reproduction. Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding. The other kind is in which an animal reproduces by dividing into two individuals called as binary fission. Eg.: amoeba.

The changes which occur at adolescence are controlled by hormones. Hormones are chemical substances. These are secretions from endocrine glands, or endocrine system. The male hormone or testosterone begins to be released by the testes at the onset of puberty. Once puberty is reached in girls, ovaries begin to produce the female hormone or estrogen.

The production of these hormones is under the control of another hormone secreted from an endocrine gland called pituitary gland.

Force has magnitude as well as direction. Force per unit area is called pressure.

The substances which reduce friction are called lubricants.

Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by the use of ball bearings.

Fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is also called drag.

The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch.

For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.

It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. The other kind of charge is said to be negative.

A major earthquake occurred in India on 8th October 2005 in Uri and Tangdhar towns of North Kashmir. Before that a major earthquake occurred on 26th January 2001 in Bhuj District of Gujarat.

Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plane.

Richter scale is not linear like decibel.

Angle of incidence is always equal to the angle of reflection. This is known as the law of reflection. The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. This is another law of reflection.

Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.

Saturn is the least dense of all planets.

Asteroid orbits are in between Mars and Jupiter.

Smog is made up of smoke and fog. Smoke may contain oxides of nitrogen which combine with other air pollutants and fog to form smog. The smog causes breathing difficulties such as asthma, cough and wheezing in children.

Other gases like methane, nitrous oxide and water vapour also contribute towards global warming. Like CO2, they are also called greenhouse gases.

Ganga Action Plan was launched in 1985.

Water which is suitable for drinking is called potable water.

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