Current Affairs 8th Class

Pollution of Air and Water   Synopsis  
  • Pollution is the introduction of harmful substances into the environment, which can harm the health, survival or activities of living organisms. Pollutants may be biodegradable or non-biodegradable. Biodegradable pollutants can be broken down by biological agents, whereas non-biodegradable pollutants cannot be broken down by any agents and remain in the environment for a long time.
  • The major causes of air pollution are industries and automobiles. The burning of fossil fuels releases carbon dioxide, carbon monoxide, sulphur dioxide, oxides of nitrogen, hydrocarbons and particulate matter. Industrial processes release polluting gases and particulate matter.
  • The oxides of nitrogen and sulphur combine with rainwater to form acid rain, which affects soil fertility, vegetation, buildings and monuments.
  • Carbon dioxide and some other gases, like methane are called greenhouse gases. They trap radiations from the earth and are considered to be the cause of global warming.
  • Air pollution can be reduced by modifying automobile engines, using CNG in the place of petrol or diesel, using unleaded petrol, removing pollutants from industrial waste gases before their release into the atmosphere and using cleaner sources of energy.
  • Water pollution can be classified into two types - chemical and biological.
  • Chemical pollution of water is caused by the discharge of unwanted chemicals into water bodies.
  • Biological pollution of water bodies is caused by oxygen-demanding wastes and disease- causing micro-organisms.
  • The major causes of water pollution are industrial effluents and urban sewage. Disease- causing organisms, agricultural and industrial chemicals, oxygen-demanding brganic wastes and hot water are some of the major categories of pollutants.
  • The accumulation of plant nutrients in water bodies enhances the rapid growth of algae. The algae cut off the supply of light to organisms and their decomposition by bacteria uses up dissolved oxygen from water.
  • Eutrophication is an increase in the rate of supply of organic matter in an ecosystem.
  • Water pollution can be prevented or reduced by the treatment of sewage, treatment of industrial wastes, limited use of pesticides and fertilisers, burning of hospital wastes and proper disposal of dead bodies.
  • Water fit for drinking is called potable water.
  • Alum facilitates the removal of suspended particles from water by sedimentation.
  • Water can be disinfected by boiling, chlorination, ozonisation or irradiation with ultraviolet rays.
  • Sustainable development implies a change in all aspects of life. The public must be educated on the 3 Rs concept that is recycle, reuse and reduce to conserve and preserve resources.

Food Production and Management   Synopsis  
  • Agriculture is the science or practice of growing crops.
  • Plants of the same kind are grown and cultivated at one place on a large scale are called crops.
  • There are three main crop seasons -
(i) Kharif (June-September), e.g., rice, jute, maize, groundnut and cotton. (ii) Rabi (October-March), e.g., wheat, mustard, potato, barley and gram. (iii) Summer crops.  
  • The steps involved in cultivating a crop are as follows.
  • Ploughing, levelling and manuring the soil.
  • Sowing seeds at the correct depth and with right spaces between them. Some seeds are sown in nurseries and the seedlings are then transplanted to the main field.
  • Improving soil fertility by adding manure and chemical fertilizers and also by adopting methods like crop rotation and leaving the field fallow.
  • Ensuring irrigation at the right time.
  • Protecting crops from weeds, pests and diseases either by using chemicals or by using natural methods.
  • Harvesting, threshing and winnowing.
  • Legumes are often used in crop rotation, because the nitrogen fixing bacteria which live in their roots improve soil fertility.
  • Nitrogen fixation is a part of the nitrogen cycle, which is, continued cycling of nitrogen from the air to the soil and to living organisms.
  • Grains are stored in silos or god owns that have been fumigated. Buffer stock is maintained for emergencies.
  • Scientists have developed hybridisation processes to grow disease resistant varieties of plants. The earliest success is the production of high-yielding varieties of plants which led to increase in the production of food crops. This is often referred to as the Green Revolution.
  • The branch of agriculture dealing with the rearing of farm animals is called animal husbandry.
  • Animals give us milk, meat and eggs. Animal products are an excellent source of protein. Animal proteins are superior to plant proteins. Egg white contains the protein albumen.

Cell   Synopsis  
  • The structural and functional unit of living organisms is a cell. Organisms that consist of only one cell are called unicellular. A tissue is made of cells. Tissues organise to form organs which in turn form organ systems and the organism.
  • Cells without well organised nucleus, i.e., lacking nuclear membrane, are called prokaryotic cells and cells with definite nucleus are called eukaryotic cells.
  • The cell is enclosed by a cell membrane enclosing the cytoplasm, nucleus and cell organelles.
  • The cell membrane is selectively permeable, i.e., it lets the passage of certain substances only.
  • The cytoplasm contains organelles, water and many dissolved substances such as proteins, carbohydrates, inorganic substances, fats, etc.
  • The nucleus is bound by the nuclear membrane. Inside it, the nucleoplasm and a dense network of chromatin are present. Chromatin condenses during cell division and separates into chromosomes which bear genes.
  • Mitochondria are the power houses of the cell. They use oxygen to oxidise food and release energy
  • Endoplasmic reticulum helps in the transport of substances within the cell. It is a network of tube like structures.
  • Golgi body is a stack of tubes and vesicles. It helps in the synthesis and storage of many substances. Some vesicles form lysosomes that digest and destroy cells. Hence they are called suicidal bags.
  • Centrioles are present in animal cells. These help in the formation of astral fibres during cell division.
  • Plastids are exclusively present in plants. They are of three types. Chloroplasts are photosynthetic in function. Leucoplasts are for storage and chromoplastsare coloured.
  • Vacuoles are present in plant cells. It is covered by a tonoplast and has a vacuolar sap which stores food, wastes and water.
  • Plant cells have a cell wall. It is thick, rigid and is made of cellulose. It gives support, shape and protection to the cell.
  • The outermost region of the stem of a plant is called the epidermis. Ground tissue occupies most of the inner region. It consists of different types of tissues. The central core is called the pith, while the region inside the epidermis and extending up to the vascular bundles is called the cortex.
  • The vascular bundles are distinct groups of cells arranged in a ring inside the stem. They consist of xylem and phloem tissues. Xylem cells transport water and minerals, while phloem cells transport food.
  • There are four types of animal tissues epithelial, muscular, connective and nervous. Epithelial tissue is protective. Connective tissue has cells embedded in a matrix. Muscular tissue helps in movement. Nervous tissue consists of nerve cells. It makes up more...

Microorganisms   Synopsis  
  • Microorganisms are tiny organisms that can be seen only under a microscope.
  • Microorganisms are classified into: bacteria, fungi, protozoa, viruses and algae.
  Useful mocroorganisms:  
  • coli bacteria produce vitamins \[{{B}_{12}}\] and K. In herbivores and in some insects, bacteria and protozoa digest cellulose.
  Decomposition and recycling of materials:  
  • Microorganisms such as bacteria, protozoa and fungi break down the harmful organic portion of raw sewage to less harmful sludge.
  Medicine and health supplements:  
  • Antibiotic penicillin is produced by a type of fungus Penicillium notatum. Insulin used for treating diabetics is produced by genetically modified bacteria, a vaccine, consisting of dead or weakened harmful microorganisms.
  • Nitrogen fixation by Rhizobium bacteria, decomposition of organic wastes by saprophytic bacteria and fungi are some of the uses.
  • In food industry lactic bacteria is used in the preparation of curd and cheese. In making bread and in brewing alcoholic drinks, yeast is used.
  Harmful Microorganisms:   Diseases caused by bacteria in humans.      
Name of the disease Name of the Pathogen Symstom of the disease
1. Cholera Vibrio comma Vomitings, loose motions
2. Leprosy Mycobacterium leprae Deformities of limbs
3. Gonorrhoea Coccus bacteria Pain in reproductive organs
4. Pneumonia Diplococcus pneumoniae Fluid in the lungs
5. Tetanus (lockjaw) Clostridium tetani Muscular spasms
6. Tuberculosis Mycobacterium tuberculosis Persistent cough, weight loss
7. Typhoid Salmonella typhi Fever
  Diseases caused by bacteria in plants. Blackrot in cabbage, fire blight in paddy Diseases caused by protozoans    
Name of the disease Name of the Pathogen Symptoms of the disease
Dysentry Malaria Sleeping sickness Entamoeba histolytica Plasmodium sps Trypanosoma gambiense Diarrhoea more...
Conservation of plant and Animals   Synopsis  
  • The physical and biological world we live in is called our environment.
  • Biodiversity refers to the variety of living organisms in a specific area.
  • Plants and animals of a particular area are known as the flora and fauna of that area.
  • The term wildlife refers to plants and animals living in the natural conditions. It serves as a source of genes or a gene bank.
  • Over-exploitation of wildlife for economically useful products, deforestation, in dustrialisation and pollution resulted in extinction of several plant and animal species.
  • Red Data Book contains a record of endangered species. Conservation involves keeping the natural environment in its balanced state as far as possible.
  • Preservation involves keeping some of the earth's resources for future generations.
  • The conservation of wildlife is linked with the conservation of forests and wetlands. India has 13 biosphere reserves, 92 national parks and 500 wildlife sanctuaries to help conserve wildlife.
  • We should save, reuse and recycle paper to save trees, energy and water.
  • In the whole universe earth is the only planet where life exists. It is our duty to make sure it remains beautiful, varied and a worthwhile place to live in.
  • Natural disasters and human activities result in degeneration of biodiversity.
  • There are endangered species exposed to danger of extinction. Endemic species are more endangered. They need conservation.
  • Government at National and at State levels is taking all the care to conserve biodiversity.
  • Forests must be preserved for biodiversity conservation.
  • There are nearly 10 million of species of organisms on earth of which only 2 million have been identified and listed.

Reproduction in Animals   Synopsis  
  • Reproduction is the production of new individuals more or less similar to the parent organisms. This may be achieved by a number of means and serves to perpetuate increase of species.
  • There are two main methods in which organisms give rise to new individuals - Asexual reproduction and sexual reproduction.
  • Asexual reproduction is the process of producing new organism(s) from a single parent without the involvement of sex cells or gametes.
  e.g., Binary fission in amoeba, regeneration in planaria, budding in hydra.  
  • Sexual reproduction is the process of producing new organism(s) from two parents with the involvement of sex cells or gametes. Male sexual unit is known as male gamete or sperm while female sexual unit is termed as female gamete or ova.
  • The fusion of sperm and ovum is known as fertilisation. Thus, the two major processes, i.e., formation of gametes and fusion of gametes constitute sexual reproduction.
  • The reproductive organs of human beings, i.e., testes is in male and ovaries in female produce gametes and also secrete hormones like testosterone (male hormone), estrogen and progesterone (female hormones).
  • Fertilisation takes place in the fallopian tube. The embryo develops in the uterus, and receives oxygen and nutrients through the placenta.
  • Animals such as human beings, cows, dogs which give birth to young ones are called viviparous animals. Animals such as hen, frog, lizard which lay eggs are oviparous animals.
  • The transformation from the larval stage to the adult stage in the life cycles of frog and insects is called metamorphosis.
  • The cloning of animals produces offspring with genetic materials which are identical to the parent. The most famous animal clone is Dolly, the sheep.

Reaching the Age of Adolescence   Synopsis  
  • Humans become capable of reproduction after puberty sets in. Between the ages of 11 years and 19 years children are called adolescents.
  • The period of time when a person changes from a child into an adult is called puberty.
  • A boy reaches puberty when he is 13-14 years old. Girls reach puberty earlier than boys at the age of 11 -12 years.
  • During puberty, physical and emotional changes take place. These are called secondary sexual characters. In girls, puberty is marked by the onset of menstruation and the development of secondary sexual characters, namely, enlargement of breasts, widening of pelvic girdle and growth of hair in the armpits and pubic region.
  • In boys, puberty begins with the production of sperms and is marked by secondary sexual characters, namely, deepening of the voice, growth of hair on the face, armpits and pubic region and enlargement of the scrotum and penis.
  • Menstruation is the periodic discharge of blood from the uterus through the vagina. The discharged blood usually contains a dead egg cell and some tissues from the lining of the uterus.
  • If the egg cell is fertilized, it will form a zygote which will implant itself on the wall of the uterus. It will eventually develop into an embryo. Menstruation does not take place during pregnancy.
  • The onset of puberty and maturity of reproductive parts are controlled by hormones.
  • Hormones are secretions of endocrine glands which are poured directly into the blood stream.
  • Pituitary gland secretes hormones which include growth hormone and hormones that make other glands such as those of the testes, ovaries, thyroids and adrenals secrete hormones. Pancreas secretes insulin/thyroid produces thyroxine and adrenals produce adrenaline.
  • Testosterone is the male hormone and estrogen is the female hormone. The uterine wall in females prepares itself to receive the developing fertilised egg. In case there is no fertilisation, the thickened lining of the uterine wall breaks down and goes out of the body along with blood. This is called menstruation.
  • The sex of the unborn child depends on whether the zygote has XX or XY chromosomes.
  • It is important to eat balanced food and maintain personal hygiene during adolescence.
  • Personal hygiene must be maintained during menstruation to avoid itch, unpleasant odour, bacterial and fungal infection.

  Elements   Elements  
  • An element is a substance which can neither be broken nor built from two or more simple substances by any physical or chemical method. Or in other words an elements is a pure substance which contains only one kind of atom, e.g., Fe (Iron), Na (Sodium), etc.
  Types of Elements (a) Metals (b) Non-Metals (c) Metalloids   COMPOUNDS
  • A compound is a pure substance which contains more than one kind of elements or atom in fixed proportion by weight e.g., NaCl (Sulphur dioxide), (Sodium chloride),\[S{{O}_{2}}\]etc.
  Types of Compounds (a) Organic compounds (b) Inorganic compounds
  • The properties of a compound are completely different from those of its constituents.
  • A material containing two or more elements or compounds in any proportion is a mixture. It can be seperated into constituents, e.g., air, milk, paints, cements, etc.
  Types of Mixture (a) Homogeneous (b)  Heterogeneous (a) A mixture is said to be homogeneous if its composition is uniform throughout. (b) A mixure is said to be heterogeneous if its composition is not uniform.
  • The properties of a mixture are the properties of its constituents.
    • A mixture with definite boiling point is known as azeotropic mixture.
  Separation of Mixtures (a) Sublimation: In this process, a solid substance passes directly into its vapours on application of heat, and when vapours are cooled, they give back the original substance, e.g., iodine, napthalene, benzoic acid, etc. (b) Sedimentation and decantation: This method is used when one component is liquid and the other is insoluble solid. (c) Crystallisation: This method is based on the difference in solubility of the various compounds in a solvent, e.g., mixture of \[KN{{O}_{3}}\] and \[NaCl\] can be separated by this process. (d) Filtration: It is used for quick and complete removal of solid suspended particles from a liquid (or gas) by passing the suspension through a filter. (e) Evaporation: In this method the solution is heated so that the solvent vapourises to give the solute (solid substance), e.g., salt can be obtained from salt solution. (f)   Distillation: A mixture of two substances, only one of which is volatile, can be separated by this process, e.g., \[NaCI\]can be separated from water by distillation. (g) Fractional distillation: This process is used if both the components of a mixture are volatile. It is based on the difference of boiling points, e.g., the various fractions of crude petroleum can be separated by this process. (h) Steam distillation: It is used to separate a liquid (should be immiscible with water) from a mixture by heating with steam, e.g., lemon oil, toluene, nitrobenzene, etc. (i)   Mechanical separation: Two immiscible liquids can be separated by using a separately funnel, e.g., oil and water. (j)   Magnetic separation: Two solids one of which is magnetic substance can be separated by this method. more...

  Metals and its Physical Properties     Metals and its Physical Properties   (a) Physical Change: A physical change is a change of state only and is not accompanied by an alteration in the chemical composition, weight or chemical properties of any substance, e.g., melting of ice, magnetising a needle, etc. (b) Chemical Change: A chemical change gives rise to a new substance having composition and properties altogether different from those of the original substance, e.g., radioactive decay, rusting of iron, photosynthesis, etc. Energy is absorbed or released during a chemical change.   PHYSICAL AND CHEMICAL PROPERTIES (a) Physical Properties: The properties which do not depend on reaction with any other substance, e.g., colour, melting point, boiling point, density, etc. (b) Chemical Properties: The chemical properties of a substance are those that describe its reactions with other substances.   EMPIRICAL FORMULA
  • The empirical formula gives the simple whole number ratio between the number of atoms of the different elements forming the compound, e.g., \[{{H}_{2}}O\] tells that there are twice as many H atoms as O atoms.
  • It indicates the number of atoms in a molecule of a molecular substance. Elements as well as compounds can be represented by a molecular formula, e.g.,\[{{H}_{2}},\]\[{{O}_{2}},\]\[{{N}_{2}},\] etc.
  • Colloidal particles may be crystalline or non-crystalline. When these are dispersed in a liquid, solid or gas they result in the formation of a colloidal system, e.g., top soil of earth, protoplasm, etc.
(a) Sol: When a solid is dispersed in a liquid, the particles of a Sol will not settle out. (b) Gel: In a gel, the liquid contains a colloidal solid dispersed throughout the system, but set in a structure which does not flow, e.g., gelatin, jellies, etc. (c) Aerosol: The dispersion of either a solid or a liquid in a gas. When the dispersed colloidal particle is solid, the result is smoke and when it is liquid, the result is fog. (d) Emulsion: When one liquid is dispersed into another in which it is not soluble, e.g., milk, paint, etc.   Properties of Colloids
  • When the beam of light is passed through a true solution, the path of the beam is not visible but when it is passed through a colloidal solution, its path is visible. This effect is known as Tyndall Effect.
  • It comprises the various processes involved in the production of metal from the ore which generally contains a large percentage of rocky materials, called gangue.
  • It is caused in iron due to presence of moisture, oxygen, \[C{{O}_{2}}\]in the air. Rusting is prevented by surface coating with film of oil, paint or metal coating such as chromium coating, nickel plating, tin plating and copper plating.
  • It is a mixture of two or more metals, and small amount of non-metals also.
  • These are minerals from which metals are produced.

Carbon The Carbon   The Carbon
  • Group 14 (IVA) consists of carbon, silicon, germanium, tin and lead. Carbon is a non-metal, silicon and germanium are metalloids, tin and lead are metals.
    • Carbon is a very important element. This is due to 3 reasons:
  • Carbon is a tetravalent atom. It may form four covalent bonds in different directions tetrahedrally.
    • Carbon has several allotropes, three of which are common.
(i)   Amorphous carbon is coal and soot. The carbon molecules are covalently bonded but there is no order of arrangement. (ii)   Graphite occurs when carbon forms flat covalent networks. These flat "sheets" are not bonded to each other, making them free to slide past each other. Graphite composes the "lead" in pencils. Since graphite contains free electrons, it is a good conductor of heat and electricity. Graphite is used as a lubricant in fast-moving machinery since graphite is soft and slippery. (iii) Diamond occurs when carbon forms a three-dimensional covalent network. Diamonds are different from graphite and amorphous carbon. They are transparent, brilliant-looking and incredibly hard. Diamond only forms at heat and intense pressure. It is a bad conductor of heat and electricity, as there are no free electrons present in it.
  • Carbon compounds are of two types: inorganic and organic.
  Organic and Inorganic Compounds
  • The compounds that have a mineral origin fall under the category of inorganic compounds.
  • The compounds having a plant or animal origin, are classified as organic compounds.
  • In spite of the fact that organic compounds were originally derived from living sources. Today, most of these compounds can be synthesised.
  • Organic compounds are known to contain mainly hydrogen and carbon.
    • Application of Organic Compounds in Daily Life:
  • Ethylene: This gas is produced naturally by most fruits, and it promotes ripening.
  • Alkanes: They are used in hydrocarbon fuels (petrol, diesel, propane, butane, kerosene, etc.)
  • Alkenes: They occur in fats (unsaturated fats have C = C bonds in)
  • Polymers: These long chain molecules feature in proteins, plastics, rubber and many other synthetic materials.
  • Alcohols: To make alcoholic drinks (the obvious case with ethanol), to act as general solvents (methanol is in methylated spirit, isopropyl alcohol is in a lot of cleaning agents).
  • Carboxylic acid: Fatty acids (a type of carboxylic acid) are used to make soaps and detergents, acetic acid is in vinegar.

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