8th Class Science Carbon and its Compounds NCERT Summary - Elements and Compounds

NCERT Summary - Elements and Compounds

Category : 8th Class

Elements and Compounds

 

Hydrogen

  • It is colourless, odourless, tasteless, inflammable and lightest known substance (gas). It is found in water\[{{H}_{2}}O\], organic compounds and all living things. It is neutral to litmus.
  • It can be produced in the laboratory by Bosch process and by electrolysis. It is used in balloons, ships, for ammonia and vanaspati ghee preparation, etc.

 

Oxygen

  • It is colourless, tasteless, odourless, combustible, slightly heavier than air and somewhat soluble in water. Atmospheric air contains oxygen by about 21% by weight. It can not only be prepared in the laboratory but also in factories on commercial scale. It can be liquefied and solidified. It is employed in welding process and also used in hospitals for artificial respiration.

 

Nitrogen

  • It is colourless, tasteless, odourless, non-combustible, inactive, non-poisonous gas, forming about 80% of the atmospheric air by volume and 75% by weight. It is slightly lighter than air and only slightly soluble in water. It is used for filling electric bulbs, for making fertilisers, ammonia, nitrates, etc.

 

Ozone

  • It is an allotropic form of oxygen containing three atoms in the molecule and is formed when oxygen or air is subjected to silent electric charge. It is bluish gas, very active chemically, and a powerful oxidizing agent.
  • It is found in the upper atmosphere some 25 to 40 km from the Earth's surface, called ozonosphere. It is this layer which absorbs a large proportion of the Sun's ultraviolet radiation. Ozone is used for purifying air and water and in bleaching.

 

Carbon

  • It is a universal constituent of living matter. It can be mainly classified into two forms allotropic form and amorphous form. Diamond and graphite are two of its allotropic forms whereas charcoal, lamp black, coke, etc. belong to amorphous form. Carbon atoms are capable of uniting with each other to form very large molecules upon which life is based.

 

Diamond

  • It is the hardest naturally occurring substance. It is transparent to X-rays only. It is very costly and used in jewellery, drilling and cutting tools. It can be cut only by a diamond.

 

Graphite

  • It is soft, easily powdered and gives a greasy feeling. It is a good conductor of heat and electricity. Used in lead pencils, electrical machines and as lubricant for heavy machines. Also used as a moderator in nuclear reactors.

 

Coal

  • Over long periods of time, trees, bones get buried under the ground by violent geological changes. As a result of chemical reaction with clay sand, water, etc., these get transformed to coal in nature. This process is known as carbonisation. Due to this process, we get substances like peat, lignite, bituminous (soft) and anthracite (hard), depending upon degree of carbonisation.

 

Organic Compounds

  • Constitute substances like petroleum, coal, food components (protein, fats, carbohydrates, vitamins), anesthetics, antiseptic, antibiotics, cotton, wool, silk, synthetic fibres.

 

Hydrocarbons

  • These are compounds of hydrogen and carbon viz. methane, ethane, propane, butane, benzene. Hydrocarbons are found abundantly in nature viz., petroleum, natural gas, coal, etc. From these natural hydrocarbons or parent organic compounds many other organic compounds can be derived.

 

Petroleum

  • It is a hydrocarbon compound extracted from the ground or sea bed by deep drilling. Petroleum is then refined in distilleries and converted into petroleum products such as petrol, diesel, grease, lubricating oil. The residue of this is used for making man-made fibre like nylon, terylene, plastic, etc. and a wide range of drugs. Natural gas, CNG are another range of products made of hydrocarbon compounds.

 

Noble gases

  • Noble gases are gaseous elements and are also known as rare gases as these are of low amount in the atmosphere. They are called noble gases because of their chemical inertness. The noble gases are: helium, neon, argon, krypton, xenon and Radon, of course, is not present in the atmosphere. However, it is produced in the radioactive decay of radium. Helium is present in Sun's atmosphere as well as in natural gas up to maximum of 10%.
  • Noble gases are colourless and odourless and are exclusively used for making inert atmosphere in welding and cutting, inside electric bulbs and in metallurgical operation. In liquefied form, the natural gases are used for creating low temperature.

 

Halogens

Halogens are the four elements, viz., fluorine, chlorine, bromine and iodine. Fluorine and chlorine are gases, bromine is a volatile liquid and iodine is a volatile solid. Halogens are highly reactive. They do not occur in nature in free state but only as compounds. Their colours are:

Fluorine             -           Pale yellow

Chlorine -           Yellowish green

Bromine -           Reddish brown or orange

Iodine                -           Violet black

 

Chlorine

  • It is widely used in drinking water supply as germicide. It is also used for manufacturing bleaching powder, disinfectants, hydrochloric acid and many organic compounds.

 

Sulphur

  • It is a non-metallic element. It burns with a blue flame emitting sulphur dioxide. It occurs in many allotropic forms, as the elements in many volcanic regions and as sulphides of many metals. It is employed in vulcanising rubber, manufacturing of dyes and chemicals in medicines and for killing moulds as well as pests.

 

Phosphorus

  • It is necessary for life. It too occurs in many allotropic forms, mainly as calcium phosphate. White phosphorus is very inflammable and poisonous solid. Its compounds are employed as fertilisers and detergents.

 

Silicon

  • It is a non-metal, found abundantly in earth's crust and in rocks as silica or silicates. Semi-conductors are made of pure elements of silicon. As silicates, it is widely used for manufacturing glass. It is used in alloys.

 

Alkalis

  • Bases soluble in water are called alkalis, viz. sodium hydroxide and potassium hydroxide. They have a soapy touch, bitter taste and turn red litmus to blue and yellow turmeric powder (Haldi) to brown.

 

Uranium

  • Its main ore is pitchblende. It is a radioactive metal, occurring in nature, comprising largely of two isotopes: 99.3% of U-238 and 0.7% (U-235). The isotope (U-235) has the capacity of sustaining a nuclear chain reaction and is used in nuclear reactors and nuclear weapons.

 

Thorium

  • It is a dark grey radioactive metal used in alloys and as a source of nuclear energy. Its compounds occur in monazite and thorite.

 

Plutonium

  • It is a transuranic element (element having atomic number more than 92) which do not occur in nature but may be obtained by nuclear reaction. It is radioactive. The isotope \[({}_{94}^{239}\text{Pu})\] is produced in nuclear reactors and is of great importance as it undergoes nuclear fission when bombarded by slow neutrons. This isotope is employed in nuclear weapons.

 

Iron

  • It is extracted from its ores using blast furnace. Iron obtained from blast furnace is called pig iron or cast iron containing about 5% carbon. Pure iron is called wrought iron which does not contain carbon more than 0.2%, or any other impurities or constituents. Wrought iron is soft, malleable, and easy to work on. It is used for making chains, wires, furniture, items of decoration and electromagnets.

 

Copper

  • It is a metal elements, malleable, ductile and best conductor of electricity after silver. It is widely used for making electrical wires and in steam boilers being not affected by water or steam. It is used for alloying, e.g., bronze, brass, gunmetal, German silver, bell metal, Dutch metal, etc.

 

Zinc

  • It is a metal element, bluish white in colour. It occurs as calamine, zincite and zinc blende. It is used for alloying, e.g., brass and in galvanising iron.

 

Aluminium

  • It is a metal element, light white in colour, occurring widely in nature in clays, extracted mainly from ore: bauxite. It is ductile, malleable, can be drawn into wire, widely used in electric transmission and distribution, being a good conductor of electricity. It is quite light. It is also used as an alloy. Alloyed Aluminium is extensively used in manufacturing utensils, electrical apparatus, automobile engines, pistons and also in aircrafts.

 

Silver

  • It is a metal element, soft, white malleable, best conductor of electricity. It is used in jewellery and coins. Its compounds are used in photography.

 

Gold

  • It is a metal elements, bright yellow, soft, malleable, non-corrodible by air and unaffected by most acids, but dissolves in aqua regia. It is alloyed with silver and copper. It is the best conductor of electricity but is not used being very costly. Gold, particularly in India, is extensively used for jewellery and dentistry. Its compounds are employed in photography and medicines. It is also used for surface plating.

 

Potassium

  • It is a metal used extensively in the form of various salts which are further used as fertilisers. It is necessary for life and is found in all living matter.

 

Calcium

  • It occurs in nature in the form of calcium sulphate (gypsum) and calcium bones and teeth. Some of its compounds are used in industry.

 

Magnesium

  • It occurs as magnesite, dolomite, carnallite as well as in many compounds. Its compounds are used in medicine. It is used in light weight alloys, in photography and incendiary bombs. It is essential to life also as it occurs in chlorophyll.

 

Mercury

  • It is a silver white, liquid form metal, widely used in thermometers, barometers, manometers and many scientific apparatus. Its compounds are poisonous and are used in medicines. It is a very heavy metal. Its specific gravity is 13.6.

 

Notes- Elements and Its Compounds


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