WBCS Preliminary(Chemistry): Some Common Elements & Compounds

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  1. Hydrogen: Symbol H, formula H2. The first element in the periodic table and the most basic and common of all elements in the universe. Over ninety percent of all the atoms in the universe are hydrogen atoms and they are the lightest of all elements. The name hydrogen comes from the Latin word "hydro" which means water. Scientists use the letter "H" to represent hydrogen in all chemical equations and descriptions.

    • Hydrogen atom has one electron in its valence shell like alkali metals.
    • Hydrogen generally shows + 1 valency like alkali metals.
    • Hydrogen is a good reducing agent like other alkali metals.
    • The isotopes of hydrogen: Protium has an atomic number 1, and mass number 1, Deuterium, has an atomic number 1, and mass number 2 and Tritium has an atomic number 1, and mass number 3.
    • It has a vapour density of 1, which is 14.4 times lighter than air.
  2. Carbon: The sixth element in the periodic table. It is a very stable element. Because it is stable, it can be found in many naturally occurring compounds and by itself. Scientists describe the three states of carbon as diamond, amorphous, and graphite.
    • Carbon exhibits allotropy and shows maximum catenation.
    • Normal valency of carbon is four due to the presence of four valence electrons. Thus all four bonds are generally covalent.
    • Carbon occurs both in free state as diamond, coal etc. and also in the combined form as CO2.
    • Diamond is one of the allotropic forms of carbon and is the purest form of natural carbon. It is the hardest natural substance. Diamond is a giant framework that forms a rigid structure with no free electrons to conduct electricity.
    • Graphite is also an allotropic form of carbon, which is very soft and slippery. Graphite has a mobile cloud of electrons on the horizontal planes, which makes it a good conductor of electricity.
    • Apart from diamond and graphite, which are crystalline forms of carbon, all other forms of carbon are amorphous allotropes of carbon. Destructive distillation of coal gives products like coal gas, gas carbon, coal tar and ammonical liquor.
    • Lamp Black is also known as Soot. Soot is obtained by the incomplete combustion of carbonaceous, fuels, especially oil fuels, in limited supply of air. The soot settles on the cooler parts of the chamber, and can be collected by scrapping it.
    • Wood charcoal is obtained by the destructive distillation of wood. The chief products formed are wood charcoal, wood tar, pyroligneous acid and wood gas .
    • Sugar charcoal can be obtained by dehydrating cane sugar, either by treating it with concentrated sulphuric acid or by heating it in the absence of air.
    • Bone charcoal is a black powder called as 'ivory black'. It is porous and can adsorb colouring matter. It is mostly used in sugar industry to decolourise sugar.
  3. Nitrogen: It is the seventh element of the periodic table located between carbon and oxygen. Almost eighty percent of Earth's atmosphere is made of nitrogen gas. Nitrogen is a clear gas that has no smell when it is in its pure form. It is not very reactive when it is in a pure molecule, but it can create very reactive compounds when combined with other elements including hydrogen (ammonia). There are 7 electrons in a nitrogen atom.
    • Nitrogen has 5 electrons in its valence shell. It has a valency of 3 with respect to hydrogen and a valency upto 5 with respect to oxygen.
    • In the laboratory nitrogen is prepared by the action of heat on a mixture of ammonium nitrite and ammonium chloride. Nitrogen is collected by the downward displacement of water and is called chemical nitrogen.
    • Nitrogen is a neutral gas and is neither combustible nor a supporter of combustion.
  4. Oxygen :Symbol O, formula O2. Alone, oxygen is a colorless and odorless compound that is a gas at room temperature. Oxygen molecules are not the only form of oxygen in the atmosphere; you will also find oxygen as ozone and carbon dioxide. There are 8 electrons in an oxygen atom. In the laboratory oxygen is usually obtained by heating a mixture of potassium chlorate and manganese dioxide. Manganese dioxide facilitates the decomposition of potassium chlorate, but it itself remain unchanged in mass and composition and hence acts as a catalyst in the reaction. Oxygen is non-combustible but a good supporter of combustion. An oxide is a compound of two elements, one of which is oxygen. It can be liquefied and solidified. It is employed in welding process and also used in hospitals for artificial respiration. Oxygen shows a valency of -2.
  5. Chlorine: Chlorine belongs to group VII A. Members of this group are called halogens which means 'salt producers'. Chlorine has seven electrons in its outer most shell and so has a valency of 1. Chlorine is prepared by the oxidation of concentrated hydrochloric acid using oxidising agents like manganese dioxide, lead dioxide, trilead tetra oxide, potassium permanganate and potassium dichromate. Chlorine is a non combustible gas but supports the burning of certain metals and non-metals. Chlorine is highly reactive. It reacts with hydrogen, other non metals and metals to form the corresponding chlorides. Chlorine being an acidic gas turns moist blue litmus paper to red and then bleaches it.
  6. Water( H2O):
    • Water is the only substance that can exist simultaneously in all the three states of matter, i.e., solid, liquid and gaseous on this earth.
    • Pure water is a colourless, odourless and tasteless liquid.
    • The density of water is 1 g cm-3 at 4oC.
    • The boiling point of water is 100oC at a pressure of 760 mm of Hg. The melting point of ice is 0oC at a pressure of 1 atmosphere.
    • Ice has a relative density of 0.92. The specific heat capacity of water is 1 cal/g at 15oC.
    • Water is called the "Universal Solvent". Almost all substances dissolve in water to a certain extent. Hence, it known as a universal solvent. Because of this property, it is impossible to get chemically pure water on the earth.
    • Metals such as gold, silver, copper, tin, etc. do not react with water. Ordinary iron gets rusted and aluminium gets tarnished.
    • Water is described as being 'hard' if it does not lather readily with soap. 'Soft water', on the other hand, is described as the one, which lathers readily with soap. Chemically, natural water is never pure and contains varying amounts of the dissolved impurities absorbed from the natural or man made environment. Temporary hardness and permanent hardness are the two types of hardness occurring in hard water: Water is said to be temporarily hard when it contains bicarbonates of calcium and magnesium (or hydrogen carbonates). This type of hardness can be easily removed by boiling. Water is said to be permanently hard when it contains sulphates and chlorides of calcium and magensium. Water becomes permanently hard when it passes over the rocks, which contain sulphates or chlorides of calcium and magnesium to form insoluble calcium bicarbonates or magnesium bicarbonates (or hydrogen carbonates). This hardness cannot be removed by boiling.
    • Heavy water is prepared either by prolonged electrolysis or by fractional distillation of ordinary water. Heavy water(D2O) is colourless, tasteless and odourless liquid. It has all higher values for physical constants than the corresponding values of ordinary water. Fission in uranium-235 is brought by slow speed neutron. Heavy water is used for this purpose in nuclear reactors as moderators.
  7. Ammonia(NH3):
    • Ammonia is present in atmospheric air and in natural water in trace amounts. However in sewage water, it is present in greater proportion. Ammonia is present in the combined form as various ammonium salts. The two most popular salts are ammonium chloride and ammonium sulphate.
    • Ammonia is generally obtained from Ammoniacal liquor obtained by the destructive distillation of coal, destructive distillation of nitrogenous organic matters such as horns, hoofs, bones etc. of animals, Ammonium salts.
    • In the laboratory, ammonia is usually prepared by heating a mixture of ammonium chloride and slaked lime in the ratio of 2 : 3 by mass.
    • Ammonia is a colorless gas. Its vapor density is 8.5. Hence it is lighter than air (vapor density of air = 14.4). When cooled under pressure ammonia condenses to a colorless liquid, which boils at -33.4oC. When further cooled, it freezes to a white crystalline snow-like solid, which melts at -77.7oC. Ammonia is one of the most soluble gases in water. At 0oC and 760 mm of Hg pressure one volume of water can dissolve nearly 1200 volumes of ammonia. This high solubility of ammonia can be demonstrated by the fountain experiment. Ammonia is neither combustible in air nor does it support combustion. However it burns in oxygen with a greenish-yellowish flame producing water and nitrogen. Ammonia reacts with the acids to form their respective ammonium salts. Ammonia is highly soluble in water and forms ammonium hydroxide.
  8. Hydrochloric Acid(HCL):
    • Hydrochloric acid is prepared by dissolving hydrogen chloride gas in water. Hydrogen chloride is a covalent compound, but when dissolved in water it ionizes to form hydrogen ions and chloride ions
    • Hydrochloric acid is produced along with the industrial preparation of caustic soda (sodium hydroxide). During the electrolysis of sodium chloride, large quantities of hydrogen and chlorine gas are obtained as by-products. These two gases are burnt to form hydrogen chloride gas. The hydrogen chloride gas so formed is dissolved in water to form hydrochloric acid. A saturated solution of the acid has a density of 1.2 g cm-3. It contains about 40% by mass of hydrogen chloride.
      1. It turns litmus paper from blue to red.
      2. It turns methyl orange from yellow to pink.
      3. It reacts with metals to form their respective chlorides and liberates hydrogen.
      4. It reacts with bases to form their respective chlorides and water.
      5. It combines with carbonates and hydrogen carbonates to form their respective chlorides and liberate carbon dioxide.
      6. Hydrochloric acid is used in the production of dyes, drugs, paints, photographic chemicals and in the preparation of aqua-regia for dissolving metals like gold and platinum.
  9. Nitric Acid(HNO3):
    • Nitric acid is produced in large quantities in the atmosphere during thunder storms. It is manufactured by the Ostwald's Process by the reaction of ammonia and air in presence of platinum as catalyst at 700-800o C.
    • Nitric acid is colourless in pure form. Commercial nitric acid is yellowish due to the presence of dissolved nitrogen dioxide.
    • Pure nitric acid is not very stable. Even at ordinary temperature, in presence of sunlight it undergoes slight decomposition. As the temperature increases, the rate of decomposition also increases. On strong heating it decomposes completely to give nitrogen dioxide, water and oxygen.
    • Nitric acid is a strong monobasic acid. It ionizes in water readily.
    • Nitric acid usually does not behave as an acid, with metals to form the corresponding salt and liberate hydrogen. However, magnesium and manganese are the only two metals, which react with cold and very dilute (1%) nitric acid to evolve hydrogen.
    • Nitric acid is a strong oxidizing agent. When it undergoes thermal decomposition, it yields nascent oxygen
  10. Sodium(Na):
    • Sodium belongs to Group I in the periodic table. This group is otherwise known as the alkali metals group. Since the atomic number of sodium is 11, its electronic configuration is 2,8,1. Sodium easily loses the lone electron to attain the stable configuration of neon. Therefore alkali metals like sodium that are univalent can easily form ionic compounds.
    • Since alkali metals like sodium are highly electropositive (tendency to lose an electron and become a cation), their carbonates and bicarbonates are highly stable to the action of heat.
    • Some of the important sodium compounds are:
      1. Sodium Carbonate (Na2CO3): Popularly known as washing soda or soda ash, sodium carbonate is a commercially important compound.(a) Transparent crystalline solid with ten molecules of water per molecule. (b) Soluble in water. (c) Washing soda solution is alkaline due to hydrolysis.(d) Has detergent or cleansing properties. (e) Sodium carbonate is used as washing soda in laundry as a cleansing agent, for softening hard water, in manufacturing glass, paper, soap and caustic soda.
      2. Sodium Bicarbonate (NaHCO3): Sodium bicarbonate is commonly called baking soda. Sodium bicarbonate is prepared in the laboratory by saturating a cold solution of sodium carbonate with carbon dioxide. (a) Sodium bicarbonate separates as white crystals. This is because it is very sparingly soluble in water. (b) Sodium bicarbonate is sparingly soluble in water.(c) Used in the preparation of carbon dioxide. (d) Used as a constituent of baking powder, and in effervescent drinks. Baking powder has sodium bicarbonate and tartaric or citric acid. When it is dissolved in water or heated carbon dioxide is produced. This carbon dioxide gas causes the puffiness and lightness of cakes, biscuits etc.(e) Sodium bicarbonate is used to extinguish fire as it produces carbon dioxide gas.
  11. Calcium(Ca):
    • The elements of Group II like calcium are called the alkaline earth metals. The atomic number of calcium is 20 and its configuration is 2,8,8,2. Calcium loses two electrons and becomes Ca2+ ion with the stable configuration of argon. Calcium is therefore bivalent in nature.
    • Some of the important calcium compounds:

      1. Bleaching Powder (CaOCl2):(a)Calcium oxychloride is the chemical name of bleaching powder.(b) Passing chlorine gas over dry slaked lime (Ca(OH)2), gives bleaching powder.(c) It is soluble in water. The lime present is always left behind as an insoluble salt. For this reason it is also called chloride of lime.(d) Bleaching powder is commonly used for bleaching clothes. It is also used in bleaching wood pulp in the paper industry, to disinfect drinking water, to manufacture of chloroform (CHCl3), an anaesthetic
      2. Plaster of Paris (CaSO4) 2.H2O: Chemically, plaster of paris is known as calcium sulphate hemihydrate (hemi means half). When gypsum is heated to 120oC, it loses 75% of its water of crystallization to form plaster of Paris. It is a white powder. When mixed with water, it forms a plastic mass. After about half an hour, this mass sets into a hard solid mass constituting interlaced gypsum crystals. Plaster of Paris is used to set fractured bones due to its setting property on hydration, as a sealant in laboratories, manufacture of black-board chalk.

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