   The air in our atmosphere is composed of molecules of different gases.
   It consists of 78 % nitrogen gas, 21 % oxygen, 0.9 % argon, and 0.03 % carbon dioxide, water
    vapor-floating molecules of water and trace gases.
   Earth"s atmosphere is divided into five main layers.
                             Layers                       Ranges in km
                             Troposphere                  0 to 12
                             Stratosphere                 12 to 50
                             Mesosphere                   50 to 80
                             Thermosphere                 80 to 700
                             Exosphere                    700 to 10000
                                              Some Important Gases
  Gases              Symbol &              Discovered by                 Fact
                     Atomic No.
  Oxygen (O2)        O&8                   Carl Wilhelm Scheele and      Approximately 2/3 of the mass of the
                                           named by Antoine              human body is oxygen.
                                           Lavoisier (1777).             It is the gas we inhale.
  Ozone (O3)         It is a form of       Christian Friedrich           It is blue in color and has a strong odor.
                     elemental             Schönbein                     It is very reactive gas and toxic.
  Nitrogen (N2)      N&7                   Daniel Rutherford (1772)      It is estimated at about seventh in total
                                           and named by Jean-            abundance in our galaxy and the Solar
                                           Antoine Chaptal (1790).       System.
  Carbon             It is a               Joseph Black in 1750          It is the gas we exhale.
  dioxide (CO2)      compound.                                           CO2 as a gas is heavier then air and
                                                                         therefore can be layered as blanket of
                                                                         inert gas.
Water (H2O)
   It can occur in three states: solid (ice), liquid, or gas (vapor).
   It is a good polar solvent and is often referred to as the universal solvent.
   It covers 71% of the Earth"s surface, and is vital for all known forms of life.
   Its boiling point is 1000C and freezing point is 00C.
   It is tasteless and odorless.
   The intrinsic colour of water and ice is a very slight blue hue, although both appear colorless in
    small quantities.
  Types of water             Description                                           pH value          hardness
  Soft water                 It is a pure form of water in which the only ion      5.5 to 7.7        0-60 gm/L
                             is Na+, and a natural source is rain water.
  Hard water                 It is an impure form of water having high             7.8 as 8          > 120 gm/L
                             mineral contents, like Ca, Mg, sulphides,
                             chlorides, bicarbonates etc.
  Heavy water                It contains either one or both the hydrogen           7.43                     -
  D2O                        atoms in water replaced by deuterium (D).             at 25 °C