Water

The student will be able to explain Water — physical and chemical properties Hardness of water and methods to remove it. Heavy water

Introduction A major part of all living organisms is made up of water. Human body has about 65% and some plants have as much as 95% water. It is a crucial compound for the survival of all life forms. It is a solvent of great importance. The distribution of water over the earth’s surface is not uniform.

Structure of water Gaseous state Solid state

Structure of ice

Chemical Properties Amphoteric Nature: It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the Brönsted sense it acts as an acid with NH3 and a base with H2S.

Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive metals. Water is oxidised to O2 during photosynthesis. 6CO2(g) + 12H2O(l) → C6H12O6(aq) +6H2O(l) + 6O2(g)

Hydrolysis Reaction: Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water. Hydrates Formation From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types. Example- CuSO4.5H2O

Hard water Contains dissolved salts of bicarbonates, sulphates and chlorides of calcium and magnesium. Hard water does not produce lather with soap solution. Soft water Soft water is free from bicarbonates, sulphates and chlorides of calcium and magnisum. It produce lather with soap solution easily. e.g., distilled water, rain water

Types of hardness The hardness of water is of two types (i)Temporary hardness • due to presence of soluble bicarbonates of calcium and magnesium. • can be removed by simple boiling. (ii)Permanent hardness • due to presence of chlorides and sulphates of calcium and magnesium. • requires treatment of water to remove this type hardness.

Temporary hardness is also called carbonate hardness Permanent hardness is also called non-carbonate hardness

Softening of water The process of removal of Ca2+ and Mg2+ ions from water is called softening of water. Boiling: Removal of temporary hardness:- (M = Ca or Mg) Clark’s method or calcium hydroxide method

Removal of permanent hardness (i) By washing soda (Na2CO3.10H2O) treatment

(ii) Calgon’s method Sodium hexametaphosphate (Na6P6O18), commercially called ‘calgon’, when added to hard water, the following reactions take place.

Removal of permanent hardness iii)By using inorganic cation exchanger (permutit method or Zeolite method): The zeolite can be regenerated by treatment with sodium chloride solution.

Removal of permanent hardness (iV) By organic ion exchanger: Cation exchanger

Anion exchanger

Heavy Water (D2O) Was discovered by Urey in 1932. Ordinary water contains one part of heavy water in 600 parts of it. Used as moderator in nuclear reactions. Used as tracer in study of mechanism of physiological process. Used in preparation of deuterium compounds.

Hydrogen economy (Hydrogen as fuel) With advancement of science and technology we realise in order to make our lives comfortable fossil fuels are depleating at an alarming rate and will be exahausted soon. The electricity cannot be stored to run automobiles. It is not possible to store and transport nuclear energy. Hydrogen is another alternative source of energy and hence called as ‘hydrogen economy’. Hydrogen has some advantages as fuel

Hydrogen economy (Hydrogen as fuel) Available in abundance in combined form as water. On combustion produces H2O. Hence pollution free. H2-O2 fuel cell give more power. Excellent reducing agent. Therefore can be used as substitute of carbon in reduction for processes in industry.