Water (H2O)

Physical properties

• The hydrogen bonding between the water molecules leads to its high freezing point, high boiling point, high heat of vaporization and fusion.
• Covalent compounds dissolve in water due to hydrogen bonding with polar molecules.

Structure of water

1. In gas phase:

• Bent molecule with bond angle 104.5˚
• O-H bond length: 95.7pm
• Highly polar molecule
  
  

  

2. In liquid state:

• Hydrogen bonds between molecules are present in the liquid state
  

  

3. In solid state:

• Water in solid state exists as ice.
• Due to the intermolecular interactions it has a highly ordered but loose 3-D structure
• Each oxygen atom is surrounded by four hydrogen atoms. Two of these hydrogen atoms are covalently bonded to the oxygen atom, and the two others (at longer distances) are bonded to the oxygen atom’s unshared electron pairs by hydrogen bond
• This open structure of ice causes its density to be less than that of the liquid state, in which the ordered structure is partially broken down and the water molecules are (on average) closer together

Chemical properties

1. Water is amphoteric. Its pH is 7.

• Acts as acid in a basic medium
  H₂O + NH₃ → OH- + NH+₄
• Acts as base in acidic medium
  H2O + H2S → H3O+ + HS-
• Water also undergoes self-ionization
  H2O + H2O → H3O+ + OH-
  

2. Water is reduced by highly electropositive metals

2H2O + Mg → Mg (OH)2 + H2

3. Water as a solvent

1. Hydrophylic Molecules

• Substances that dissolve readily in water are termed hydrophilic.
• They are composed of ions or polar molecules that attract water molecules through electrical charge effects.
• Water molecules surround each ion or polar molecule on the surface of a solid substance and carry it into solution.
• Ionic substances such as sodium chloride dissolve because water molecules are attracted to the positive (Na+) or negative (Cl-) charge of each ion.
• Polar substances such as urea dissolve because their molecules form hydrogen bonds with the surrounding water molecules.

2. Hydrophobic Molecules

• Molecules that contain a preponderance of nonpolar bonds are usually insoluble in water and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain many C-H bonds.
• Water molecules are not attracted to such molecules as much as they are to other water molecules and so have little tendency to surround them and carry them into solution.

Water is classified based on reaction with soap solution.

1. Soft water
2. Hard water: Hardness of water is due to the presence of calcium and magnesium salts

	Hard Water	Soft Water
Contains	Minerals such as calcium and magnesium	Sodium
Reaction with soap	Film	Suds
Problems	Leaves deposit called “scale”	None
Lather formation	Doesn't form lather with detergents	Forms lather with detergents
Removed by	permutit process, by exchange of ions	none

    Hardness is of two types:

1. Temporary hardness
2. Permanent hardness

Temporary hardness:

• Temporary hardness is caused by Ca and Mg Bicarbonates.
• Temporary hardness can be removed by
  Boiling: the precipitates are removed by filtering
  
• Clark's process (Treating with Ca(OH)2) – forms CaCO3 and Mg(OH)2 as precipitates

Permanent hardness:
Permanent hardness is caused by Ca and Mg chlorides and sulphates
Permanent hardness can be removed by:
a. Treating with washing soda (Na2CO3) – forms insoluble carbonates
b. Permutit process (Na2Al2Si2O8.xH2O),

• Permutit or zeolites or sodium aluminium silicates are insoluble in water and have the property of exchanging ions present in them with the ions present in the solution.
• Calcium and magnesium ions present in hard water are exchanged with sodium ions in the permutit (Na + Z). The outgoing water contains sodium salts, which do not cause hardness

CaCl₂ + 2 Na⁺ Z⁻ → CaZ₂ + 2 NaCl

CaSO₄ + 2 Na⁺ Z⁻ → CaZ₂ + Na₂SO₄

1. Calgon's process ((NaPO3)6 ≡ Na4[Na2(PO3)6])

• Calgon ionizes to give a complex anion: Na₄P₆O₁₈²⁻
• The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon.
• This results in the removal of calcium and magnesium ions from hard water in the form of a complex with Calgon. The water is softened and sodium ions are released into water.

Ca²⁺ + Na₄P₆O₁₈²⁻ → 2 Na⁺ + CaNa₂P₆O₁₈²⁻

2. Organic molecules having acidic or basic groups are known as Ion-exchange resins.

• Acid resins contain the acid group (- COOH).
• Acid resins exchange their H+ ions with other cations such as Ca²⁺, Mg²⁺, etc., present in hard water. Acid resins are, therefore known as base-exchange resins
  2RCOO⁻H⁺ + Ca²⁺ → (RCOO) ₂Ca + 2H⁺
• Basic resins exchange their OH-ions with the other anions such as HCO3, Cl-, SO₄²⁻ present in hard water. Basic resins, therefore, are also known as acid exchange resins.
  2RNH₃⁺OH⁻ + SO₄²⁻ → (RNH₃)₂SO₄ + 2OH⁻
  RNH₃⁺OH⁻ + Cl⁻ → RNH₃Cl + H⁺