Water

WATER

Physical Properties of Water

  • Water is a colorless, odorless, and tasteless liquid.
  • It is a polar molecule, which means it has a partial positive charge on one end and a partial negative charge on the other end.
  • It has a high surface tension, which allows it to form drops and hold onto objects like a bug on a water surface.
  • Water has a high specific heat, which means it can absorb a lot of heat without changing temperature, making it a good coolant.
  • It has a high heat of vaporization, which means it takes a lot of heat to turn liquid water into vapor, making it a good coolant.
  • Water has a high boiling point and freezing point, which makes it a stable solvent.

 

Structure of Water

  • Water is a bent molecule, with an angle of 104.5 degrees between the two hydrogen atoms and the oxygen atom.
  • The oxygen atom has two lone pairs of electrons, which give it a partial negative charge, and the hydrogen atoms have a partial positive charge.
  • The molecule has a dipole moment, which means it has a partial positive charge on one end and a partial negative charge on the other end.

 

Structure of Ice

  • Ice is a solid form of water, with a regular crystalline structure.
  • The water molecules in ice are held together by hydrogen bonds, which form a hexagonal lattice.
  • The hydrogen bonds cause the water molecules to be farther apart in ice than in liquid water, which makes ice less dense than water.
  • Ice has a lower melting point than water, which makes it a useful coolant.

 

 Chemical Properties of Water

 Water reacts with a large number of substances. Some of the important reactions are given below.

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.

 

The auto-protolysis (self-ionization) of water takes place as follows :

 

acid-1 base-2       acid-2 base-1

                        (acid)                (base)              (conjugate acid)        (conjugate base)

 

Redox Reactions Involving Water

Water can be easily reduced to dihydrogen by highly electropositive metals.

Thus, it is a great source of dihydrogen.

Water is oxidised to O2 during photosynthesis.

6CO2(g) + 12H2O(l)  C6H12O6(aq) + 6H2O(l) + 6O2(g)

With fluorine also it is oxidised to O2.

2F2(g) + 2H2O(l)  4H+ (aq) + 4F(aq) + O2(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 viz.,

(i) coordinated water e.g.,

 

(ii) interstitial water e.g., 

 

(iii) hydrogen-bonded water e.g.,

in

 

Hard and Soft Water

Water is classified into two types based on the presence or absence of dissolved minerals and salts, such as calcium and magnesium ions. These ions are responsible for making the water "hard" or "soft".

Hard water

Hard water contains a high concentration of dissolved minerals and salts, primarily calcium and magnesium ions. Hard water is often characterized by a buildup of mineral deposits in plumbing and appliances, such as water heaters and dishwashers. It can also make soap less effective and leave a residue on clothes after washing.

 

Soft water

Soft water, on the other hand, has a low concentration of dissolved minerals and salts, and is often preferred for household use. It tends to be more effective for cleaning and washing, and does not leave mineral buildup in plumbing or appliances.

 

Removal of Temporary Hardness of water

Temporary hardness is due to the presence of magnesium and calcium hydrogen-carbonates. It can be removed by :

 

Boiling

During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg(OH)2 and Ca(HCO3)2 is changed to insoluble CaCO3. It is because of high solubility product of Mg(OH)2 as compared to that of MgCO3, that Mg(OH)2 is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

 

 

Clark’s method

In this method calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

 

 

Removal of Permanent Hardness of water

 It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness is not removed by boiling. It can be removed by the following methods:

 

 

Treatment with washing soda

 Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.

 

Calgon’s method

Sodium hexameta-phosphate (Na6P6O18), commercially called ‘calgon’, when added to hard water, the following reactions take place.

 

The complex anion keeps the Mg2+ and Ca2+ ions in solution.

 

Ion-exchange method

This method is also called zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit. For the sake of simplicity, sodium aluminium silicate (NaAlSiO4) can be written as NaZ. When this is added in hard water, exchange reactions take place.

 

Permutit/zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution.

Synthetic resins method

Nowadays hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process. Cation exchange resins contain large organic molecule with - SO3H group and are water insoluble. Ion exchange resin (RSO3H) is changed to RNa by treating it with NaCl. The resin exchanges Na+ ions with Ca2+ and Mg2+ ions present in hard water to make the water soft. Here R is resin anion.

 

The resin can be regenerated by adding aqueous NaCl solution.

Pure de-mineralised (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the H+ form) and an anion-exchange (in the OH form) resins:

 

In this cation exchange process, H+ exchanges for Na+, Ca2+, Mg2+ and other cations present in water. This process results in proton release and thus makes the water acidic. In the anion exchange process:

 

 

OHexchanges for anions like Cl, HCO3, SO42–etc. present in water. OH ions, thus, liberated neutralise the H+ ions set free in the cation exchange.

 

The exhausted cation and anion exchange resin beds are regenerated by treatment with dilute acid and alkali solutions respectively.