Introduction to Water

Water

Introduction to Water

Molecular formula = H₂O

Molecular mass = 18

Melting point = 0°C

Boiling point = 100°C.

Water is a very common substance and is the monoxide of hydrogen. It only feebly dissociates to hydrogen and hydroxyl ions. It exists in all the three states: ice (solid), water (liquid) and steam (gas).

Anomalous Behavior of water

Water exhibits many unusual properties.

1. It has specific heat, the high latent heat of fusion and latent heat of vaporization and high dielectric constant.
2. Its melting point and boiling point are abnormally higher than those of hydrides of other members of group VIA.
3. It has the higher density than ice. This means ice floats on water and this useful for aquatic lives. It has a maximum density at 4°C (1 gm/cc).

The anomalous property of water can be well understood in light of its structure.

Due to the strongly electronegative character of the oxygen atom, the water molecules is highly polarized. Therefore, there's formation of inter-molecular hydrogen bonding between the oxygen of one water molecule and hydrogen of another water molecule. This bonding leads to the association of several water molecules both in the liquid and solid states. In the absence of hydrogen bonding, water would exist in the gaseous state like H₂S. Due to hydrogen bonding, water is in polymeric form i.e. (H₂O)_n. Some extra energy is needed to break the hydrogen bonding and hence hydrogen bonding is the reason for unusual properties of water.

X-ray studies have shown that water molecules in ice are arranged so as to form the loose open cage-like structure with vacant spaces due to the interplay of hydrogen bonding. The oxygen atoms are situated tetrahedrally to one another in this structure. Therefore, when ice is formed the volume increases and then ice has the lesser density than water. When ice melts, some of the hydrogen bonds are broken and its open cage-like structure is partially destroyed causing the water molecules to come closer to each other. This makes the water more compact and denser than ice at this melting point.

In the water molecule, central atom oxygen undergoessp³hybridization. This leads to the tetrahedral shape of the molecule in which two H atoms should lie at two the concerns of the tetrahedron and the rest of two concerns are occupied by two lone pairs of electrons of the oxygen atom. But as lone par-lone pair repulsion is greater than that between lone pair-bond pair and bond pair-bond pair, the tetrahedral shape is distorted. Thus, HOH bond angle decreases from 109.5° (which is a regular tetrahedral bond angle) to 104.5°. So, the water molecule has a distorted tetrahedral structure. It has an angular or bent (V-shaped) structure if atomic nuclei only are considered.

Types of Water

By investigating the dissolved salt ions, water classified as soft water and hard water.

1. Soft water: Water which is free from soluble salt ions like Cl^-, SO₃^{- -}, HCO₃^-, CO₃^{- -} of calcium (Ca^{+ +}) or magnesium (Mg^{+ +}) is called soft water. Soft water easily produces enough lather with ordinary soap. For example rain water, distilled water and demineralized water etc.

2. Hard water: Water containing dissolved salt ions like SO₄^--, Cl^-, HCO₃^- of calcium (Ca⁺⁺) or magnesium (Mg⁺⁺) which does not produce enough lather easily with soap is called hard water.

How is hard water formed?

The property due to which water is unable to produce lather with soap is known as the hardness of sulphate (SO₄^{- -}) and bicarbonate (HCO₃^-) of calcium and magnesium. During rainfall, CO₂ of air dissolves in water to form carbonic acid.

$$CO_2+H_2O\longrightarrow{H_2CO_3}$$

As the water containing carbonic acid flows on the surface of the earth, it reacts with CaCO₃ present in the rock to produce bicarbonate salts.

$$CaCO_3+H_2CO_3\longrightarrow{Ca(HCO_3)_2}$$

Similarly, water is contaminated with MgSO₄, MgCl₂, CaSO₄ and CaCl₂ salts when it passes over the beds of rocks.

Why is hard water unable to produce enough lather?

Soap contains sodium salt of higher fatty acid like stearic acid, palmitic acid, oleic acid etc. The general formula of the soap is given by RCOONa. When hard water comes in contact with soap, calcium or magnesium salts of fatty acid are formed which are insoluble in water.

$$2C_{17}H_{35}COONa+CaSO_4\longrightarrow{(C_{17}H_{35}COO)_2Ca+Na_2SO_4}$$

$$2C_{15}H_{31}COONa+MgCl_2\longrightarrow{(C_{15}H_{31}COO)_2Mg+2NaCl}$$

Until the Ca^{+ +} and Mg^{+ +} ions are precipitated, no lather is produced with the soap and a lot of soap is washed before getting lather from the hard water.

Types of Hardness of Water

By knowing the nature of dissolved ions, hard water is classified as temporary hard water and permanent hard water.

Temporary Hardness

The hardness caused by dissolution of soluble bicarbonates of calcium or magnesium in water is called temporary hardness can be removed simply by heating. So, such type of hardness is called temporary hardness.

Permanent Hardness

The hardness caused by dissolution of soluble sulphates and chlorides of magnesium or calcium in water is called permanent hardness. This type of hardness can't be removed by the simple method and needs special chemical methods. Therefore, such type of hardness is called permanent hardness.

Methods of Removal of Hardness

Removal of Temporary Hardness

The temporary hardness of water is due to the presence of soluble bicarbonates of calcium and magnesium. It can simply be removed by 1) boiling and 2) Clark's method.

1. By boiling: When water containing bicarbonates of calcium or magnesium is heated, insoluble CaCO₃ or MgCO₃ are formed which can be removed by the filtration process.

$$Mg(HCO_3)_2\xrightarrow\Delta{MgCO_3+H_2O+CO_2}$$

$$Ca(HCO_3)_2\xrightarrow\Delta{CaCO_3+H_2O+CO_2}$$

2. By Clark's method:It is a simple chemical method developed by Clark.

When lime water is added to the temporary hard water, insoluble carbonate with precipitate out. The precipitate is removed by the filtration process.

$$Mg(HCO_3)_2+Ca(OH)_2\longrightarrow{MgCO_3↓+CaCO_3↓+2H_2O}$$

$$Ca(HCO_3)_2+Ca(OH)_2\longrightarrow{2CaCO_3↓+2H_2O}$$

The use of an excess of lime water will reverse the path of reaction by absorbing CO₂ from the atmosphere.

Removal of Permanent Hardness

There are two chemical methods to soften the permanent hard water which are described below.

1. Using washing soda: When calculated amount of washing soda (Na₂CO₃) is added to the permanent hard water, chlorides or sulphates of calcium or magnesium change into insoluble carbonates of calcium or magnesium as well as soluble sodium chloride (NaCl) and sodium sulphate (Na₂SO₄) salts. These soluble salts are separated by the distillation process.

$$MgCl_2+NaCO_3\longrightarrow{MgCO_3↓+2NaCl}$$

$$MgSO_4+Na_2CO_3\longrightarrow{MgCO_3↓+Na_2SO_4}$$

$$CaCl_2+Na_2CO_3\longrightarrow{CaCO_3↓+2NaCl}$$

$$CaSO_4+Na_2CO_3\longrightarrow{CaCO_3↓+Na_2SO_4}$$

Thus, Ca^{+ +} or Mg^{+ +} ions are removed as a residue.

2. By ion-exchange method: This method is quite useful and more reliable method. The principle of this method is simple. The ions which are responsible for the hardness of water are exchanged by certain less damaging ions present in some chemical compounds called ion-exchangers which may be an organic or inorganic compounds.

a) Using inorganic ion exchangers (permutit method): Hydrated sodium aluminum silicate, Na₂Al₂Si₂O₈.xH₂O, is a large molecule or complex compound, known as permutit or zeolite complex.

When hard water containing CaSO₄, CaCl₂, MgSO₄ or MgCl₂ is treated with permutit compound, sodium ion of the compound is exchanged by Ca^{+ +} or Mg^{+ +} ions of hard water to produce calcium or magnesium aluminum silicate and soluble NaCl or Na₂SO₄ salts. Thus, formed calcium or magnesium aluminum silicates are insoluble which are left in the tank as a residue. Soluble NaCl and Na₂SO₄ are separated by the distillation process.

$$CaCl_2+Na_2Z\longrightarrow{CaZ↓ +2NaCl}$$

$$MgSO_4+Na_2Z\longrightarrow{MgZ↓+Na_2SO_4}$$

Regeneration of permutit: After some time, the whole of the Na2Al2Si2O8 gets changed into CaAl2Si2O8 or MgAl2Si2O8 and the sodium ions are regenerated by adding 10% NaCl solution to continue the reaction.

$$CaAl_2Si_2O_8+2NaCl\longrightarrow{Na_2Al_2SiO_8+CaCl_2}$$

$$MgAl_2Si_2O_8+2NaCl\longrightarrow{Na_2Al_2Si_2O_8+MgCl_2}$$

b)Using organic ion-exchangers:This is more advanced method than the permutit process in the method, big organic molecules having high molecular mass, permeable molecular structure and acidic groups (-COOH, -SO₃H) or basic groups (-OH, -NH₂) are attached. These are known as ion-exchange resins. These ion-exchange resins are superior to zeolites because they can remove all kinds of dissolved ions in water. The resulting water is known as deionised (demineralized) water.

The ion exchange resins which contain replaceable H⁺ ions are called cation exchange resins. For example, resins- COOH, resin- SO₃H. Another types of synthetic organic ion-exchange resins which contain replaceable -OH group are called anion exchange resins. It is represented by resin- OH. When hard water is added to the tank containing the ion-exchange resin (resin-H), the cations of the water are exchanged by hydrogen of the resin-H.

$$CaCl_2+2resin\;H\rightleftharpoons{(resin)_2Ca+2H^++2Cl^-}$$

$$MgSO_4+2resin\;H\rightleftharpoons{(resin)_2\;Mg+2H^++SO_4^{-\;-}}$$

The above reactions are reversible. To make it irreversible water is flown down the column. Ca⁺⁺ & Mg⁺⁺ are trapped by the resin. The H⁺, Cl^- and SO₄^{- -} ions pass into another tank which contains anion exchange resin. (resin-OH). The resin_OH reacts with free SO₄^{- -} and Cl^- ions liberating free OH^- ions.

$$Cl^-+resin-OH\rightleftharpoons{resin-Cl+OH^-}$$

$$SO_4^{-\;-}+2resin-OH\rightleftharpoons{(resin)_2SO_4+2OH^-}$$

To make the reaction irreversible, water is allowed to flow down the column.

The first two reactions occur in cation exchange tank & the second two reactions occur in anion exchange tank. Thus formed free H⁺ ions and free OH^- ions combine together to produce water.

$$H^++OH^-\longrightarrow{H_2O}$$

Regeneration of resins: As the reaction proceed, all the H⁺ or OH^- ions are consumed. In order to regenerate the resin, the entry of hard water is stopped and dilute HCl is introduced into the cation-exchange tank and dilute NaOH is added to the anion-exchange tank.
$$(resin)_2Ca+2HCl\longrightarrow{CaCl_2+2resin-H}$$

$$(resin)_2Mg+2HCl\longrightarrow{MgCl_2+2resin-H}$$

Similarly,

$$resin-Cl+NaOH\longrightarrow{NaCl+resin-OH}$$

$$(resin)_2+2NaOH\longrightarrow{Na_2SO_4+2resin-OH}$$

Solvent Property of Water

Water has a high dielectric constant value (82). That's why many of the substances are dissociated in it and hence water is known as the universal solvent. In fact, water is a good solvent for ionic substances and poor for covalent substances. As it's polar in nature, it dissolves polar (mainly ionic) substances but cannot dissolve non-polar substances.

1. The solubility of ionic substances: Let us consider the dissolution of an ionic substance, say sodium chloride, in water. When NaCl crystals are placed in water, the partially positively charged hydrogen atoms of polar H₂O molecules surround the negatively charged chloride ions. Similarly, the partially negatively charged oxygen atoms of water molecules surround the positively charged sodium ions a shown below.

In other words, Na+ ions and Cl- ions get hydrated as Na+ (H2O)m and Cl- (H2O)n from the action of NaCl with (n+m) water molecule. This process involves ion-dipole attractive interactions and energy is released, which is called hydration energy. This energy is used to break the lattice of the crystals and the ions pass into solution. If the hydration energy of an ionic solid is grater than the crystal energy (also called lattice energy which is the energy required to bind the ions in crystal) then it will dissolve, otherwise not. Thus PbSO4 is insoluble in H2O because its hydration energy is less than its crystal energy while the hydration energy exceeds the lattice energy in case of NaCl.

2. The solubility of some polar covalent substances: Some polar organic substances like alcohols, sugar, and carboxylic acids dissolve in water as inter molecular hydrogen bonding takes place between the function group of these compounds and polar water molecules.

The non-polar covalent substances like benzene, methane are not soluble in water as these molecules do not interact with water from H-bonds and again the energy released due to interaction is not sufficient enough to overcome the weak Vander Wall's force of attraction existing between the molecules of these covalent substances.

Detergents and Water Pollution

A synthetic detergent which does not form any precipitate with water is one of the major pollutants of water. The alkyl benzene sulphonate from the detergents is non-degradable and causes foaming. The phosphate ion of detergent is an essential plant nutrient whose presence in water promotes the growth of algae. Algae reduce the concentration of the dissolved oxygen of water due to which the growth of aquatic animals is retarded.

Bibliography:

Gewali, Mohan Bikram and Rishi Tiwari. Principles of Chemistry. second edition. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2009.