Structure and properties of water

 Introduction

Water is an essential component of life both biologically and chemically. It plays a vital role in almost every process irrespective of whether it is metabolic, organic, inorganic and physiological in nature. Water plays a crucial role in all the bodily processes occurring inside and outside of the human body. As far as the chemistry of water is concerned; it is a chemical compound consisting of 2 hydrogen atoms and 1 oxygen atom. Water is the term used when it is in its liquid state. However, under certain conditions, water can exist in a supercritical fluid state.

 

Table of Contents

1. Introduction

2. Structure of water

3. Physical properties of water

4. Chemical properties of water

    4.1 Dissociation of water

    4.2 Amphoteric nature of water

    4.3Water as a reducing and an oxidizing agent

5. Difference between hard and soft water

6. Causes of hardness of water

7. Methods of  softening water

    7.1 Boiling

   7.2 Use of washing soda

   7.3 Permutit technique

8. Heavy Water

 

Structure of water

Water is a tiny bent molecule denoted by the symbol H2O. A water molecule has 2 light hydrogen atoms attached to the heavier oxygen atom. In a water molecule, the oxygen is 16-fold heavier than the hydrogen. From the above structure, we can see that the geometry of water is distorted and the H-O bond angle formed is 104.5, which is slightly less than the normal tetrahedral angle of 109.5. The geometry of the water molecule is regarded as angular where each O-H bond is polar due to the high electronegativity of oxygen in comparison to that of hydrogen. In a water molecule, each hydrogen atom has a nucleus with a single positively-charged proton surrounded by a 'cloud' of a single negatively-charged electron. On the other side, the oxygen atom`s nucleus has 8 positively-charged protons and 8 neutrons with neutral charge which are further surrounded by a 'cloud' of 8 negatively-charged electrons. 

 

Physical properties of water

  1. Water is colourless, odourless and tasteless liquid.

  2. At STP(Standard Temperature and Pressure), the water remains in its state of balance (equilibrium) with ice and vapour. This condition is referred to as a triple point.

  3. Water is characterized by a high specific heat. Specific heat is the energy required to change the temperature of a substance.

  4. In its pure form, water has a neutral pH. Hence it is neither acidic nor basic. However, the type of solvents decides the acidity or alkalinity of water. For instance, if you add salt( sodium), it turns acidic as the sodium is acidic.

  5. Water is a good conductor of heat. Except for mercury, no other element can be as good as the water in its conductivity.  

  6. Water molecules exist in liquid form between the range of 0 - 100° Celsius. This allows water molecules to exist in its liquid form in most of the places around the world.

  7. Water is a universal solvent hence it is possible to dissolve most of the chemical compounds by using the solubility factor of water.

  8. Water has a high surface tension hence it is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film.

  9. The table below summarizes some of the specific physical properties of water.

 

Constant

Molecular mass

Melting point in K

Boiling point in K

Heat of fusion

Heat of vaporization

H2O

18.015

273.2

373.2

6.01

40.66

D2O

20.028

276.8

374.4

6.28

41.61

 

 

Chemical properties of water

Dissociation of water: 

At high temperatures, water doesn't dissociate into its elements because of its stability. However, pure water dissociates to form hydroxonium ion and the hydrogen ion as shown below.

 

Amphoteric nature of water:

 An amphoteric substance is the one in which, it can be used both as an acid and a base. This is because of the tendency of the water being in a neutral stage. 

Quick question: Distinguish between hydrolysis and hydration?

Answer: Hydrolysis is the chemical reaction in which H+ and OH- ions of water molecules react with compounds to form products while hydration refers to the addition of one or more water molecules to ions or molecules to form hydrated compounds.

 

 

 

 

 

 

 

 

Water as a reducing and an oxidizing agent: 

Water reacts with sodium metal to form sodium hydroxide and hydrogen gas. I,e  Li (s) + H2O (l)     LiOH (aq) + H2 (g). You can see in this reaction, water acts as an oxidizing agent as it oxidizes lithium to lithium hydroxide while itself is reduced to hydrogen gas. On the other hand, water can be used as a reducing agent. I,e F (g) + H2O (l)     HF (aq) + O2 (g).              

Difference between hard and soft water

Soft water produces enough lather when soap or soap solutions are used while hard water fail to produce lather with the soap solution readily. Examples of soft water include; distilled water, rainwater, and demineralized water. Seawater, river water, and tap water are examples of hard water.

 

Causes of the hardness of water

The primary cause of water hardness is the presence of the bicarbonates, chlorides and the sulfates of group 2A elements such as calcium and magnesium. For instance, both magnesium carbonate and calcium sulfate react with soap to form the corresponding precipitate that hinders the formation of lather. There are two types of hardness of water; Temporary hardness and the permanent hardness.  Temporary hardness is due to the presence of bicarbonates of calcium and magnesium. Temporary hardness is also known as carbonate hardness. Permanent hardness is caused by the presence of chlorides and sulfates of calcium along with the magnesium. Permanent hardness is also known as non-carbonate hardness.

 

Methods of  softening water

Water softening refers to the process of removing the harness from the water. There are several methods that we can use to soften hard water. These include:

 

Boiling

By boiling, the bicarbonates of calcium and magnesium decompose into insoluble carbonates and carbon dioxide. The insoluble carbonates can, in turn, be removed through the filtration process. Boiling is mainly used to remove temporary hardness. The boiling reaction is written as under.

Ca (HCO3)2(S) CaCO3(S) + CO2 (g) + H2O (l)

Mg (HCO3)2(S) MgCO3(S) + CO2 (g) + H2O (l)

 

Use of washing soda

Washing soda is used to remove permanent water. In this process, water is treated with a calculated amount of washing soda which converts the chlorides and sulfates of calcium and magnesium into the respective carbonates which are easily precipitated. The reaction with washing soda is written as under.

Ca (CO3)2(S) + Na2 CO3 (S)  CaCO3(S) + 2NaCl

MgSO42(S) + Na2 CO3 (S)  MgCO3(S) + 2Na2SO4

 

Permutit technique

Permutit technique is one of the modern methods that are used by most water plants to soften hard water. In this process, hydrated sodium aluminium silicate is loosely packed in a huge tank over a layer of coarse sand. Hard water is then introduced into the tank from above. Once the water reaches the bottom of the tank, it slowly rises through the permutite layer in the container. The cations present in hard water are then exchanged for Na+ ions.

 

Heavy Water

The term heavy water is used to refer to the presence of deuterium oxide.  Heavy water is obtained as a by-product in the electrolysis of normal water. It is used as a moderator and coolant in most of the nuclear reactors. It can also be used in the study of the mechanism for chemical reactions.  As a  raw material, heavy water is used in the preparation of deuterium compounds ( such as deuteriosulphuric acid, nitromethane, and deuterioacetylene). 

 





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