WATER

 

Week:                                                                   Date:                                                     Time:

Period:                                 Duration: 1 HR 20 MIN.                                                  Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                         Class: SS TWO

Topic:               WATER

Sub topic: 

Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials: WATER

Entry behavior: The students have been taught energy change and chemical reaction.

Behavioural objective: At the end of the lesson the students should be able to:

1.       Explain the structure of water

2.       State 4 sources of water

3.       List physical properties of water

4.       Explain hardness of water and their causes

5.       State 4 basic process used for the purification of water

CONTENT

WATER

Water is one of the most common substances known. It is a good solvent for many substances and rarely occurs in its pure form in nature.

STRUCTURE OF WATER

A water molecule consists of two hydrogen atoms and one oxygen atom. The three atoms make an angle, the H-O-H angle is appropriately 104.5o. The centre of each hydrogen atom is approximately 0.0957nm from the centre of the oxygen atom. The structure of a single water molecule is shown below:

 

Because oxygen is more electronegative than hydrogen ( in other word, electrons tend to be in the neighborhood of the oxygen), the hydrogen atoms end up with a partial positive charge and the oxygen atom with a partial negative charge. This separation of charge produces a net dipole moment of the molecule.

SOURCES OF WATER

Sources of water include rainwater, spring water, well water, river water, lake water and sea water.

PHYSICAL PROPERTIES OF WATER

Pure water is a clear, colourless, tasteless, and odourless liquid with the following properties:

I.                    A boiling point of 100o C

II.                  A freezing point of 0o C

III.                Maximum density of 1 gcm-1 at 4o C

IV.                Neutral to litmus

HARDNESS OF WATER

Hard water will not form lather readily with soap. It contains a number of dissolved salt, the most important being calcium tetraoxosulphate (vi), magnesium tetraoxosulphate (vi) , calcium hydrogen trioxocarbonate (iv), magnesium hydrogen trioxocarbonate (iv) and iron (III)chloride.

Types of hard water

There are two types of hard water, viz:

I.                    Temporary hardness                              II. Permanent hardness

Temporary hardness: it is caused by the presence of dissolved  calcium hydrogen trioxocarbonate (iv) [Ca(HCO3)2] which decomposes on heating.

Method of removal of temporary hardness of water

1.       Boiling/distillation - Ca(HCO3)2                                     CaCO3 (insoluble)  + H2O + CO2

2.        Using calculated amount of slaked lime, Ca(OH)2  [Clark’s  process]

Ca(HCO3)2  + Ca(OH)2                CaCO3  +   2H2O

3.       Ca(HCO3)2  + Na2CO3 (washing soda)             CaCO3  +   2NaHCO3

4.       Ion exchange resin / permutit method

Effect of temporary hardness of water

I.                    Furring of kettles and boilers

II.                  Stalagmites and stalactites

Permanent hardness: it is cause by the presence of calcium and magnesium ions in the form of soluble tetraoxosulphate (vi) and chlorides.

Method of removing permanent hardness

This type of the hardness is removed by the use of chemicals. The chemical used are all soluble sodium compounds which will form insoluble precipitates with the calcium and magnesium ions. Na2CO3 (washing soda) , caustic soda (NaOH), and permutit or zeolite are some of the chemical used in the removal of permanent hardness.

                Na2CO3 +   CuSO4 (soluble)               CaCO3 (insoluble)  +  Na2SO4

                NaOH +   MgSO4 (soluble)               Ca(OH)2 (insoluble)  +  Na2SO4

Advantages of hard water

I.                    Hard water taste better than soft water because of the dissolved mineral in it.

II.                  The calcium salt present in hard water when taken in by animals, help to build strong teeth and bones.

III.                It assists animals such as snails and crabs to make their shells.

IV.                Does not dissolve lead pipe

V.                  Helps to reduce heart diseases

VI.                Provides calcium salt used in manufacture of food in plants.

Disadvantages of hard water

I.                    It waste soap             II. It form scum                 III. Cost a lot of money to soften

VII.              Does not easily lather with soap                        V. cause furring of kettles and boilers

V.                  It cannot be used in dyeing and tanning as the salts in the interfere with the modes of action of these processes.

PURIFICATION OF WATER

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids and gases from contaminated water. The goal is to produce water fit for a specific purpose. Most water is purified for human consumption (drinking water). Other purposes include medical, chemicals and industrial purposes.

Basic process that are used for purification of water

·         Aeration (exposure to sunlight)

·         Coagulation / flocculation (add alum)

·         Sedimentation

·         Filtration

·         Disinfection (add chlorine)

·         Add fluoride / iodide

·         Add Ca(OH)2 ( for PH control)

Chemical used for purification of water supply

·         Potash alum KAl(SO4)2 / NaAlO2; chlorine; iodine / iodide; Fluorine / Fluoride ; Ca(OH)2 / CaO; CuSO4.

MANICIPAL WATER SUPPLY

Pipe- borne water is prepared in a water – treatment plant. This water is usually germ-free but it contains mineral solutes like sodium chloride.

Water from rainfall, river or lakes is stored in reservoir. This water is purified by various methods which include: coagulation, sedimentation, filtration and disinfection. The purified water is then distributed to town  and cities via underground pipes for domestic and industrial uses.

The treatment of water to make it fit for our use can be done in the following way: first, the untreated water is passed through large settling tanks where chemicals like potash alum ,KAl(SO4)2 , or sodium aluminate (iii), NaAlO2 are added to cause coagulation or flocculation. The impurities clump together to form particles of dirt or flocs which settle down rapidly. Next, the water is passed through a filter bed to remove the remaining fine particles of dirt. Then the water is treated with chemicals like chlorine to kill germs. Other useful chemicals such as iodine and fluorine, may be added in the correct amounts as food supplements to prevent goiter and tooth decay respectively. Equally calcium oxide (calcium hydroxide) may be added to reduce acidity and remove hardness of water. Finally, the treated water, which is now clear and free from germs, is stored in a reservoir and distributed to the town for use.

PRODUCTION OF DISTILLED WATER

Distilled water is chemically pure water. It is prepared by condensing steam using Liebig Condenser. Water prepared using exchange resin is called deionized water.

Distilled water is used in the laboratory for preparing reagents and analytical work; certain industrial processes; preparation of drugs and car batteries.

I.                    erywhere.          

PRESENTATION

i.The teacher explains water and its structure.

ii. The teacher states sources of water and its physical properties.

iv. The students chorus sources of water and its physical properties.

v.The teacher explains types, effect, advantage and disadvantages of hardness of water.

vi. The teacher explains all the processes in water purification.

vii. The students chorus the basic processes in water purification.

 

EVALUATION

 The teacher evaluates the lessons by asking the following questions:-

1.       Explain the structure of water

2.       State 4 sources of water

3.       List physical properties of water

4.       Explain hardness of water and their causes

5.       State 4 basic process used for the purification of water

 ASSIGNMENT

1.       Explain test of water

2.       List 3 properties of water make it a universal solvent

solution

1. TEST FOR WATER

When a few drops of water are added to:

I.                    White anhydrous copper (ii) tetraoxosulphate (vi) it turn blue.

CuSO4 (white anhydrous)   +  5H2O                       CuSO4 . 5H2O (blue hydrated)

II.                  Blue cobalt (ii) Chloride, it turn pink

CoCl2 (blue anhydrous)   +  6H2O                        CoCl2 .6H2O (pink hydrated)

However, these two tests are not specific for water. They only indicate the presence of water. Any aqueous solution or substance containing water will give a positive test for water.

WATER AS UNIVERSAL SOLVENT

The following properties of water make it a universal solvent:

I.Water is a polar molecule.

II.Water can form hydrogen bond with other polar compound.

III.Water dissolves many substances by reacting with them chemically.

IV.Water is cheap and easily available everywhere.

3a.          State how you would carry out the following procedure in the laboratory:

      i.Remove the sediment in a sample of water.

                ii.Soften temporarily hard water without heating.

                iii.Obtain pure water from muddy water.

4a.          Define hard water.

b.            Name two substances responsible for hardness in water.

c.             Give two methods for the removal of hardness in water.

               

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