WATER

TOPIC: WATER

CONTENT

·        Sources, Types, Uses of water

·        Structure of Water.

·         Laboratory Preparation of Water.

·        Test for Water

·        Causes/ Removal of Hardness of Water.

·         Purification of Water for Municipal Supply.

WATER

Water is regarded as a universal solvent, because it can dissolve almost all other substances.

SOURCES OF WATER

The following are the sources of water:

1.             Natural water: - Rainwater, Well water, Spring water and Sea water

2.             Treated water: - Distilled water, Pipe-borne water and chlorinated water.

STRUCTURE OF WATER

In a molecule of water, H₂O, the central atom is Oxygen. Oxygen has the following electronic configuration: 1s² 2s² 2p⁴.

There are two lone pairs electrons in the valence shell of oxygen (2s²2p²) and two unpaired electrons (2p_y¹2p_z¹). Each unpaired electron forms a covalent bond with one electron from a hydrogen atom. The water molecule has two lone pairs and two bonded pairs of electrons in the valence shell of its central atom, thereby satisfying the octet rule for stability.

Ideally, the four electron pairs should be directed towards corners of a tetrahedron. However, when lone pairs of electrons are located near another lone pair, the repulsion between them is so great that they tend to push the two bond pairs of electrons closer together. As a result, the bond angle in water is compressed to approximately 105^o, such that the structure of the water molecule is V-shaped or angular shape.

                                                                            O

                                                                   H              H

LABORATORY PREPARATION OF WATER 

To prepare water in laboratory, dry hydrogen gas is ignited in air. It burns with a faint blue flame to give steam, which will condense on contact with any cold surface to form water.

PHYSICAL PROPERTIES OF WATER

1.   Water boils at 100^oC and freezes at 0^oC

2.   It has a maximum density of 1gcm^-3 at 4^oC

3.   It is neutral to litmus.

CHEMICAL PROPERTIES

1.  Water reacts with electropositive metals to form alkalis and liberate hydrogen gas. E.g

                              Na_(s) + H₂O_(aq)→ NaOH_(aq)+ H_2(g)

  Mg & Zn react with steam to also liberate hydrogen gas

 Mg_(s) + H₂O_(g) → Mg(OH)_2(s) + H_2(g)

       Cu, Au, Ag, Hg do not react with water to form alkaline solution

2.   Non-metal like chlorine reacts with water to form acid solution.

                           H₂O_(aq) + Cl_2(g)→ HCl_(aq) + HOCl_(aq)

TEST FOR WATER

Both white anhydrous copper II tetraoxosulphate (VI) and blue colbalt II chloride paper are used to test for the presence of water. When few drops of water are added to

1. White anhydrous copper (II) tetraoxosulphate (VI), it turns blue and 

2. Blue cobalt (II) chloride, it turns pink.

 

TREATMENT OF WATER FOR MUNICIPAL SUPPLY

The following are the processes of treating river water for town supply

1. Coagulation (or Floculation): Chemicals like potash alum, KAl(SO₄)₂, or sodium aluminate III, NaAlO₂ is added to water in a large settling tank. This causes the large dirt to coagulate or clog together

2. Sedimentation: The coagulated solid particles or flocs are allowed to settle in the settling tank to form sediments at the bottom of the tank.

3. Filtration: The water above the sediment still contains some suspended particles.  The water is passed through a filter bed of layers of sand to remove the remaining fine dirt particles.

4.  Chlorination (Disinfection): Chemicals like chlorine is then added to the water to kill germs. Iodine and fluorine are also added as food supplements to prevent goiter and tooth decay respectively.  The treated water is then stored in a reservoir and distributed to the town.

 

REMOVAL OF TEMPORARY HARDNESS

1.  Physical method: By boiling

Ca(HCO₃)_2(aq) → CaCO_3(s) + H₂O_(l) + CO_2(g)

2.  Chemical method: By using of slaked lime (calcium hydroxide solution)

                   Ca(HCO₃)_2(aq) + Ca(OH)_2(aq) →2CaCO_3(s)  + 2H₂O_(l)

3 Addition of washing soda :-

Ca(HCO₃)_2(aq)  +  Na₂CO_3(aq) → CaCO_3(s)  +  2NaHCO_{3 (aq)}

EFFECTS OF TEMPORARY HARDNESS:  It causes

1. Furring of kettles and boilers.

2. Stalagmites and stalactites in caves.

PERMANENT HARDNESS

Permanent hardness in water is caused by the presence of Calcium and Magnesium ions in the form of soluble tetraoxosulphate (VI) and chlorides (i.e. CaSO₄, MgSO₄, MgCl₂, CaCl₂)

Removal of permanent hardness: 

1. Physical method: by distillation.

By chemical method only

1. Addition of washing soda          Na₂CO_3(aq) + CaSO_4(aq) → CaCO_3(s) + Na₂SO_4(aq)

2.  Addition of caustic soda

  2NaOH_(aq)+ CaSO_4(aq) → Ca(OH)_2(s)+  Na₂SO_4(aq)

3. Ion exchange resin

  CaSO_4(aq) + Sodium zeolite →Calcium zeolite + NaSO_4(aq)

           (insoluble)

ADVANTAGES OF HARD WATER

i.   It taste better than soft water because of the presence of ions

ii. Provides calcium for the development of bones and teeth of animals

iii.  It provides CaCO₃, that crab and snail use to build their shells.

iv. Reduce heart disease

v.  It does not dissolve lead, hence, it can be supplied in lead pipes. (it is less likely to cause corrosion in pipes.

DISADVANTAGES OF HARD WATER

i. It causes furring of kettles and boilers.

ii.  It wastes soap.

iii. It cannot be used in dying and tanning.

iv. Effects is seen in stalactites and stalagmite

v. It cause dry itchy skin

   Objective questions 

1. Treated town water undergoes the following steps except 

(A). coagulation 

(B). precipitation 

(C). sedimentation 

(D). chlorination

 

2.  Water is temporarily hard because it contains 

(A). CaSO₄

(B). MgSO₄

(C). Chlorine 

(D). Ca(HCO₃)₂

 

3. Temporary hardness of water is removed by the use of one of the following 

(A). boiling

 (B). use of use of Ca(OH)₂  

(C). use of Na₂CO₃ 

(D). use of alum

 

4. A substance that turns white anhydrous CuSO₄ blue is

 (A). water 

(B) liquid ammonia 

(C). hydrochloric acid 

(D). molten sulphur

 

5. Distilled water is different from deionized water because 

(A). distilled water is a product of condensed steam while deionized water is filtered laboratory water 

(B). distilled water is always pure and sold in packs while deionized is not packaged for consumption 

(C). distilled water is condensed steam, but deionized water is produced using ion-exchange resins which absorbs undesired ions. 

(D). distilled water is man-made while deionized water is both natural and artificial

6. When a sample of water was boiled, it lathered more readily with soap. it can be concluded that the sample most likely contained 

(a). magnesium and tetraoxosulphate (VI) 

(b). suspended solids 

(c). organic impurities 

(d). calcium hydrogentetraoxocarbonate (IV)

7.  What are the gases associated with the formation of acid rain? 

(a). CO₂ and HCl 

(a)  CO₂ and NO

(c). SO₂ and NO 

(d). which of the following processes will pollute water? 

(a) Exposure of a body of water to ultraviolet rays

(b) Discharge of industrial effluents into waterways

(c). Passage of river water through a sand bed 

(d). Addition of measured quantity of chlorine                                                    

SECTION B

1.     State the steps involved in the treatment of river water for town supply ?

2.     Give two ions that causes hardness of water?

3.      Write two equations to show the removal of permanent hardness of water?

4.      Mention two compounds that causes permanent hardness in water ?

5.      State two ways of removing permanent hardness in water?

6.       List two advantages of hard water and two disadvantages of hard water?