WATER at a glance

 

WATER

CONTENT

·     Types, Uses and 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 said to be a universal solvent , because it can dissolve almost all other substances.

SOURCES OF WATER

The following are the sources of water: the sources of water may be grouped into two

1.             Natural water: Rainwater, Well water, Spring water and Sea water, rivers and lakes

2.             Treated water: Distilled water, Pipe – borne water, deionized water and chlorinated water

STRUCTURE OF WATER

Because of the repulsion between the two lone pairs of electrons in the oxygen atom the two bonding pair are pushed towards each other resulting to a V-shape or angular shape or bent shape for water.

 

                     O
                 ∕    \
               H       H

 

LABORATORY PREPARATION OF WATER 

When dry hydrogen gas is lighted in air. It burns with a faint blue flame to give steam, which condenses when it comes in contact with any cold surface to form water.

 

PHYSICAL PROPERTIES OF WATER

1.  Water has a boiling point of  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.

 4. It is a liquid at room temperature

 

CHEMICAL PROPERTIES

1.     Water reacts with electropositive metals like K, Na and Ca to form alkali and liberate hydrogen gas. E.g                   Na_(s)+ H₂O_(aq) →NaOH_(aq) +  H_2(g)

  Mg & Zn react with steam to form an alkaline solution while Cu, Hg, Ag, Au, do not react with water

2.       Non-metal especially the halogens chlorine reacts with water to form acid solution

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

 ii.   H₂O_(aq)  +Br_2(g) →HBr_(aq)  +  HOBr_(aq)

    

TEST FOR WATER

When few drops of water are added to

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

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

NOTE: 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

 HARDNESS OF WATER

Hard water is any water that does not form lather (foam) readily with soap. 

There are two types of hardness / hard water 

I. Temporary hardness or temporary hard water

II. Permanent hardness/ permanent hard water.

I. Temporary hardness is caused by the presence of Ca(HCO3) or Mg(HCO3) in any water sample and these can be removed by boiling the water.

II. permanent hardness is caused by the presence of CaSO4 or MgSO4 or CaCl2 or MgCl2 it can not be removed by boiling

 

REMOVAL OF TEMPORARY HARDNESS

1.  Physical method: By boiling

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

2.      Chemical method: By using  calculated amount 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)+ NaHCO_{3 (aq)}

EFFECTS OF TEMPORARY HARDNESS:  

Hard water causes

1.         Furring of kettles and boilers.

2.        Stalagmite and stalactites in caves.

 

Removal of permanent hardness: 

1. by physical method : Distillation 

2.By chemical method only

i.           Addition of washing soda

    Na₂CO_3(aq) + CaSO_4(aq) →CaCO_3(s) + Na₂SO_4(aq)

ii.   Addition of caustic soda

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

iii.    Ion exchange resin

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

ADVANTAGES OF HARD WATER

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

ii.  Calcium salts in it helps to build strong teeth and bones.

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

iv. It does not dissolve Lead, hence it can be supplied in lead pipes.

 

DISADVANTAGES OF HARD WATER

1.             It causes furring of kettles and boilers.

2.             It wastes soap.

3.             It cannot be used in dying and tanning.

4.             Effects is seen in stalactites and stalagmite

 

TREATMENT OF WATER FOR MUNICIPAL/ TOWN SUPPLY

The following are the processes involved in the treatment of river water for town supply

1.    Coagulation: Chemicals like potash alum, KAl(SO₄)₂, or sodium aluminate III, NaAlO₂ is added to water in a large settling tank.

 

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 to remove the remaining fine dirt particles.

 

4.     Chlorination (Disinfection): Chemicals like chlorine is then added to the water to kill germs. 

5. Calculated amount of iodine to prevent goiter and fluorine to prevent tooth decay are 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.

OBJECTIVE QUESTIONS

1. Water that readily forms lather with soap is known as A. Hard water

B. Soft water
C. Distilled water
D. Rain water

2. The hardness of water is mainly caused by the presence of A. Sodium and potassium salts

B. Calcium and magnesium salts
C. Iron and copper salts
D. Zinc and lead salts

3. Which of the following causes temporary hardness in water? A. Calcium bicarbonate

B. Calcium sulfate
C. Magnesium chloride
D. Sodium chloride

4. Permanent hardness of water is caused by the presence of A. Calcium bicarbonate only

B. Magnesium bicarbonate only
C. Calcium sulfate and magnesium sulfate
D. Sodium bicarbonate

5. Temporary hardness can be removed by A. Filtration

B. Boiling
C. Distillation
D. Sedimentation

6. Which of the following is a characteristic of hard water? A. Forms lather easily

B. Has no dissolved salts
C. Produces scum with soap
D. Is always colorless

7. The substance formed when soap reacts with hard water is called A. Salt

B. Foam
C. Scum
D. Sludge

8. Which method can remove both temporary and permanent hardness?                                                                                       A. Boiling

B. Sedimentation
C. Distillation
D. Decantation

9. The process of removing hardness using sodium carbonate is known as.                                                                   A. Chlorination

B. Washing soda treatment
C. Aeration
D. Filtration

10. Which of the following is NOT a source of natural water? A. Rivers

B. Lakes
C. Springs
D. Plastic tanks

11. Water suitable for drinking is called.                                    A. Hard water

B. Distilled water
C. Potable water
D. Mineral water

12. The hardness caused by calcium sulfate is.                       A. Temporary hardness

B. Permanent hardness
C. Softness
D. Alkalinity

13. Which ion is commonly present in hard water?                  A. Na⁺

B. K⁺
C. Ca²⁺
D. NH₄⁺

14. The zeolite process is used for.                                            A. Purifying air
B. Softening water
C. Producing oxygen
D. Testing water

15. Rainwater is generally regarded as A. Hard water

B. Soft water
C. Salty water
D. Permanent hard water

16. Which of the following removes permanent hardness most effectively?                                                                           A. Boiling

B. Addition of soap
C. Ion-exchange process
D. Decantation

17. Water is called a universal solvent because it                     A. Is always pure

B. Dissolves many substances
C. Is colorless
D. Is tasteless

18. Which chemical is commonly known as washing soda?  A. Sodium chloride

B. Sodium hydroxide
C. Sodium carbonate
D. Sodium nitrate

19. During boiling, temporary hardness is removed because bicarbonates decompose to form                                               A. Carbonates that precipitate out

B. Sulfates
C. Chlorides
D. Nitrates

20. The best laboratory method for obtaining pure water from hard water is A. Filtration

B. Sedimentation
C. Distillation
D. Chlorination

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.

 

 

THEORY QUESTIONS

1.a i    Mention two compounds that causes permanent hardness in water

  ii.    State two ways of removing permanent hardness in water

  iii.     List two advantages of hard water

  b.(ai )    State the steps involved in the treatment of river water for town supply.

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

      iii. Name two cations that causes hardness of water?

c.i. Give two methods of removal