Water – SS2 Chemistry Lesson Note (Third Term)

Water (H₂O) is a compound of hydrogen and oxygen, regarded as the universal solvent because it dissolves many substances. It occurs naturally as rainwater, well water, spring water and seawater, and can be treated for use. Water is either soft or hard, depending on whether it forms lather readily with soap.

Behavioural objectives

By the end of the lesson, students should be able to:

• state the sources of water and the two types of water;
• describe the structure of the water molecule and explain why it is V-shaped;
• state the physical and chemical properties of water and the tests for water;
• distinguish between temporary and permanent hardness and state how each is removed;
• list the advantages and disadvantages of hard water;
• describe the stages of treating river water for municipal supply.

What are the sources of water?

The following are the sources of water:

1. Natural water: rainwater, well water, spring water and seawater.
2. Treated water: distilled water, pipe-borne water and chlorinated water.

What are the types of water?

Water is of two types, namely soft water and hard water. Soft water forms lather with soap easily. Hard water does not form lather readily with soap because it contains some dissolved salts.

What is the structure of a water molecule?

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

The valence shell of oxygen has two lone pairs of electrons (2s² 2p²) and two unpaired electrons (2p_y¹ 2p_z¹). Each unpaired electron forms a covalent bond with an electron from a hydrogen atom. The water molecule has two lone pairs and two bond pairs of electrons in the valence shell of its central atom, so the octet rule is satisfied and the molecule is stable.

Ideally, the four electron pairs should be directed towards the 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 squeeze the two bond pairs of electrons closer together. As a result, the bond angle in water is compressed to approximately 105°, and the water molecule is V-shaped or angular.

How is water prepared in the laboratory?

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

What are the physical properties of water?

1. Water boils at 100°C and freezes at 0°C.
2. It has a maximum density of 1 g cm^-3 at 4°C.
3. It is neutral to litmus.

What are the chemical properties of water?

Reaction with metals

Water reacts with electropositive metals to form an alkali and liberate hydrogen gas. For example:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Magnesium and zinc react with steam.

Copper, gold, silver and mercury do not react with water to form an alkaline solution.

Reaction with non-metals

Non-metals such as chlorine react with water to form an acid solution.

H₂O(l) + Cl₂(g) → HCl(aq) + HOCl(aq)

How do you test for water?

When a 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.

What is hardness of water?

Hard water is water that does not form lather readily with soap. Water acquires hardness when soluble salts such as CaSO₄, MgSO₄ and Ca(HCO₃)₂ dissolve in it from the soil which it flows through.

There are two types of hardness: temporary hardness and permanent hardness.

Temporary hardness

Temporary hardness is caused by the presence of Ca²⁺ and Mg²⁺ in the form of hydrogen trioxocarbonate (IV), for example Ca(HCO₃)₂.

Removal of temporary hardness

1. Physical method: by boiling.
   
   Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g) (on heating)
2. Chemical method: by using slaked lime (calcium hydroxide solution).
   
   Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) + 2H₂O(l)

Effects of temporary hardness

Temporary hardness causes:

1. furring of kettles and boilers;
2. stalagmites and stalactites in caves.

Permanent hardness

Permanent hardness is caused by the presence of calcium and magnesium ions in the form of soluble tetraoxosulphate (VI) salts and chlorides (CaSO₄, MgSO₄, MgCl₂ and CaCl₂).

Removal of permanent hardness

Permanent hardness is removed by chemical methods only:

1. Addition of washing soda.
   
   Na₂CO₃(aq) + CaSO₄(aq) → CaCO₃(s) + Na₂SO₄(aq)
2. Addition of caustic soda.
   
   2NaOH(aq) + CaSO₄(aq) → Ca(OH)₂(s) + Na₂SO₄(aq)
3. Use of ion exchange resin.
   
   CaSO₄(aq) + sodium zeolite → calcium zeolite (insoluble) + Na₂SO₄(aq)

What are the advantages of hard water?

1. It tastes better than soft water.
2. Calcium salts in it help to build strong teeth and bones.
3. It provides CaCO₃, which crabs and snails use to build their shells.
4. It does not dissolve lead, so it can be supplied in lead pipes.

What are the disadvantages of hard water?

1. It causes furring of kettles and boilers.
2. It wastes soap.
3. It cannot be used in dyeing and tanning.

How is water treated for municipal supply?

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

1. Coagulation: chemicals like potash alum, KAl(SO₄)₂, or sodium aluminate (III), NaAlO₂, are added to the water in a large settling tank.
2. Sedimentation: the coagulated solid particles, or flocs, are allowed to settle at the bottom of the settling tank to form sediments.
3. Filtration: the water above the sediment still contains some suspended particles. It is passed through a filter bed to remove the remaining fine dirt particles.
4. Chlorination (disinfection): chemicals like chlorine are then added to the water to kill germs. Iodine and fluorine are also added as food supplements to prevent goitre and tooth decay respectively. The treated water is then stored in a reservoir and distributed to the town.

Evaluation

1. Name two natural sources and two treated sources of water.
2. Explain why the water molecule is V-shaped.
3. Write the equation for the removal of temporary hardness by boiling.
4. State two tests for water and the colour change in each.
5. List the four processes in the treatment of river water for town supply.

Assignment

Write the correct option ONLY.

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 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 deionised water because
   • A. distilled water is a product of condensed steam while deionised water is filtered laboratory water
   • B. distilled water is always pure and sold in packs while deionised water is not packaged for consumption
   • C. distilled water is condensed steam but deionised water is produced using ion-exchange resins which absorb undesired ions
   • D. distilled water is man-made while deionised water is both natural and artificial