Hard water and water pollution (O-level chemistry)

Hard water

Hard water: is water which does not form lather easily with soap. This is due to the formation of insoluble precipitates (scum) when soap molecules react with calcium or magnesium ions in water. The insoluble precipitates are called scum.

How does water become hard?

Rain dissolves carbon dioxide as it falls and forms very dilute carbonic acid

H₂O _(l) +CO_{2 (g)}            ↔        H₂CO_{3 (aq)}

If this water passes through rocks containing calcium carbonate or magnesium carbonate, the carbonates dissolves to form hydrogen carbonate forming temporary hard water.

CaCO_3(s) + H₂CO_{3 (aq)}  →     Ca(HCO₃)_2(aq)

MgCO_3(s) + H₂CO_{3 (aq)}   ↔    Mg(HCO₃)_2(aq)

If rainwater passes through the rocks containing calcium or magnesium sulphate, the sulphates dissolve in water forming permanently hard water.

NB; calcium sulphate-2-water naturally occurs as gypsum ‘CaSO₄.7H₂0’

Types of water hardness

Temporally hardness: this is caused by presence of calcium hydrogen carbonate or magnesium hydrogen carbonate

Removal of temporally hardness.

1. Boiling: decomposes the soluble calcium hydrogen carbonate to insoluble calcium carbonate

Ca (HCO₃)_2(aq)   →    CaCO_3(s) + H₂O _(l) + CO_{2 (g)}

After boiling, water is filtered to remove the insoluble carbonate.

N.B: boiling removes only temporally hardness.

2. Addition of lime water: calculated amount calcium hydroxide solution precipitates all the calcium and magnesium ions as calcium or magnesium carbonates.

Ca(OH) _2(aq) +Ca (HCO₃)_2(aq)     →     CaCO_3(s) + H₂O _(l)

Mg(OH) _2(aq) + Mg(HCO₃)_2(aq)    →     MgCO_3(s) + H₂O _(l)

3. Addition of sodium carbonate (washing soda): sodium carbonate precipitates all the calcium ions / magnesium ions as calcium carbonate or magnesium carbonate, which can be filtered

Ca(HCO₃)_2(aq) + Na₂CO_{3 (aq)}    →   CaCO_3(s) + 2NaHCO_{3 (aq)}

Mg(HCO₃)_2(aq) + Na₂CO_{3 (aq)}   →    MgCO_3(s) + 2NaHCO_{3 (aq)}

Permanent hardness:

this is caused by the presence of calcium sulphate or magnesium sulphate.

Removal of permanent hardness

1.  Addition of sodium carbonate (washing soda): sodium carbonate precipitates all the calcium ions as calcium carbonate, which can be filtered.

CaSO_{4 (aq)} +Na₂CO_3(aq)   →    CaCO_3(s) + Na₂SO_{4 (aq)}

2. Distillation: this process removes all the types of hardness; it is expensive to distil large amount of water.

3. Ion exchange:  this method is used for softening water on a large scale. In this method the hard water containing calcium and magnesium ions  is passed through Permutt (sodium aluminium silicate) NaY. As it does so, ion exchange occurs, that is, calcium ions are exchanged with sodium ions.

CaSO_{4 (aq)} + Na₂Y _(aq)                →           CaY_(s) + Na₂SO_{4 (aq)}

Soap

Soap is soluble sodium or potassium stearate.

To prepare soap

–  Mix sodium hydroxide (about 10cm³ of is 5M) with castor oil (about 2 cm³).

– Warm to boiling, and stir with a glass rod while it is boiling gently for 5-10 minutes.

– The soap remains in solution.

– Add saturated sodium chloride solution, boil gently and stir for further 2min and allow to cool. Soap is insoluble in sodium chloride. Filter off the solid soap.

Fat + alkali   →    soap + glycerine

i.e. glycerol stearate + sodium hydroxide       →        sodium stearate + glycerine

Manufacture of soap on large scale:

Oil and fats (coconut oil, palm oil, mutton fat, fats made by hydrogenation of oil, etc.) are heated by steam with sodium or potassium hydroxide. Sodium chloride is added to separate the soap as an upper layer (the ‘salting out’ of soap). Potassium hydroxide forms soft soap. The glycerine or glycerol is obtained by distillation.

NB: the process of manufacturing of soap is called saponification.

The Action of soap with hard water

When soap is added to hard water containing calcium or magnesium salts. It first precipitates calcium or magnesium stearate as scum. (SCUM: is the milky substance formed when soap reacts with hard water) and no lather is formed until the entire calcium ion or magnesium ions are removed.

2Nast _(aq) + CaSO_{4 (aq)            →}                     Na₂SO_{4 (aq)}   + Cast _(s) (scum)

Detergents: is any substance that facilitates the emulsification and removal of grease. Soap is a detergent, but the name is usually used for synthetic substituent of soap.

Advantages of detergents:

• Synthetic detergents are more soluble in water than soap.
•  Synthetic detergents do not form scum with hard water unlike soap.

             Disadvantages of detergents.

• Detergents are non-biodegradable i.e. cannot be decomposed /acted on by bacteria. (Soap is bio gradable). Therefore, synthetic detergents can pollute rivers, lakes or seas more than soap.
• Detergents are expensive.

 

Disadvantages of hard water.

Wastes soap: hard water needs much soap to form leather.

• With hard water, soap form scum, this makes / leaves dirt marks on clothes.
• Hard water form boiler scale inside the boiler tube and fur in kettles which is band conductor of heat and therefore wastes fuel.

Advantages of hard water.

• Hard water contains calcium used in formation of animal shells, teeth and bones.
• Water pipe; lead pipes cannot be used with soft water because soft water dissolves some lead, which is poison. Hard water forms a coating of lead carbonate and sulphate which prevents it from reacting

Pollution

This is the addition of a substances or energy which is potentially harmful to life, to the environment at a rate higher than the environment can accommodate.

Types of pollution.

Air pollution

Air pollution is produced by

(i) Power and generating plants: fuel and coal used in power generating plants contain sulphur and when burnt, sulphur is oxidized to sulphur dioxide and sulphur trioxide.

S _(s) + O_{2 (g)}         →              SO_{2 (g)}

2SO_{2 (g)} + O_{2 (g)            →}                   2SO_{2 (g)}

(ii) Motor vehicles:  fumes from the car exhaust contain; carbon dioxide             carbon monoxide, water vapour, soot, nitrogen oxides, lead compounds and unburnt carbon

(iii) Industrial processes: they emit into the air various pollutants e.g. paper making process use sulphides and emit hydrogen sulphide

Water pollution

Streams, rivers and lakes may be polluted by sewage, fertilizers, oil and hot water.

(i) Sewage: 

is the product formed when waste matter enters the water. Sewage originates mainly from domestic and industrial sources. Untreated sewage may contain organisms which cause diseases such as cholera and typhoid.

Treatment of sewage

Sewage is first passed through screens to remove large solid particles such as wood, plastics and rugs. Then sewage is passed through a chamber where inorganic or mineral matter such as sand and gravel are made to settle. This is called primary treatment. An effluent from

Primary treatment is passed to secondary treatment process. Here the effluent is brought into contact with oxygen and aerobic microorganisms. The metabolic activities of these organisms break down organic matter to harmless substances such as carbon dioxide and water. The product from secondary treatment is chlorinated to reduce the content of bacteria, the effluent is released into a river. The solid left after sewage treatment is called sludge and can be used as organic manure.

(ii) Oil pollution

Comes from ships, engine oil from a motor vehicle. It prevents oxygen from entering the water. This leads to death of fish and other aquatic organisms that use oxygen.

(iii) Fertilizer

Fertilizer contains mainly nitrate and phosphorus which when leached into rivers and lakes cause EUTROPHICATION (overgrowth of algae). When the algae die the bacteria responsible for the decaying process use a lot of oxygen, hence depletes oxygen leading to the death of fish

(iv) Hot water

Hot water from power station raises the temperature of water reducing its ability to dissolve oxygen. It also causes high temperatures in aquatic organisms blocking their enzymes from functioning properly.

Water treatment/purification

The process of removing pollutants from water to produce domestic water involves the following stages:

(i) Filtration

Impure water is first passed through screens to filter out floating debris.

It is then passed through courses of sand filter to remove large insoluble particles. The sand also contains microbes which remove some of the bacteria.

(ii) Sedimentation

Water obtained after filtration is not pure enough, it contains fine particles that must be removed. This is done by adding powdered potash alum (potassium aluminium sulphate) to water. The addition of alum coagulates/ precipitates thin/fine particles and settle at the bottom of the container. This method is called sedimentation. These particles are removed by further filtration through a fine sand bed.

(iii) Chlorination

The clear liquid or water obtained after sedimentation may contain harmful bacteria. Adding recommended quantities of chlorine to the water kills these bacteria.

   H₂O (s)  +  Cl₂(g)       →         HOCl (aq) + HCl (aq)

(iv) pH adjustment

Addition of chlorine to water lowers its pH which is corrected by adding sodium carbonate (soda ash).

HOCl (aq)  +   Na₂CO₃ (aq)  →  NaCl (aq)    + CO₂ (g)

 HCl (aq)   +    Na₂CO₃ (aq)  → NaCl (aq)     + CO₂ (g)

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Compiled by Dr. Bbosa Science