Chapter 15: Water

Unit 15. Water (FBISE SSC-II Chemistry Keybook)

2. Give short answers.

  1. List the impurities present in rain water. 
     Rainwater contains pollutants, soil, plant parts, insect parts, bacteria, algae, and sometimes radioactive materials that the rain/snow has washed out of the air. 
  2. List toxic substances present in household wastes. 
    Household wastes include, human wastes, livestock wastes, soaps and detergents, paints and oil, food and vegetable wastes, garbage etc. 
  3. In what ways, industrial wastes pollute water. 
    Manufacturing of industrial products are always accompanied by some by-products and waste effluents. These wastes may contain highly toxic compounds and heavy metals such as Pd, Cd, Cr, Hg, As, Sb etc. Water from leather tanneries contains large quantities of chromium (VI) salts. Chromium (VI) ions are highly toxic and known to cause cancer. 
  4. What is water pollution? 
    Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans, aquifers and groundwater). Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds. 
  5. List some waterborne diseases. 
  • Cholera
  • Dysentery
  • Jaundice
  • Typhoid
  • Hepatitis
  1. What are pathogenic microorganisms? 
    An organism of microscopic size, usually a bacteria or virus, that causes disease. These organisms are transmitted through food, water and direct contact. These organism may cause typhoid fever, dysentery and hepatitis. 

3. What is hard water? Why is it sometimes undesirable? 
 Water that gives little lather or forms scum with soap is called hard water. 

Hard water is undesirable: 

  • Hard water wastes a lot of soap, when used for washing.
  • The soap forms scum with hard water, which adhere to the clothes being washed. Scum can spoil the finish of some fabrics.
  • Cause kettles to fur.
  • Can cause hot water pipes, boilers and car radiators to block due to the formation of insoluble calcium and magnesium salts, causing great damage

4. List two ways in which lakes and streams become polluted. 
Industrial units generally discharge their wastes either to open land or into water bodies, lakes, ponds, rivers or oceans.

  • Water from leather tanneries contains large quantities of chromium (VI) salts. Chromium (VI) ions are highly toxic and known to cause cancer.
  • Industrial wastes cause irreversible degeneration of the environment causing serious health problems for public and marine life.

5. Give chemical equations for the: 

a. Reaction of slaked lime with alum: 

3Ca (OH)2 + Al2 (SO4)3  2Al (OH)3 + 3CaSO4 
b. Carbonated rain water with lime stone: 

CaCO3 + H2CO3 Ca(HCO3)2 
c. Reaction that occurs when temporary hard water is boiled: 

Ca(HCO3)2  2CaCO3 + CO2 + 2H2O 
Mg (HCO3)2  2MgCO3 + CO2 + 2H2O 

d. Ca+2 ions interact with sodium zeolite: 

Ca+2 + Na2Z 2Na+ + CaZ 

6. How can buildings made of limestone be affected by acid rain? 

  • Acid rain has a corrosive effect on limestone or marble buildings or sculptures. It is well established that either wet or dry deposition of sulphur dioxide significantly increases the rate of corrosion on limestone, sandstone, and marble.
  • Sulphur trioxide plus water makes sulphuric acid. Marble buildings and statues are disintegrated by this acid.

CaCo3 (s) + H2SO4 (aq) CaSO4 (aq) + H2O (aq) + CO2 (g)

Therefore buildings made of limestone are affected by acid rain.

  • The acid reacts with carbonates in the lime stone. The lime stone dissolves and the marble buildings gradually crumbles away.

7. Make a list of main methods of softening hard water. In each case write a chemical equation to summarize the chemical reactions involved.

Methods to remove temporary hardness:

a. By Boiling: 
Hardness of water can be removed simply by boiling. During boiling the soluble calcium and magnesium hydrogen carbonates are decomposed forming insoluble carbonates. Since Ca+2 and Mg+2 ions are removed as insoluble carbonates, water becomes soft. 

Ca (HCO3)2  CaCO3 + CO2 + H2O 

Mg (HCO3)2  MgCO3 + CO2 + H2O 

b. By adding slaked lime (Clark’s method): 
Temporary hardness in water on the large scale can be removed by adding an estimated amount of slaked lime in it. The slaked lime reacts with the hydrogen carbonates to form insoluble carbonates. 

Ca (HCO3)2 + Ca (OH)2  2CaCO3 + 2H2O 
Mg (HCO3)2 + Ca (OH)2 CaCO3 + MgCO3 + 2H2O 

Methods to remove permanent hardness:

a. By adding washing soda: 

On the large scale permanent hardness in water can be removed by adding washing soda (Na2CO3.1OH2O). Ca+2 and Mg+2 ions are removed as their insoluble carbonates. 

Ca+2 + CO3-2  CaCO3 

Mg+2 + CO3-2  MgCO3 

b. By ion Exchange Resins: 

The hard water is passed through a container filled with a suitable resin containing sodium ions. Zeolite is one of the natural ion exchanger. Chemically it is sodium aluminium silicate. It is usually written as Na2Z. The Ca+2 or Mg+2 ions causing the hardness are exchanged with Na+ ions in the resin. 

Ca+2 + Na2Z 2Na+ + CaZ 

Mg+2 + Na2Z 2Na+ + MgZ 

The used up zeolite can be regenerated by heating with concentrated solution of NaCl. This makes the process economical.

CaZ + 2NaCl Na2Z + CaCl2 

8. List some disadvantages of water hardness. 

  • Hard water wastes a lot of soap, when used for washing.
  • The soap forms scum with hard water, which adhere to the clothes being washed. Scum can spoil the finish of some fabrics.
  • Cause kettles to fur.
  • Can cause hot water pipes, boilers and car radiators to block due to the formation of insoluble calcium and magnesium salts, causing great damage.

9. What are the Earth’s four main water sources?

Earth main water sources are:

  • Atmospheric Water
  • Rivers and Lakes
  • Groundwater
  • Glaciers and icecaps

10. How does hard water differ from soft water? 

Soft Water

Hard Water

Water that easily gives lather with soap and does not form scum is called soft water. 

Water that gives little lather or forms scum with soap is called hard water. 

Does not contain dissolved salt.

It contain dissolved salt, i.e Ca and Mg.

It does not waste soap when used for washing.

It waste lot of soap when used for washing.

11. What is the purpose of coagulation in water treatment? 
Coagulation is the process in which water is treated with slaked lime and alum. These materials react to form a gelatinous mass of aluminum hydroxide. 

3Ca (OH)2 + Al2 (SO4)3  2Al (OH)3 + 3CaSO4 

The aluminium hydroxide carries down dirt particles and bacteria. 

12. Explain how hard water hampers the cleansing action of soap. 

Hardness in a water is due to the presence of dissolved salts for example Ca and Mg. Soap is good cleaning agent, but Ca and Mg reacts with soap and form scum. Which hampers cleansing action of soap.

13. Why are municipal water supplies treated with aluminum sulphate and slaked lime?

Municipal water supplies treated with aluminum sulphate and slaked lime, these materials react to form a gelatinous mass of aluminum hydroxide. 

3Ca (OH)2 + Al2 (SO4)3  2Al (OH)3 + 3CaSO4 

The aluminium hydroxide carries down dirt particles and bacteria. In this way water become purified.

14. What are some health effects of biological contamination of water?

Human wastes are dumped on the ground or into the nearest stream. Human waste contains pathogenic microorganisms. These organisms may cause typhoid fever, dysentery and hepatitis. Hepatitis is a viral disease occasionally spread through drinking water. Some waterborne diseases are cholera, dysentery and jaundice.

15. Write a word and balanced chemical equation to show the effect of heat on magnesium hydrogen carbonate in an aqueous solution.

Heat 

Magnesium hydrogen carbonate Magnesium carbonate + Carbon dioxide + Water

Heat 

Mg (HCO3)2  MgCO3 + CO2 + H2O 

16. Why is it cooler near a lake than inland during summer?

Water has a high heat of vaporization. So large amount of heat is required to evaporate small amount of water. This property accounts for climate-modifying property of lakes and oceans. Thus in summer it is cooler near a lake than in interior land areas.

17. Evaluate the option, ion exchange resins can be used to remove temporary hardness.

In ion exchange method the hard water is passed through a container filled with a suitable resin containing sodium ions. Zeolite is one of the natural ion exchanger. Chemically it is sodium aluminium silicate. It is usually written as Na2Z. The Ca+2 or Mg+2 ions causing the hardness are exchanged with Na+ ions in the resin. 

Ca+2 + Na2Z 2Na+ + CaZ 

Mg+2 + Na2Z 2Na+ + MgZ 

The used up zeolite can be regenerated by heating with concentrated solution of NaCl. This makes the process economical.

CaZ + 2NaCl Na2Z + CaCl2 

18. Public health depends on water quality. Give arguments.

Unclean water supplies, poor sanitation and poor hygiene kill 2, 668, 000 people worldwide each year. Water is crucial for sustaining the reactions that keep us alive. For instance digestion, distribution of food through blood, removal of waste matter from the body. We also need water for drinking, cooking and cleaning. Clean water is very important for public health.

19. Hard water causes kettles to fur. This fur can be removed by using an acid. Justify.

Temporary hardness is caused by the presence of soluble bicarbonates of calcium and magnesium. Boiling will decompose these bicarbonates into carbonates, which are insoluble. Calcium and magnesium carbonate make up the fur on kettles. Carbonates are soluble in acids, so adding some acid will decompose the carbonate salts and the fur is removed from kettles.

20. The following chemical equation is about a calcium compound.

Comp A + 3Ca (OH)2   Comp B + H2O

Comp B + H2CO3 Comp C

  1. Name and give the formula of.
  2. Compound A

Ca (HCO3)2 + Ca (OH)2  2CaCO3 + 2H2O 

Compound A is Ca (HCO3)2 which is calcium hydrogen carbonate.

  1. Compound C

CaCO3 + H2CO3 Ca (HCO3)2 

Compound C is Ca (HCO3)2 which is calcium hydrogen carbonate.

  1. Describe with the aid of balanced chemical equation. What happens when compound C is heated?

Heat 

Ca (HCO3)2  CaCO3 + CO2 + H2O 

When calcium hydrogen carbonate is heated it changes into calcium carbonate, carbon dioxide and water.

  1. Compound C is soluble in water. Write a balanced chemical equation to show what happens when its aqueous solution is treated with washing soda?

Ca (HCO3)2  + Na2CO3 CaCO3 + NaHCO3

21. How chemistry helps maintain a dean swimming pool? Explain
 Water in swimming pools is purified from pathogenic organisms by aeration and chlorination. Chlorination is a probably the best and the cheapest method of sterilization of water and it is the most effective in destroying pathogenic bacteria. For chlorination, liquid chlorine may be added directly in the swimming pool water. 

22. Why it is advisable to wash hands well with soap after using bathrooms? 
 If your hands are not clean and you touch your face or public surfaces, you may be infecting yourself and others by spreading germs and disease. Colds, flus, and infectious diarrhea are all known to be spread by hand-to-hand contact. Washing your hands regularly can help keep you and those around you healthy by controlling the spread of germs (bacteria and viruses). 

23. Differentiate between raw water treatment and sewage treatment process.

Raw water treatment:

The raw water is treated in a municipal water purification plant, to make it fit for drinking and domestic purposes. Various stages in this treatment are; 

Sedimentation: 

It is the process in which water is allowed to stand in a reservoir. The suspended matter sinks to the bottom. 

Coagulation: 

It is the process in which water is treated with slaked lime and alum. These materials react to form a gelatinous mass of aluminum hydroxide. 

3Ca(OH)2 + Al2(SO4)3  2Al(OH)3 + 3CaSO4 

The aluminum hydroxide carries down dirt particles and bacteria. 

Filtration: 

The water is then filtered through sand and gravel. Sometimes it is filtered through charcoal to remove coloured and odorous compounds. 

Chlorination: 

In the final step, chlorine is added to kill any remaining bacteria. Chlorine reacts with water to form hypochlorous acid HClO which kills bacteria. 

Cl2 + H2O HClO + H+ + Cl– 

Sewage Water Treatment:

In many countries, sewage water is passed through certain treatment stages before it is discharged into a lake, stream, river or ocean. This treatment involves following steps.

Primary Sewage Treatment:

Primary treatment removes some of the solids as sludge. For this purpose waste water is allowed to stand in a large sedimentation tank to remove suspended particles.

Secondary Sewage Treatment:

Effluent from the primary treatment is passed through sand and gravel filters. There is some aeration in this step, and aerobic bacteria convert most of the organic matter to stable inorganic materials.

Activated Sludge Treatment:

The sewage is then placed in tanks and aerated with large blowers. This results in the formation of large, porous clumps, which absorbs contaminants. The aerobic bacteria further convert the organic material to sludge. This sludge is stored on land or sometimes used as fertilizer.

Chlorination:

The effluent from sewage plant is treated with chlorine to kill any remaining pathogenic microorganism.

24. Evaluate the advantages of waste water treatments.

Water treatment is essential for many reasons.

  • Through water purification, we can avoid drinking impure and containimated water, which causes many epidemic diseases and unsafe for healthy life.
  • It removes bacteria, viruses and parasites which may cause serious health problems.
  • It also removes heavy metals like As, Bs, Cr and Pb which can cause long term neurological problems, kidney diseases and cancer.
  • It also improves the flavour and appearance.

25. Water borne diseases are no longer common in developed countries. Defend this statement.

Human wastes are dumped on the ground or into the nearest stream. Human waste contains pathogenic microorganisms. These organisms may cause typhoid fever, dysentery and hepatitis. In developed countries raw water is treated in a municipal water purification plant, to make it fit for drinking and domestic purposes.