Water Technology: Important Two Marks Questions and Answers

Applied Chemistry I:

UNIT 1: Water Technology

 

Two Marks Questions and Answers

 

1. Give some important suspended impurities present in water

Sand, oil droplets, vegetable and animal matter, impart turbidity to water

 

2. Name any four chemical impurities present in water.

Bicarbonates, sulphates and chlorides of Ca and Mg and carbonates, bicarbonates of Na and K.

 

3. What are the biological impurities present in water?

Fungi, bacteria and other microorganisms cause diseases.

 

4. Give any four important sources of water.

The main sources of water are

(i) rain

(ii) rivers and lakes (surface water)

(iii) wells and springs (underground water)

(iv) sea water.

 

5. Define Taste and Odour.

Taste

Taste is the sensation of flavour perceived in the mouth and throat on contact with a substance.

Odour

Odour is a smell (or) scent caused by one (or) more volatilized chemical compounds that are generally found in low concentration.

 

6. What is meant by turbidity?

Turbidity is the reduction of clarity of natural water due to the presence of finely divided, insoluble impurities suspended in water.

 

7. What is the significance of pH in water?

(i) pH determines the solubility (amount that can be dissolved in water).

(ii) It also determines the biological availability (amount that can be utilized by aquatic life).

(iii) A rise (or) fall in pH can indicate chemical pollution (or) acid rain. Many animals cannot live in water at rat a pH level below 5 (or) above 9.

 

8. Define hardness.

Hardness is the property (or) characteristics of water, which does not produce lather with soap.

 

9. Why do we express hardness of water in terms of calcium carbonate equivalent?

CaCO₃ is chosen as a standard because

(i) Its molecular weight (100) and equivalent weight (50) is a whole number, so the calculations in water analysis can be simplified.

(ii) It is the most insoluble salt, that can be precipitated in water treatment.

Amount equivalent to CaCO₃ = [ Amount of hardness producing salt × Equivalent weight of CaCO₃ ] / Equivalent weight of hardness producing salt

 

10. What is temporary hardness and permanent hardness. How are they removed?

(i) Temporary hardness

This is due to the presence of bicarbonates of calcium and magnesium.

It can be removed by, (i) boiling the water (ii) adding lime to the water.

(ii) Permanent hardness

This is due to the presence of chlorides and sulphates of calcium and magnesium. It cannot be removed by boiling the water.

But, it can be removed by (i) Lime‒soda process (ii) Zeolite process.

 

11. What is BOD?

BOD is defined as, "the amount of free oxygen required by bacteria for the biological oxidation of the organic matter under aerobic conditions at 20°C for a period of 5 days".

 

12. What is the significance of BOD?

(i) It indicates the amount of decomposable organic matter present in the sewage.

(ii) It enables us to determine the degree of pollution at any time in the sewage stream.

(iii) Lesser the BOD, better is the quality of water. ie. the water sample with BOD of less than 3 ppm is considered as pure water, whereas the water more than 4 ppm is considered as polluted water.

 

13. Define COD.

COD is defined as, "the measure of amount of oxygen required to chemically oxidise all the oxidisable impurities present in the sewage using an oxidising agent like acidified K₂Cr₂O₇".

 

14. What are the advantages of COD?

(i) Determination of COD is carried out only in 3 hours, but determination of BOD is carried out after 5 days.

(ii) It measures both the biologically oxidisable and biologically inert organic matter.

 

15. Give a brief note on BOD and COD.

(i) BOD

BOD is defined as, "the amount of free oxygen required by bacteria for the biological oxidation of the organic matter under aerobic conditions at 20°C for a period of 5 days".

It indicates the amount of decomposable organic matter present in the sewage.

(ii) COD

COD is defined as, "the measure of amount of oxygen required to chemically oxidise all the oxidisable impurities present in the sewage using an oxidising agent like acidified K₂Cr₂O₇".

It measures both the biologically oxidisable and biologically inert organic matter.

 

16. What is chlorination?

The process of adding chlorine to water is called chlorination. Chlorination can be done by the following methods.

(a) By adding chlorine gas

(b) By adding chloramine

(c) By adding bleaching powder

 

17. What is sterilisation.

The process of destroying the harmful bacterias is known as sterilisation or disinfection. The chemicals used for this purpose are called disinfectants.

 

18. What is break‒point chlorination? Explain.

It is the point where all the bacterias, reducing substances, organic compounds and ammonia are completely removed and the applied chlorine begins to act as a powerful disinfectant.

 

19. What is blow‒down operation?

Blow‒down operation is a process of removing a portion of concentrated water by fresh water frequently from the boiler during steam production.

 

20. What is brackish water?

The water containing dissolved salts with a peculiar salty (or) brackish taste is called brackish water.

Brackish water - Contains > 1000 but < 35,000 ppm of dissolved solids.

 

21. Define desalination.

The process of removing common salt (sodium chloride) from the water is known as desalination. The water containing dissolved salts with a peculiar salty or brackish taste is called brackish water.

 

22. Write the principle involved in the desalination of water by reverse osmosis. (or)

What is meant by 'Reverse osmosis"? How is it applied in the desalination of water?

If pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow is reversed ie., solvent flows from higher concentration to lower concentration. This process is called reverse osmosis.

Salt water is taken as higher concentration and water is taken as solvent. If pressure is applied on the salt water, the water flows from salt water to water side.

 

23. Name the method which separate both ionic and non‒ionic impurities from water.

Reverse osmosis process.

 

24. Name some of the membranes employed in reverse osmosis process. State the advantages of this process.

Example: Cellulose acetate, cellulose butrate.

Advantages

(i) It removes ionic as well as non‒ionic, colloidal impurities.

(ii) The life time of the membrane is high and it can be replaced within few minutes.

 

25. What are the advantages of reverse osmosis method.

(i) The life time of the membrane is high, and it can be replaced within few minutes.

(ii) It removes ionic as well as non‒ionic, colloidal impurities.

(iii) Due to low capital cost, simplicity, low operating, this process is used for converting sea water into sea water into drinking water.

 

26. Why is water softened before using in boiler?

If hard water obtained from natural sources is fed directly into the boilers, the following troubles may arise.

1. Scale and sludge formation.

2. Priming and foaming (carry over).

3. Caustic embrittlement.

4. Boiler corrosion.

 

27. What are scales and sludges?

(i) Sludge

If the precipitate is loose and slimy it is called sludge. Sludges are formed by substances like MgCl₂, MgCO₃, MgSO₄ and CaCl₂. They have greater solubilities in hot water than cold water.

(ii) Scale

On the other hand, if the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale. Scales are formed by substances like Ca(HCO₃)₂, CaSO₄ and MgCl₂.

 

28. What are the differences between scales and sludges?

Scales

1. Hard and adherent deposit.

2. Difficult to remove

3. Formed by Ca(HCO₃)₂ CaSO₄, Mg(OH)₂

4. Acts as thermal insulator

Sludges

1. Loose and non‒adherent deposit

2. Easy to remove.

3. Formed by MgCl₂, MgCO₃, CaCl₂.

4. Acts as poor conductor of heat.

 

29. Mention any two disadvantages of formation of deposits in steam boilers

(or)

What are the disadvantages of scale formation.

Scales act as thermal insulators. It decreases the efficiency of boiler. Any crack developed on the scale, leads to explosion.

 

30. What is meant by priming and foaming? How can they be prevented.

(i) Priming

It is the process of production of wet steam. Priming can be prevented by controlling the velocity of steam and keeping the water level lower.

(ii) Foaming

It is the formation of stable bubbles above the surface of water. Foaming can be prevented by adding coagulants like sodium aluminate and antifoaming agents like synthetic polyamides.

 

31. What is meant by priming and mention its causes.

Priming is the process of production of wet steam. Priming is caused by

1) High steam velocity.

(ii) Very high water level in the boiler.

(iii) Sudden boiling of water.

(iv) Very poor boiler design.

 

32. List two disadvantages of using hard water in boilers.

(i) Scale and sludge formation

(ii) Priming and foaming (carry over)

(iii) Caustic embrittlement

(iv) Boiler corrosion

 

33. What is the reasons for the caustic embrittlement in boilers? Give examples.

Presence of alkali metal carbonates and bicarbonates like Na₂CO₃, K₂CO₃, NaHCO₃, KHCO₃ (or) NaOH cause caustic embrittlement.

 

34. What is caustic embrittlement? Mention any one method to prevent it.

Caustic embrittlement means intercrystalline cracking of boiler metal.

Prevention

Caustic embrittlement can be prevented by

(i) using sodium phosphate as softening agent instead of sodium carbonate.

(ii) by adding tannin, lignin to the boiler water, which blocks the hair cracks.

 

35. Indicate the reasons for boiler corrosion

Boiler corrosion arises due to the presence of

(i) dissolved oxygen,

(ii) dissolved carbon dioxide,

(iii) dissolved salts.

 

36. What are the requisites of drinking and boiler feed water?

(i) Boiler feedwater

Must have zero hardness and free from dissolved gases like O₂, CO₂, suspended matter, TDS and turbidity.

(ii) Drinking water

. H of water should be in the range of 7.0‒8.5.

. Total hardness and dissolved solids of water should be less than 500 ppm.

 

37. State the harmful effect of silica present in boiler feed water.

. Formation of silicate scale on the boiler acts as thermal insulator.

. It forms heavy deposit on the blades and nozzles of turbines.

 

38. Define softening of water. How is it caried out.

The process of removing hardness producing salts from water is known as softening (or) conditioning of water.

Softening of water can be done in two methods

(i) External treatment. (ii) Internal treatment.

 

39. Soft water is not DM water whereas DM water is soft water Justify.

(or)

Distinguish between soft water and demineralised water,

The soft water, produced by lime‒soda and zeolite processes, does not contain hardness producing Ca²⁺ and Mg²⁺ ions, but it will contain other ions like Na⁺, K⁺, SO₄^2‒, Cl^‒ etc., On the other hand D.M. (Demineralised) water does not contain both anions and cations.

 

40. What are the advantages of ion‒exchange process.

(1) Highly acidic or alkaline water can be treated by this process.

(ii) The water obtained by this process will have be very low hardness (nearly 2 ppm).

 

41. How is exhausted resin regenerated in ion‒exchange process.

When the cation exchange resin is exhausted, it can be regenerated by passing a solution of dil HCl or dil H₂SO₄

RCa + 2HC1 → RH₂+ CaCl₂

RNa + HCl → RH + NaCl

Similarly, when the anion exchange resin is exhausted, it can be regenerated by passing a solution of dil NaOH.

R'Cl₂ + 2NaOH → R'(OH)₂ + 2NaCl.

 

42. Give some examples for cation exchange resin.

(i) Sulphonated coals.

(ii) Sulphonated polystyrene.

 

43. Give some examples for anion exchange resin.

(i) Cross - linked quaternary ammonium salts.

(ii) Urea - formaldehyde resin.

 

44. How is boiler corrosion, due to dissolved oxygen, removed.

Sodium sulphite, hydrazine are some of the chemicals used for removing dissolved oxygen from water.

 2Na₂SO₃ + O₂ → 2Na₂SO₄

 N₂H₄ + O₂ → N₂ + 2H₂O

 

45. Name the gases dissolved in water that cause corrosion?

(i) Dissolved oxygen (DO)

DO in water attacks the boiler material at higher temperature.

4Fe + 6H₂O + 3O₂  → 4Fe(OH)₃↓

(ii) Dissolved carbon dioxide

Dissolved CO₂ in water produces carbonic acid, which is acidic and corrosive in nature.

CO₂ + H₂O ‒‒‒‒> H₂CO₃

 

46. How does carbon dioxide cause boiler corrosion.

Dissolved carbon dioxide in water produces carbonic acid, which is acidic and corrosive in nature.

CO₂ + H₂O → H₂CO₃

Carbon dioxide gas is also produced from the decomposition of bicarbonate salts present in water.

Ca(HCO₃)₂ ---^Δ→ CaCO₃↓ + H₂O + CO₂↑

 

47. What are ion exchange resins?

Ion exchange resins are long chain, cross linked. insoluble organic polymers with a microporous structure. The functional groups attached chains are responsible for the ion exchanging properties.

 

48. What are the disadvantages of ion‒exchange process.

(i) Water containing turbidity, Fe and Mn cannot be treated, because turbidity reduces the output and Fe, Mn form stable compound with the resin.

(ii) The equipment is costly and more expensive chemicals are needed.

 

49. How is water demineralised in an ion‒exchanger?

When the water containing ions (both anion and cation) are passed through ion exchange columns, it absorbs all the ions (anions and cations) as shown below.

Cation exchanger: R(H)₂ + CaCl₂ → RCa + 2HC1.

Anion exchanger: R(OH)₂ + 2HCl → RC1₂ + 2H₂O.

 

50. Distinguish between the Zeolite process and the ion exchange process.

Zeolite process

1. Only cations are removed.

2. Acidic water cannot be treated because it decomposes zeolite bed.

3. Zero hardness cannot be obtained.

Ion‒exchange process

1. Both anions and cations are removed.

2. Acidic water can be treated,

3. Zero hardness can be obtained.

 

51. What is meant by internal conditioning of water.

Internal conditioning is the process which involves the removal of scale forming substance by adding chemicals directly into the boiler.

 

52. Explain the function of a coagulant with example.

When the coagulant is added to water, it gets hydrolysed to form a gelatinous precipitate of coagulatant Al(OH)₃. The gelatinous precipitate, Al(OH)₃, entraps the finely divided and colloidal impurities, settles to the bottom and can be removed easily.

 

53. What is phosphate conditioning

(or)

What is the role of phosphates in the internal treatment of water?

Scale formation can be avoided by adding sodium phosphate. It is used in high pressure boilers. The phosphate reacts with Ca²⁺ and Mg²⁺ salts to give soft sludges of calcium & magnesium phosphates.

 3CaSO₄ + 2Na₃PO₄ →  Ca₃(PO₄)₂ + 3Na₂SO₄

 

54. What are boiler compounds? Mention two different boiler compounds and their actions.

Scale forming substances can be removed by adding chemicals directly to the boiler. These chemicals are called boiled compounds.

Examples: Sodium carbonate and sodium phosphate.

(i) CaSO₄ + Na₂CO₃ → CaCO₃ + Na₂SO₄

(ii) 3CaSO₄ + 2Na₃PO₄ → Ca₃(PO₄)₂ + 3Na₂SO₄

 

55. What is calgon conditioning? How is it functioning in water treatment?

(or)

Write the chemical reaction involved in calgon conditioning.

Calgon is sodim hexa meta phosphate Na₂[Na₄(PO₃)₆]. This substance interacts with calcium ions forming a highly soluble complex and thus prevents the precipation of scale forming salt.

2CaSO₄ + Na₂[Na₄(PO₃)₆] → Na₂[Ca₂(PO₃)₆] + 2Na₂SO₄

 

56. Distinguish between internal and external conditioning of water.

External treatment

1. It is expensive

2. No chemicals are used

3. It is carried out before feeding the water into the boiler.

4. It is used for high pressure boilers.

5. Blow down operation is not required.

Internal treatment

1. It is cheap

2. Chemicals are used

3. It is carried out within the boiler.

4. It is used for low pressure boilers.

5. It requires blow down operation.

 

57. Why calgon conditioning is better than phosphate conditioning?

In calgon conditioning calgon forms highly soluble complex, but in phosphate conditioning, it gives sludge. So periodical disposal of sludge is important in phosphate conditioning, but in calgon conditioning no problem of disposal.