Water Treatment Viva Questions

Water Treatment Viva Questions

Water Treatment Viva Questions, Viva Questions on Water Treatment, Water Technology Viva QuestionsEngineering Chemistry Viva Questions, Engineering Viva Questions, Short Answer Questions on Water Treatment

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Corrosion Science Viva Questions

Short Answer Questions

Q.1. Name the main sources of water.

Ans. Sea water, rain water, ground water, and surface water.

Q.2. What is the cause for the alkalinity of natural water?

Ans. Due to the presence of dissolved bicarbonates of Ca and Mg in water.

Q.3. Define the hardness of the water.

Ans. Hardness is the characteristic property, which produces white scum on treating with the soap solution.

Q.4. Why does not hard water give lather with soap?

Ans. Because hard water produces insoluble white precipitate on treating with soap.

2C17 H35 COONa + CaCl2    →     (C17 H35 COO)2 Ca↓ + 2NaCl

Soap                       Hardness             White Scum

Q.5. How is the hardness of water expressed?

Ans. The concentration of hardness-producing salts is expressed in terms of calcium carbonate (CaCO3) equivalent.

Q.6. How hardness is determined in terms of CaCO3 equivalent.

Ans. Hardness (CaCO3 equivalent) = \frac{W\times 50}{E}

W = Weight of hardness-producing substance in ppm

E = Equivalent weight of the hardness-producing substance

Q.7. Define ppm, mg/L, Clarke’s degree, and French degree.


ppm: 1 part of CaCO3 equivalent hardness present in 106 parts of water.

mg/L: Number of mg of CaCO3 equivalent hardness present in 1L of water.

Clarke’s degree: Number of parts of CaCO3 equivalent hardness present in 70,000 parts of water.

French degree: Number of parts of CaCO3 equivalent hardness present in 105 parts of water.

Q.8. What is the relationship between ppm, mg/L, °Cl, and °Fr.


1 ppm = 1 mg/L = 0.07°Cl = 0.1°Fr

Q.9. Explain why water containing Ca2+ (aq) and HCO3 (aq) ions is said to be hard.

Ans. The Ca2+ ions give precipitates with soaps. On heating HCO3 ions, they are converted to

CO32 ions, which precipitate in kettles/boilers with Ca2+ ions.

Q.10. Why do we express the hardness of the water in terms of CaCO3 equivalent?

Ans. Because addition and subtraction of concentration of hardness-causing constituents are

easy. Its molecular mass is 100.

Q.11. What are the salts responsible for the temporary and permanent hardness of water?

Ans. Temporary hardness: Mg(HCO3)2 and Ca(HCO3)2

Permanent hardness: CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, etc.

Q.12. Name the gases that dissolve in water and cause corrosion.

Ans. Oxygen, carbon dioxide, and sulphur dioxide.

Q.13. Name any three substances that are used for the sterilization of water.

Ans. (i) Liquid chlorine (ii) Bleaching powder (iii) Chloramine

Q.14. Why is chlorination is better than chlorine or bleaching powder for sterilization of water?

Ans. Because chloramine

(i) is quite stable

(ii) does not impart a bad taste to treated water

(iii) imparts good taste to treated water

Q.15. What is break-point chlorination?

Ans. It involves the addition of a sufficient amount of chlorine to water in order to

  1. oxidize organic matter
  2. reduce substance and
  3. free ammonia

and leaves behind mainly free chlorine for disinfecting disease-producing bacteria.

Q.16. What are the advantages of break-point chlorination?


  • It oxidizes organic matter, NH3, and reducing substances completely.
  • It removes colour in water.
  • It destroys all the disease-producing bacteria completely.
  • It removes odour from water.
  • It prevents any growth of weeds in water.

Q.17. Mention the impurities present in natural water.


  • Suspended impurities
  • Colloidal impurities
  • Dissolved impurities

Q.18. What is standard hard water?

Ans. Usually, it is a solution containing 1g of CaCO3 equivalent hardness in 1 liter, i.e.,

1000 ppm of hardness water.

Q.19. What is sedimentation with coagulation?

Ans. The process of removing finely suspended impurities as well as colloidal impurities by adding the required amount of coagulant to water before sedimentation.

Q.20. What is colloidal conditioning?

Ans. Scale formation can be avoided in low-pressure boilers by adding substances like kerosene, tannin, agar-agar, etc., which get adsorbed over the scale-forming precipitates, thereby yielding non-sticky and loose deposits, which can be removed by blow-down operation.

Q.21. What is the indicator used in the EDTA method? What is the end-point?

Ans. Indicator: EBT

Endpoint: Wine red to pure blue

Q.22. Why is NH4OH-NH4Cl buffer solution added during the determination of hardness of water by the EDTA method?

Ans. The indicator used in this titration (EBT) shows color change at a pH value of about 10.

So alkaline buffer (NH4OH- NH4Cl) is used.

Q.23. Soft water is not demineralized, whereas demineralized water is soft. Why?

Ans. Soft water may contain Na+ , Cl , and SO42− ions, so it is not demineralized, whereas demineralized water does not contain any cation and anion.

Q.24. Why is water softened by the zeolite process fit for use in boilers?

Ans. Because zeolite-softened water contains large quantities of sodium salts like NaCl,

Na2SO4, etc., which avoids caustic embrittlement.

Q.25. CO2 should not be present in boiler-feed water. Why?

Ans. Because CO2 forms carbonic acid (H2CO3) on reacting with water. So boiler’s wall material can be attacked slowly by carbonic acid and becomes weaker and weaker progressively.

Q.26. What is meant by softening of water?

Ans. Softening of water means removing hardness-producing salts from water.

Q.27. Why is water softened before using in boilers?

Ans. Water should be softened before using in boilers otherwise it may cause various boiler problems like

  • scale and sludge formation
  • priming and foaming
  • boiler corrosion.

Q.28. What is meant by disinfection of water by UV method?

Ans. When water is irradiated by UV radiations, microorganisms and bacteria are killed. This so-called disinfection of water by UV radiation.

Q.29. If silica is present in water, what harmful effects it can cause to boilers?

Ans. If silica is present in water, it causes the formation of very firmly sticking deposits of calcium silicate (CaSiO3) and magnesium silicate (MgSiO3) scales in the boilers, which are very difficult to remove.

Q.30. Are coagulants also used in the hot lime soda process? Give reasons.

Ans. No, because the reaction proceeds faster in the hot lime-soda process, and the precipitate and sludge formed settle down rapidly. Thus, no coagulants are required in the hot lime-soda process.

Q.31. Water should not be soft for drinking purposes. Why?

Ans. Water should not be soft for drinking purposes because soft water is plumbosolvent, i.e., it attacks lead used in plumbing.

Q.32. What is the main advantage of reverse osmosis over the ion-exchange process?

Ans. Reverse osmosis removes all ionic, non-ionic, colloidal, and high molecular weight organic matter.

Q.33. Why does the water soften by the lime-soda process cause boiler troubles?

Ans. The treated water still contains some residual hardness.

Q.34. Why can caustic embrittlement be controlled by adding Na2SO4 to boiler-feed water?

Ans. When Na2SO4 is added to boiler-feed water, it blocks hair cracks, thereby preventing infiltration of caustic soda solution in these areas. So by this way, caustic embrittlement is prevented by using Na2SO4 in boiler-feed water.

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