Engineering Chemistry S Datta

WATER TREATMENT 375
Q. 21. The presence of carbon dioxide is avoided in boiler feed water. Explain.
Ans. Dissolved carbon dioxide forms carbonic acid and attacks boiler pipes and corrodes.
Q. 22. Why does hard water consume a lot of soap?
Ans. When hard water comes in contact with soap Ca 2+ and Mg 2+ ions react with soap
and scum is formed when the reaction is overconsuming much of soap, lather is then formed.
That is why hard water consumes a lot of soap.
Q. 23. Why is calgon conditioning better than phosphate conditioning?
Ans. Calgon conditioning forms soluble complex salt
Na 2
[Na 4
(PO 3
) 6
] + CaSO 4
= Na 2
[Na 2
Ca(PO 3
) 6
] + Na 2
SO 4
Calgon Soluble complex
whereas phosphate conditioning i.e., treatment with sodium phosphates, precipitates calcium
and magnesium phosphates causing scale in boiler pipes.
Q. 24. Why is water softened before using in boiler?
Ans. Natural or hard water when fed in boiler produces scale, sludge and causes priming
and foaming and boiler corrosion. That is why water is softened before using in boiler.
Q. 25. Why is caustic embrittlement controlled by adding sodium sulphate to
boiler-feed water?
Ans. Sodium sulphate when used blocks hair cracks thus prevents infiltration of caustic
soda through the cracks. Thus sodium sulphate prevents embrittlement in boilers.
Q. 26. State two harmful effects of silica in water.
Ans. Silica reacts with Ca 2+ and Mg 2+ ions forming calcium silicate and magnesium
silicate in boiler. This type of scales are difficult to remove.
EXERCISES
1. Explain the various steps for the purification of water for municipal supply.
2. What is desalination? Name the different methods of desalination and describe any one.
3. What are boiler troubles? Why are they caused? What are the methods of their elimination?
4. Distinguish between:
(a)Temporary and permanent hardness.
(b)Sludge and Scale.
(c)Softening and demineralization.
5. Write notes on: (i) Caustic embrittlement, (ii) Reverse osmosis, (iii) Priming and foaming,
(iv) Hot lime-soda process, (v) Boiler corrosion.
6. What is the principle involved in the determination of total hardness of water by EDTA method?
7. What are the functions of lime and soda in hot lime-soda process? Give equations. Why coagulants
are not used in hot lime-soda process?
8. Why does hard water consume more soap?
9. During deionization process, water is first passed through cation exchanger and then through
anion exchanger, why?
10. Describe the methods of disinfection of water. Why is chloramine better than chlorine for sterilization
of water?