WATER CHEMISTRY - Short Questions Answer


Short Questions Answer 

1. Name the chief sources of water.

A) The chief sources of water are rainwater, surface water, ground water& seawater.

2. What are the different types of impurities present in water?

A) Different types of impurities are

i) Dissolved impurities:

a) Inorganic salts 

i) Cations: Ca+2,Mg+2, Na+ ,K+ ,Fe+2, Al+3 traces of Zn+2 etc

ii) anions: Cl- ,SO4 2-,  NO3 - ,HCO3- & sometimes F- &NO2-

b) gases: CO2, O2, N2 oxides of sometimes NH3,H2S

c) Organic salts.

ii) Suspended impurities

a) Inorganic: Clay and sand

b) Organic: Oil globules, Vegetable, and Animal matter

iii) Colloidal impurities: Finely divided Clay & Silica, Products from organic waste

iv) Bacterial impurities: Bacteria, other microorganisms, and other forms of animal &vegetable life.

3. Distinguish between hard water and soft water?

A) Soft water-which produces lather easily on shaking with soap due to the absence of calcium and magnesium salts Hard water- which doesn’t produce lather (forms white curd) or very less lather difficultly with soap due to the presence of Ca+2& Mg+2

4. Define the hardness of the water.

A) Hardness is defined as a characteristic property of water that prevents the lathering of soap. It is also defined as soap consuming capacity of water. It is mainly due to the presence of calcium and magnesium ions, which react with sodium salts of long-chain fatty acids present in the soap to form insoluble scums of calcium and magnesium soaps (which don’t have detergent value)

2C17H35COONa + CaCl2 → (C17H35COO) 2 Ca + 2NaCl

Sodium stearate calcium stearate Soap (soluble) calcium soap (insoluble)

5. What are the different types of hardness?

A. There are two types of hardness

1. Temporary hardness or carbonate hardness

2. Permanent hardness or non carbonate hardness

Total Hardness = Temporary hardness + Permanent hardness

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

A.Temporary hardness: Ca (HCO3)2, Mg (HCO3)2

Due to the presence of dissolved bicarbonates of Ca+2& Mg+2

It is removed by mere boiling of water.

Ca (HCO3)2 → CaCO3↓ + H2O + CO2↑

Mg (HCO3)2 → Mg (OH)2↓ + 2 CO2↑

Permanent hardness: CaCl2, MgCl2,CaSO4,MgSO4,FeSO4,Al2(SO4)3 etc

Due to the presence of dissolved chlorides and sulfates (Cl- & SO4-2) of a+2& Mg+2

It cannot be destroyed by boiling.

7. How hardness is expressed?

A. Generally the concentration of hardness of water is usually expressed in terms of equivalents of CaCO3

8. Why do we express hardness in terms of CaCO3 equivalents?

Reason for choosing CaCO3: It is the most insoluble salt that can be precipitated in water treatment. This mode permits easy addition and subtraction of concentration of hardness-causing constituents since its molecular weight is 100(equivalent mass=50).

9. Mention common units used for expressing the hardness of the water.

A. parts per million (ppm) or milligrams per liter (mg/L)

10. Why is Calgon conditioning better than Phosphate conditioning?

A. In Calgon conditioning, the added Calgon forms a soluble complex compound with CaSO4, thereby it prevents the scale and sludge formation in the boiler. Since the complex formed is soluble, it does not cause any problem to the boiler.

2CaSO4 + [Na4(PO3)6]2- ----> [Ca2(PO3)6]2- + 2 Na2SO4

Soluble complex

On the other hand, in phosphate conditioning, sodium phosphate is added to boiler water so that precipitate of calcium phosphate is formed. Although this precipitate is nonadherent and soft yet it has to be removed by frequent blow-down operations.

3CaCl2 +2Na3PO4 -----> Ca3 (PO4)2 ↓+ 6Nacl


Hence Calgon conditioning is better than Phosphate conditioning

11. What is meant by softening of water?

A. The process of removing hardness causing salts from water is called water softening.

12. What is the main advantage of reverse osmosis over the ion exchange process?

A. Reverse osmosis removes all ionic, nonionic, colloidal, and high molecular weight organic matter.

13. What are the requisites for potable water?

A. Potable water: means the water which is safe to drink.

Essential requirements: The water should be clear

  • Colorless and Odorless
  • Pleasant in taste
  • It should not have turbidity (not exceed 10 ppm)
  • pH should be in the range of 7.0-8.5
  • Free from gases like H2S & minerals like Pb, As, Cr & Mn salts
  • Total hardness should be less than 500 ppm
  • Free from disease-producing bacteria

14. What is sedimentation with coagulation?

A. The process of removing fine suspended and colloidal impurities by adding the required amount of coagulant to water before sedimentation.

15. What is meant by breakpoint chlorination?

A. It involves the addition of a sufficient amount of chlorine to oxidize Organic matter, reducing substances, and free ammonia leaving behind free chlorine killing pathogenic bacteria. The addition of a sufficient amount of chlorine to satisfy chlorine demand is called breakpoint chlorination

16. What are the advantages of breakpoint chlorination?

i) oxidize completely organic matter, NH3, and reducing agents,

ii) removes colors in the water,

iii) destroys completely all the disease-producing bacteria, 

iv) removes odor from water,

v) prevents any growth of weeds in the water.

17. What are electro dialysis and reverse osmosis?

A. Electro dialysis: A process in which the ions of the salts present in water are pulled out by passing a direct current, using electrodes and a thin rigid plastic membrane pair.

Reverse osmosis: A process by which pure solvent is separated from its contaminants by using a semi-permeable membrane and applying high pressure on the concentrated side.

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