Page 92 - Chemistry - XI

Q4. Which of the following methods of purifi cation is the easiest and the safest?
(a) Filtration (b) Boiling (c) Freezing (d) Adding alum

Answer key

1. (a), 2. (d), 3. (b), 4. (b)

PROJECT - 3

Introduction
Human health and life depends upon the quality of drinking water. If water contains iron, fl uoride or
chloride ion above permissible limits, it may cause many health problems. Excess of fl uoride ion cause
fl uorosis. Hardness of water is due to Ca and Mg . Hard water is not suitable for drinking and washing
2+
2+
purposes. Therefore, it is very important to know the ions present and their amount.

Objective
Testing the hardness, presence of iron, fl uoride, chloride, etc. in drinking water obtained from diff erent
regions and a study of the cause of presence of these ions above permissible limits.

Principle/Theory

Hard water contains high concentration of Ca and Mg . It is not harmful to health, but poses serious
2+
2+
problems when it exceeds the limit because it causes scales built up in pipes and appliances such as boilers,
hot water tanks, cooling towers shortening their life-span and aff ecting their effi ciency. Ca and Mg 2+
2+
react with soaps (sodium salt of fatty acids) to form calcium and magnesium salts of fatty acids which are
insoluble and form scum and prevent soap to form suffi cient lather .
The concentration of Ca and Mg can be determined by a titration with disodium salt of ethylene
2+
2+
diamine tetra acetic acid (EDTA). Ca and Mg react with this salt to form stable complexes in a solution
2+
2+
having pH = 10. A buff er solution of ammonia and ammonium ion is used to maintain pH of the solution
around 10.

Indicator
Erichrome Black T (EBT)

End Point

Wine red to sky blue

Chemical Reaction
Mg + EBT (aq) → (Mg – EBT) (aq)
2+
2+
Sky blue Wine red
[Mg – EBT] + H Y (aq) → MgY (aq) + 2H + EBT
2–
+
2–
2+
2
(Ethylene diamine Sky blue
tetra acetate ion)

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