Page 94 - Chemistry - XI

COMPETENCY BASED QUESTIONS

Multiple Choice Questions

Q1. In drinking water, Cl ion should not exceed
–
(a) 250 mg L (b) 500 mg L (c) 1 g L (d) 2 g L –1
–1
–1
–1
Q2. Very soft water has CaCO
3
(a) less than 15 ppm (b) more than 50 ppm (c) 100 to 200 ppm (d) > 200 ppm
Q3. Permanent hardness of water is caused by
(a) CaCl (b) MgCl (c) CaSO and MgSO ( d) All of these
2 2 4 4
Q4. Permissible limit of F in drinking water
–
(a) 1.5 ppm (b) 5 ppm (c) 10 ppm (d) 50 ppm

Q5. Permissible pH for drinking water is
(a) below 6.5 (b) above 8.5 (c) 6.5 to 8.5 (d) below 5

Answer key

1. (a), 2. (a), 3. (d), 4. (a), 5. (c)

PROJECT - 4

Introduction
Soaps are sodium or potassium salts of higher fatty acids like stearic acid, palmitic acid, etc. They are used

for washing clothes. When a soap is dissolved in water, it forms lather or foam. Lather carries away dirt and
grease by forming emulsion when excess of water is added. Hard water is not suitable for washing because
it does not form enough lather with a soap.
The washing or cleaning action of a soap depends upon its foaming capacity, i.e. the extent to which
lather is formed with water. Forming capacity of a soap can be compared by comparing the time taken for
disappearance of foam produced previously by the same concentration of various samples with the same
force. Lesser the time taken for the foam for disappearance of a sample, the lower will be its foaming
capacity.
Hard water does not give foam with a soap. It is because of the fact that Ca and Mg present in hard water
2+
2+
form their insoluble salts which precipitates out in the form of a scum.
2C H COONa + Ca → (C H COO) Ca + 2Na +
2+
17 35 17 35 2
Calcium stearate
2C H COONa + Mg → (C H COO) Mg+ 2Na +
2+
17
35
17
2
35
Magnesium stearate
In order to enhance the foaming capacity of a soap, these Ca and Mg ions should be removed with the
2+
2+
help of Na
help of Na CO .CO 3
2 2
Ca
Ca + Na CO → CaCO + 2Na +
2+
3
2
3
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