Page 241 - Chemistry ICSE Class IX
P. 241
Chemistry Practicals 229
Observations
Test Tube A Test Tube B
VQ YJKEJ UQCR UQNWVKQP KU CFFGF VQ YJKEJ FGVGTIGPV UQNWVKQP KU CFFGF
No lather is formed. Lather is formed.
White curdy precipitate is formed. No precipitate is formed.
Results
y Soap solution does not give lather with hard water.
y Detergent solution gives lather with hard water.
Conclusion
Detergents function very well even in hard water, whereas soaps do not.
Explanation
2+
In hard water, due to the presence of Ca and Mg ions, soaps give insoluble calcium and magnesium salts
2+
of fatty acids. These salts precipitate as scum. The scum sticks to the fabrics and make their cleaning dif cult.
–
+
RCOO Na + + Ca 2+ o (RCOO) Ca(s) + 2Na (aq)
2
soap from calcium salt of the fatty remains in water
hard water acid (insoluble ppt.)
Thus, in hard water, a soap does not give lather.
VIVA VOCE
1. What are long chain fatty acids?
Ans. The carboxylic acids containing 15 to 18 carbon atoms in their molecule are called long chain (or higher) fatty acids.
2. Give two examples of long chain fatty acids.
Ans. (i) Stearic acid (C H COOH)
17 35
(ii) Palmitic acid (C H COOH)
15 31
3. Write the chemical formula of a typical soap.
Ans. Sodium stearate is a typical soap. Its formula is C H COONa.
17 35
4. Soaps are prepared from oils. What is the starting material for detergents?
Ans. Detergents are prepared from petrochemicals. Linear alkylbenzene is the processed raw material for obtaining detergents.
5. What is the disadvantage of using detergents?
Ans. Detergents cause pollution of water in waterbodies, such as rivers, lakes and ponds.
