Answers

2015-02-19T08:59:26+05:30
In case of hard water it will precipitate calcium carbonate, activity of soap will improve. But in presence of fresh water it will reduce the inter-facial tension , but bubbles of soap will not be formed . In presence of saline water addition of sodium carbonate may increase pH resulting in scale formation of calcium carbonate. 
However, in general the foaming capacity of detergents is not affected to a very large extent by addition of sodium carbonate.
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2015-02-19T17:51:49+05:30
 bcoz rapidly neutralizes the free fatty acids present in oil and complete the saponification reaction. 
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