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Soaps and Detergents:

Soaps:

A soap is the sodium or potassium salt of long chain fatty acids/carboxylic acids which has cleansing properties in water. Example, sodium stearate (C 17 H 35 COO - Na + ) and sodium palmitate (C 15 H 31 COO - Na + ) .

Since soap solutions are basic in nature, they turn red litmus blue .

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Manufacturing of Soaps:

Soaps are made by heating animal fats or vegetable oils with concentrated sodium hydroxide solution (NaOH). Glycerol is formed as a side product. This process  is known as saponification .

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Structure of Soap Molecules: 

A soap molecule consists of 2 ends: 

1. Hydrophilic end

It is the ‘water loving’ part of the soap molecule. It is polar in nature and is readily soluble in water . This includes the hydrocarbon part .

2. Hydrophobic end  

It is the ‘water fearing’ part of the soap molecule. It is non-polar in nature and insoluble in water but soluble in oil and dirt . This includes the carboxylic part .

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Cleansing action of Soap:

When soap is dissolved in water, two types of arrangements of these molecules are seen:

1. On the surface of water: 

The soap molecules present on the surface of water align themselves in such a way that the hydrophilic end is inside water and the hydrophobic end faces outward. 

2. Inside water: 

The soap molecules which are completely inside water form a cluster. The arrangement is such that the hydrophobic tails are towards the inside of the cluster and hence their contact with wate is inhibited whereas the hydrophilic ends are present on the surface . This type of arrangement is known as micelle .

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  • When a dirty cloth is put in a soap solution (soap + water), the hydrophobic part dissolves the oil and dirt from the surface of cloth
  • This is how dirt and oil is entrapped inside the micelle with the help of the hydrophobic part. 
  • When the cloth is agitated , these micelles are removed from the surface of cloth and they disperse in water, thereby removing the oil and dirt. 
  • The cloth gets cleaned after thoroughly rinsing it with water 3-4 times.

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Note: 

Soaps cannot be used with hard water i.e. water containing calcium and magnesium salts. This is because they react with these calcium and magnesium salts present in hard water to form a white sticky precipitate known as scum . This scum sticks to the surface of clothes that have to be washed and interferes with the cleaning ability of soap. Also, soaps are biodegradable in nature.

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Detergents:

Detergents are sodium salts of long chain benzene sulphonic acids (eg. CH 3 -(CH 2 ) 11 -C 6 H 4 -SO 3 - Na + ) or sodium salts of long chain alkyl hydrogensulphates (eg. CH 3 -(CH 2 ) 10 -CH 2 -SO 4 - Na + ).

Note:

  • Detergents have a similar but stronger cleansing action as compared to that of soap. 
  • They can be used with hard water also because they do not react with the calcium and magnesium salts of hard water to form scum.
  • But most of the detergents are non-biodegradable and cause water pollution in lakes and rivers.  

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo