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Carbon and its Compounds — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceCarbon and its Compounds5 Marks
Question
Soaps and detergents are both types of salts. State the difference between the two. Write the mechanism of the cleansing action of soaps. Why do soaps not form lather (foam) with hard water? Mention any two problems that arise due to the use of detergents instead of soaps.
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Answer

The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.
Cleansing action of soap can be described as follows:
  1. A soap molecule has a tadpole-shaped structure.
  2. At one end (long non-polar end) of the soap molecule is a hydrocarbon chain which is insoluble in water but soluble in oil.
  3. At the other end (short polar end) of the soap molecule, there is a carboxylate ion which is hydrophilic, i.e. water soluble but insoluble in oil.

Soap on mixing with water forms a concentrated solution and causes foaming. The long non-polar end of soap gravitates towards and surrounds the dirt and absorbs the dust in it. The short polar end with the carboxylate ion repels the water away from the dirt. A spherical aggregate of soap molecules is formed in the soap solution in water and is called a micelle. Thus, the soap molecule dissolves the dirt and our clothes get clean.

Soaps do not form lather in hard water because Hard water contains calcium and magnesium salts. Soap molecules react with calcium and magnesium salts to form an insoluble precipitate called scum.
Two problems arise because of the use of detergents instead of soap:
  1. Soaps are biodegradable, while detergents are non-biodegradable; hence, detergents accumulate in the environment and cause problems.
  2. Certain phosphate additives are added to detergents. These phosphate additives act as nutrients for algae which form a thick green scum over the river water and upset the animal life in the river.

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