CBSE Chemistry Subject Notes

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Chemistry

Carbon and Its Compounds

Manufacture of soap

Soap is made from animal fat or vegetable oils which when heated with sodium hydroxide solution ( caustic soda solution) , split to form sodium salt of higher fatty acid ( soap) and glycerol.

Saponification  -  The process of making soap by the hydrolysis of fats and oils with alkalis .

• Common salt is added in soap making -  It is added to the mixture to make the soap come out of the solution. It precipitates out all the soap from the aqueous solution.
• The soap which is used for washing clothes or bathing works by making the oil and grease particles dissolve in water because the oil and grease are insoluble in water.

Structure of a Soap Molecule –

A soap molecule is made up of 2 parts :  a long hydrocarbon and a short ionic part .

• The hydrocarbon part of the soap molecule is soluble in oil or grease so it can attach to the oil and grease particles present on dirty clothes. The short ionic part of the soap molecule is soluble in water so it can attach to the water particles .

Micelle  -  A spherical aggregate of soap molecules in the soap solution in water.

• Soap solution is a colloidal solution. A soap solution appears cloudy because the soap micelles are large enough to scatter light.

Limitations of soaps –

 Soap is not suitable for washing clothes with hard water ---

1. Large amt. of soap is wasted in reacting with calcium and magnesium ions of hard water to from insoluble precipitate called scum . So larger amount of soap is needed.
2. The scum formed sticks to the clothes and interferes with the cleaning ability of the additional soap , making cleaning difficult.

• Hard water contains calcium and magnesium salts.

  Detergents :-  It is the sodium salt of a long chain benzene sulphonic acid ( or the sodium salt of a long chain alkyl hydrogensulphate) which has cleansing properties in water.
  They are also called soap-less soaps because though they act like a soap in having the cleansing properties, they do not contain the usual soaps like sodium stearate , etc.

• Detergents are better cleansing agents than soaps because they do not form insoluble calcium and magnesium salts with hard water and can be used for washing even with hard water.

Advantages of detergents –

1. Can be used with hard water
2. Have strong cleansing action than soap.
3. More soluble in water than soaps.

Disadvantage of detergents over soaps -

Some detergents are biodegradable , they cannot be decomposed by micro organisms like bacteria and hence cause water pollution in lakes and rivers .All  soaps  are bio- degradable and do not cause pollution.

Submitted By Mrs. Kritika Bhola
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