Class 10, Science, Chapter-4, Lecture-7, Soaps and Detergents (Notes)

DETERGENT:

Any chemical substance which has cleansing action when dissolved in water is called a detergent.

Two Types of Detergents:

1. Soaps
2. Synthetic detergents (soap-less soaps)

SOAP:

Soaps are sodium or potassium salts of higher fatty acids.

Examples of Soaps:
  1.Sodium Palmitate C₁₅H₃₁COONa
  2.Sodium Stearate C₁₇H₃₅COONa
  3.Sodium Oleate C₁₇H₃₃COONa

Two Parts of a Soap Molecule:

1. Non-Polar Tail
   A long hydrocarbon chain which is Hydrophobic (water-repelling) and Non–ionic
2. Polar Head
   A carboxylate group ( COONa⁺ ) which is Hydrophilic (Water-attracting) and Ionic.

SAPONIFICATION:

The hydrolysis of esters to obtain soap (sodium salt of the parent acid) and the parent alcohol by heating them with alkalies (NaOH, KOH) is called saponification.

$\mathop {{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}{{\rm{C}}_2}{{\rm{H}}_5}}\limits_{{\rm{Ethyl ethanoate}}} \mathrel{\mathop{\kern0pt\longrightarrow} \limits_{{\rm{Heat}}}^{{\rm{NaOH}}}}$ $ \mathop {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOHNa}}}\limits_{{\rm{Sodium ethaonate}}}  + \mathop {{{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}}\limits_{{\rm{Ethanol}}} $

LIMITATIONS OF SOAP:

1. Soap does not form lather with hard water,
2. Soaps react with hard water to form insoluble precipitates called scum which sticks to the cloth.

SOAP DOES NOT FORM LATHER IN HARD WATER:

Reason:
The soap reacts with Calcium or Magnesium ions present in hard water to form an insoluble precipitate (SCUM).

$2{\rm{ }}{{\rm{C}}_{15}}{{\rm{H}}_{31}}{\rm{COONa + MgC}}{{\rm{l}}_2}\buildrel {} \over \longrightarrow$ ${\left( {{{\rm{C}}_{15}}{{\rm{H}}_{31}}{\rm{COO}}} \right)_2}{\rm{Mg + 2 NaCl}}$

$2{\rm{ }}{{\rm{C}}_{15}}{{\rm{H}}_{31}}{\rm{COONa + CaC}}{{\rm{l}}_2}\buildrel {} \over \longrightarrow$ ${\left( {{{\rm{C}}_{15}}{{\rm{H}}_{31}}{\rm{COO}}} \right)_2}{\rm{Ca + 2 NaCl}}$

SYNTHETIC DETERGENTS:

Synthetic detergents are the sodium salt of a long-chain benzene sulphonic acid or that of a long-chain alkyl hydrogen sulphate OR ammonium salt of long-chain carboxylic acids.

Examples:   Sodium n-dodecylbenzene sulphonate
                   Sodium n-dodecyl sulphate

Two parts of a Synthetic Detergent:

1. Non-Polar Tail
   A long hydrocarbon chain which is Hydrophobic (water repelling) and Non-ionic
2. Polar Head
   A sulphonate (SO₃) OR sulphate (SO₄) group which is Hydrophilic (water-attracting) and Ionic

DISADVANTAGES OF SYNTHETIC DETERGENT:

Some of the Synthetic detergents containing branched chains are non-biodegradable and hence cause water pollution.

DIFFERENCES BETWEEN:

CLEANSING ACTION OF SOAP:

A soap consists of the following two parts

1. A non-polar tail of long-chain hydrocarbon which is water-repelling (Hydrophobic).
2. A polar head which is water-attracting (hydrophilic).

When the soap is dissolved in water and clothes are put in water the non-polar tail dissolves in oil or grease and the polar head dissolves in water.
The grease or dust particles are broken off and small spherical grease soap droplets called micelles are formed which are water-soluble.
The water-soluble micelles containing dirt are removed by the excess of water.

MICELLE:

A small water-soluble spherical soap droplet formed due to aggregation of the hydrophobic non-polar tail in the centre and hydrophilic polar head towards the periphery is called a micelle.

MICELLES DO NOT COMBINE TO FORM LARGE DROPLETS:

Reason:
The surfaces of the micelles are similarly charged due to the negatively charged heads present in water, which repel them from each other.