Question

how to test soap's degreasability? explain the step

Answer 1

Soaps are sodium or potassium salts of long chain fatty acids. Although soap is a good degreasing agent, its cleaning (degreasing) capacity is reduced when used in hard water. Thus one can test for the soap's degreasing ability by testing its action in hard water. When water contains sulfates, chlorides or bicarbonate salts of Ca²⁺ or Mg²⁺ ions, it is called as hard water.

When you add soap to hard water, the Ca²⁺ or Mg²⁺ ions in the hard water react with the added soap and replace the Na⁺/K⁺ ions present in the soap. This leads to formation of scum (insoluble in water and sticks to the substrates/clothes). This thus reduces the degreasing action of the soaps.

Ca²⁺ + RCOONa -------> RCOOCa + 2Na⁺

similarly,

Mg²⁺ + RCOONa ------> RCOOMg + 2Na⁺

A typical soap contains a polar head group and a non-polar tail (example: SDS-Sodium lauryl sulfate or sodium dodecyl sulfate). The non-polar end sticks to the greases and formes blobs leading to micelle like structures that are then wahsed away by water attached through polar head groups. This is dependent upon the concentration of the soap and also how polar the polar group really is. Thus long chain molecules in the soaps will help improve their degreasing capability.