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 Form 4 Chemistry: Organic Chemistry II Online Lessons

The role of soap in cleaning

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Answer Text:
The role of soap in cleaning
Note: functions of soap in water
1. It makes the water able to wet material more effectively by lowering the surface tension.
2. Emulsification of oil and grease.
- Soap molecules have two dissimilar ends:
-A hydrocarbon chain which is non-polar and has no attraction for waterhence oil soluble
-A Carboxylate end, which is polar and is attracted to water; hence iswater-soluble.
Note: the Carboxylate end is in fact negatively charged in water because after dissolution, the sodium ion and the carboxylate ion exist as separate entities.
Effects of soap on oil – water mixture (removal of oils and grease during washing)
Note: schematic representation of a soap molecule
-On adding soap into oil water mixture the following build up occurs:
- A molecule of soap has apolar (hydrophilic) and nonpolar (hydrophobic) parts;
- The non-polar end dissolves in oil and the polar end dissolves in water.
- When the mixture is agitated (thoroughly shaken) the hydrocarbon chain (tail) dissolves in grease while the carboxylate – sodium end of the soap molecule (the head) remains dissolved in water.
Diagrams: role of soap in cleaning.
- Each oil drop ends up with a large cloud of negative charge around it as the polar heads are negatively charged.
- Consequently the oil drops repel each other, hence preventing them from coalescing.
- The water – soluble sodium “ heads” on the surface of the droplets keep the droplets emulsified (suspended) in the water.
- During rinsing the water carries away the oil droplets.