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What is Soap?

Soaps are mixtures of sodium or potassium salts of fatty acids which can be derived from oils or fats by reacting them with an alkali (such as sodium or potassium hydroxide) at 80100 C in a process known as saponification.

fat + NaOH ---> glycerol + sodium salt of fatty acid

CH2-OOC-R - CH-OOC-R - CH2-OOC-R (fat) + 3 NaOH ( or KOH)

both heated --->

CH2-OH -CH-OH - CH2-OH (glycerol) + 3 R-CO2-Na (soap) R=(CH2)14CH3
Note about 3D molecules -- Our files on this page now use Jsmol instead of Jmol. These files make use of Javascript which permits viewing of molecules on tablets, phones and easier use on Macs. - Jsmol is best viewed with the Chrome browser.


Try this!!

Click the right mouse button with the cursor over the image-->

Labels --> Element Symbols

Click on the left mouse button and rotate the soap structure.

Notice that one end of the molecules is made up of a hydrocarbon chain -- the other end is a very polar structure containing oxygen and sodium.

Soap molecules have both properties of non-polar and polar at opposite ends of the molecule.

How does Soap Work?

Nearly all compounds fall into one of two categories: hydrophilic ('water-loving') and hydrophobic ('water-hating'). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic compounds just don't mix.

The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).

When grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution, the soap molecules work as a bridge between polar water molecules and non-polar oil molecules. Since soap molecules have both properties of non-polar and polar molecules the soap can act as an emulsifier. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. The soap will form micelles (see below) and trap the fats within the micelle. Since the micelle is soluble in water, it can easily be washed away.

[Micelle shown using the Jsmol
Molecular dynamics simulations of dodecylphosphocholine
D. P. Tieleman, D. van der Spoel, H.J.C. Berendsen] --File shown without water molecules

When you mix soap into the water the soap molecules arrange themselves into tiny clusters (called 'micelles'). The water-loving (hydrophilic) part of the soap molecules points outwards, forming the outer surface of the micelle. The oil-loving (hydrophobic) parts group together on the inside, where they don't come into contact with the water at all. Micelles can trap fats in the center.

Try this ---> click right mouse button over image --> spin --> on

  Micelle (above) shown with water molecules.

Multiple Choice Questions

1. Water is considered a polar molecule because
a) the molecule has a net positive charge
b) the molecule has a net negative charge
c) the molecule has a net zero charge
d) the ends of the molecule have partial negative and positive charges

2. Soap is formed from:
a) two hydrophobic compounds
b) a physical change when fats are heated
c) two hydrophilic compounds
d) oils or fats by reacting them with an alkali

3. Which statement is correct about hydrophobic and hydrophilic compounds
a) hydrophilic are water-hating and hydrophobic water-loving
b) hydrophilic are water-loving and hydrophobic water-hating
c) both hydrophobic and hydrophilic compounds are polar molecules
d) both hydrophobic and hydrophilic compounds are non-polar molecules

4. Soap acts by
a) digesting the fat molecules
b) forming micelles and trapping the fat within the micelles
c) releasing sodium and potassium into water
d) a chemical change

5. Which of the following statements is not correct
a) the soap molecules work as a bridge between polar water molecules and non-polar oil molecules
b) soap forms micelles
c) soap is an emulsifier
d) soap is made of just carbon and hydrogen atoms

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