NATIONAL QUALIFICATIONS CURRICULUM SUPPORT
Chemistry
Soaps and Emulsions
Section 1: Making soap
[HIGHER]
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© Learning and Teaching Scotland 2011
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SOAPS AND EMULSIONS (H, CHEMISTRY)
© Learning and Teaching Scotland 2011
Introduction
The following document has been designed as a guide for practitioners
teaching section 7 of the Consumer Chemistry component of Higher
Chemistry. This document can be used to explain specific examples to a more
in-depth level or to explain general concepts.
Section 1: Making soap
Section 1: Making soap
Making soap
Soaps are formed by the alkaline hydrolysis of fats and oils by sodium or
potassium hydroxide by boiling under reflux conditions:
The glycerol released is separated and used as a raw material for other
processes:
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SOAPS AND EMULSIONS (H, CHEMISTRY)
© Learning and Teaching Scotland 2011
Section 1: Making soap
The fatty acids are produced in the form of their sodium or potassium salts.
These salts are called soap.
Soap
The long covalent hydrocarbon chain that makes up the tail section of a soap
structure can be represented in a number of ways, either in the shorthand
notation shown below or as a bond-stick representation, shown at the bottom
of the page. The charged carboxylate group represents the head section of the
soap structure.
Soap
Section 1: Making soap
The structure of soap
The long covalent hydrocarbon chain gives rise to the hydrophobic (water
hating) and oil-soluble (non-polar) properties of the soap molecule
(represented in yellow). The charged carboxylate group (represented in blue)
is attracted to water molecules (hydrophilic) . In this way, soaps are composed
of a hydrophilic head and a hydrophobic tail:
How soaps work
The following ball (blue for hydrophilic head group) and stick (yellow for
hydrophobic tail group) diagram represents the initial interaction of soap on
addition to water and material with a grease stain:
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SOAPS AND EMULSIONS (H, CHEMISTRY)
© Learning and Teaching Scotland 2011
Section 1: Making soap
When the solution containing soap and water is agitated (stirred vigorously)
the interactions of hydrophobicity and hydrophilicity become apparent. The
hydrophobic, non-polar, tails burrow into the greasy, non -polar molecule –
like attracting like. In the same way the polar hydrophilic head groups are
attracted to polar water molecules. The head groups all point up into the
water at the top of the grease stain.
The attraction of the head group to the surrounding water, via polar-to-polar
interactions, is so strong that it causes mechanical lift of the grease molecule
away from the material on which it was deposited. The hydrophobic tails are
anchored into the grease due to non-polar to non-polar attraction. In
combination, these effects allow for the removal of the grease stain.