Page 37 - Curriculum Visions Dynamic Book

How soap works
When soap is mixed with water, it breaks up into lollipop- shaped molecules. One end is attracted to water, while the other end repels water. This means that the water-hating (hydrophobic) ends are attracted to the surfaces of all objects, because this keeps them away from the water.
When a soap molecule gets close to a piece of dirt, the water-hating end sticks to it. If the dirt is rolled around, such as it might be the case when we wash our hands, there is plenty of opportunity for soap molecules to stick all round the dirt.
In this way the molecules surround the dirt, stopping it from sticking back to the surface we are trying to clean. At the same time, the water loving ends of the molecules are attracted to the water. As a result the whole molecule, and the dirt are pulled away from the surface. In this form the dirt cannot settle out and stick to a surface and so it is easily washed away.
Soap molecules cannot clean all by themselves. They need to be pushed around, so they wrap up the dirt molecules as they are moved about. This is why we have to rub our hands or why a washing machine has to agitate to get the dirt off.
detergent: a petroleum-based chemical that removes dirt.
saponification: the term for a reaction between a fat and a base that produces a soap.
 The detergent is added to the water.
 The tails of the detergent molecules are attracted to the grease on the surface. The water- loving (hydrophilic) heads are left in the water.
 The heads of the detergent molecules are repelled by one another
and so combine, with some agitation, with the water. By holding your hand under the tap, the grease is broken up into smaller particles and can be washed away from the surface.
Dirt floats free.
Droplet of soap Water
Soap molecule
Grease
Surface with greasy dirt, skin, clothes, etc.
Soap molecules embed themselves in the grease.
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