How does Soap work?

The purpose is to remove oils, grease and dirt.

The chemical reaction (saponification) that creates soap is the reaction of an alkali with a fatty acid. This produces a molecule with one water loving end (water soluble) and one fat loving end (fat, or oil, soluble). The fat, or oil, loving end binds with the oils and 'grease' on our skin while the water loving end combines with water: the end result is what is usually a non-water soluble material being washed away. Soap acts as a surfactant and a wetting agent, allowing oil and water to mix. The rubbing action will remove accumulated oils and dirt to become dissolved in water (emulsified) and hence removed during the washing process.

Sebum is a natural lipid (fat) excreted from pores in the skin that reduces skin water loss and acts as a dis-infectant. This is why your skin will feel 'greasy' after a long day.

Odours occur when bacteria living naturally on the skin begin to break down organic material, usually dead skin.

As soap is usually alkali, it removes the "acid mantle" of the skin, making it initially feel clean and fresh. Over-cleaning is not good though, as this "acid mantle" is part of the body's natural defence mechanism against fungal or bacterial attack

Soap washes away the dead skin, removes the layer of sebum and any other grease or oily substances on the skin, removes accumulated dirt and grime; generally cleansing. This reduces the total loading of bacteria on the skin and the amount of material that the remaining bacteria can work on.

A good soap contains some glycerin, which is a humectant (attracts water); re-hydrating the skin.