Soap

Soap is the sodium or potassium salt of a long-chain fatty acid, made by the saponification of an oil or fat. Malay: sabun · Chinese: 肥皂.

Every soap particle has two very different ends, and that is the key to how it works. The long hydrocarbon chain is the hydrophobic tail, which dissolves in grease but not in water. The charged end, –COO⁻Na⁺, is the hydrophilic head, which dissolves in water. When you wash a greasy plate, the tails bury themselves in the grease while the heads point outward into the water, so the grease is lifted off the surface and held as tiny droplets that rinse away. Because the outer surface of each droplet is negatively charged, neighbouring droplets repel one another and do not join back together, so the dirt stays suspended until it is washed off. This is why soap cleans where water alone cannot.

Example. Sodium palmitate and sodium stearate are common soaps made from palm and animal fats. In water the heads face outwards and the tails cluster inwards, surrounding oil and forming an emulsion that lifts the dirt from cloth or skin. Agitating the water, by scrubbing or in a washing machine, helps break the grease into small enough droplets for the soap to surround.

Confusion to avoid. Do not confuse soap with detergent. Both clean the same way, with a hydrophilic head and a hydrophobic tail, but soap is made from natural oils and fats, while detergent is made from petroleum. The practical difference is that soap forms scum in hard water, whereas detergent does not, so soap wastes itself and lathers poorly in hard water.

Soap is the central product of this chapter, tying saponification to cleaning action, and the compare-with-detergent question is a frequent SPM Chemistry task. Our teachers often ask students to name the exact structure, “sodium salt of a long-chain fatty acid”, because the precise phrase earns the mark.