soap

Definition

Water soluble compound made by a reaction (called saponification) between caustic soda (sodium hydroxide) or caustic potash (potassium hydroxide) with animal and/or vegetable fats (oils). Soap has surface active properties (see surfactant) to wet a greasy (oily) soiled surface and suspend the oil and dirt in the water for rinsing off. Synthetic soaps (called detergents) are made from petroleum-based products, and some heavy soaps (made from lead, zinc, or other heavy-metal compounds) are water-insoluble and are used mainly in lubricating greases. Before the invention of soap (around second or third century AD), different cultures used various vegetables containing chemical compounds (called saponins) that produce suds with water. Romans used a mixture of several types of oils and sand.