What is dirt? Why won't water remove grease and dirt? Soap and detergents - bridging the gap Soap and detergents Dish washer powders - Warning! Dry cleaning

This topic explores the fascinating world of soaps and how they clean dirt from clothes and food from dishes. This topic should be read in conjunction with the topics Like dissolves like and Talking solutions.

What is dirt?
Most dirt and food stains are composed of oils and other biological molecules, sometimes mixed with inert materials like sand. Oils and other biological molecules are hydrocarbons, which chemist classify as non-polar molecules.

Like other non-polar molecules such as petrol, wax and grease, most food and dirt is not soluble in water. Soaking greasy plates has little effect!

Why won't water remove grease and dirt?
Water is a polar solvent and, as has been discussed in the topic "Like dissolves like", will mix with, and dissolve, other polar molecules and ionic salts, but will not dissolve non-polar molecules.

Hence water is not very good at cleaning plates and clothes by itself. What is required is a way of bridging the gap between the non-polar oil (which holds the dirt to the fabric / dishes) and the polar water molecules.

Soap and detergents - bridging the gap
Soap and detergents are quite unique molecules with the ability to dissolve in both polar and non-polar solvents. Both Soap and detergents are composed of long non-polar hydrocarbon molecules with a polar or ionic end, thus they combine both polar and non-polar characteristics in the same molecule.

This allows the soap and detergent molecules to bond with both oil and water molecules at the same time, forming a connection between the oil and the surrounding water molecules. Soap and detergents operate at the interface between the oil and the water and are known as "surface active substances".

With enough detergent/soap molecules present and agitation, the oil and dirt will wash off the clothes, attached to the water via the detergent/soap molecules. The agitation is important as detergent/soap molecules stick out of the oil like paddles, which catch the "current" and wash away the oil and dirt with the water.

Soap and detergents
The basic difference between Soap and detergents is simply that soaps are solid and detergents are liquid, although the boundary is often blurred. Soap molecules have longer hydrocarbon chains than detergents and have different polar ends.

Soap is made by reacting animal fat, stearic acid, with caustic soda, sodium hydroxide. The chemical name for soap is sodium stearate.

Detergents are made by reacting vegetable oils like lauric acid with sulfuric acid to produce lauryl sulfate.

Dish washer powders - Warning!
Automatic dishwasher powders are made up of a mixture of soaps and caustic chemicals like sodium hydroxide. The extra caustic chemicals react with grease and oil to form more soap molecules, giving a double cleaning action.

This combination of chemicals makes dish washing powders very efficient, but also very dangerous. Never touch dishwasher powders and keep them away from children. They are strong enough to react with oils in skin and cause damage to eyes. If ingested they will cause severe chemical burns in the throat and can be fatal.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.