When it all comes down, you can't beat soap for most cleaning jobs around the home. Can you think of a commercial cleaning product (or, to be more correct, another commercial cleaner – soap is made commercially) that is able to clean clothes (including delicates like woollens and lace), clean floors, clean toilets, clean cars and also clean skin and hair? Author and advertising copywriter Dorothy Sawyers once mused that if the advertising machine stopped singing the praises of super-duper sainitisers that we would all fall back on good old soap and water for the heavy-duty cleaning jobs.
But how does soap work? What makes it such a good cleaner? It's not magic, after all.
Soap is made by combining fat of some sort (vegetable oils make a softer, gentler soap suitable for cleaning faces and hands, or for washing wool; animal fats such as tallow make a harder soap for heavy-duty cleaning jobs) with caustic soda or lye. This makes soap qualify as a natural cleaning product par excellence, as tallow is a by-product of the meat industry while caustic soda or lye can be made from wood ash, another waste product derived from an organic source. The process is called saponification, which is the boffin's way of saying that the ingredients of soap... make soap. Caustic soda is a strong alkaline while fat is slightly acidic, so the end product is a mild alkaline with a pH of about 8 (7 is neutral).
Soap's chemistry makes it able to make water wetter. This sounds like gobbledegook until you remember about water's surface tension. Surface tension is what makes it possible for a needle to float on water or for water to stand up a millimetre or two above the top of a glass. Soap's chemical structure breaks the surface tension of water so water can't form this "skin".
Once the "skin" of the water is broken, the water can mix more readily with other substances. As water is pretty nearly a universal solvent, this means that it can begin to break down the grime and dirt molecules, floating them away from where you don't want them (e.g. carpets).
Soap's chemical structure also makes it an emulsifier – something that allows two liquids that don't usually mix (for example, oil and water) to combine. This is why soap is prized as a grease-cutter and why it can work to remove oily marks and stains.
To get a bit more technical, soap molecules are made up of long hydrocarbon chains (like most organic substances). One end of the chain is hydrophilic (loves water) while the other is hydrophobic (hates water). It's all about the minute electrical charge in the molecules. The water-hating end loves fats and oils, so this bit of the soap molecule binds to the oils. The water-loving end floats. The more soap is used, the more oil you can float out of the carpet or whatever it is.
Soap, however, doesn't work so well in hard water with lots of calcium or magnesium in it, as the soap reacts with these molecules instead, forming a white scum that gets everywhere. Here, you need a water softener to deal to these molecules and allow the soap to work. Sodium is the answer, with the most common water softener being washing soda; baking soda comes in a close second. This is why washing soda should be added to homemade cleaning products containing soap or soap gel, and why baking soda is so good at cleaning baths.