John Meissner, age 12, of Chicago, I11., for his questions
How does soap remove dirt?
Plain water does not do much to clean a dirty rag. True, some of the dirt may come out and float away. Most of it seems to be merely distributed, for the rag comes out a dirty color all over. Butt a little soap in the water makes all the difference. It makes the water bubble and froth. The dirt, by some magic, is pulled out of the cloth and, with a clear rinse, disappears down the drain.
The action of the soap takes place in three stages. First it makes water wetter. This is called the detergent action. Water, though we may not notice it, has a surface tension rather like an elastic skin. This surface tension can stretch quite far without breaking. It does not break when the wind blows the ocean surface into waves. But, when stretched too far, these waves break in foamy white caps and frothy sea spray.
The chemical action of soap weakens the surface tension of the water, breaks its skin so that it can penetrate. This wetter water carries molecules of soap into the fabric of the cloth and these soap molecules are very active. Each molecule is a long chain of atoms, one end of which is positively charged, the other negative. It is thought that one end of a soap molecule attracts grease and the other end attracts water. Step two occurs when one end of a soap molecule grabs onto a particle of grease and the other end grabs onto a particle of water.
Perhaps you are wondering where the grease comes from. Almost all grime and dirt has a greasy base. In fact, it is grease of some sort which makes dirt stick to your hands, to clothes or to anything else. Dirt sticks to your hands because your skin is covered with a fine film
When you wash your hands, countless molecules of soap go to work. Each uses one arm to pull at a particle of dirty grease and one arm to pull at the water.
This action makes an emulsion of water, soapy suds and greasy dirt. This is the time to see that no dirty spot is forgotten. For the suds must reach and soak into every area of the cloth or skin if all the dirt is to be removed. And it is always amazing how mach dirt can be loosened and pulled away by a small amount of soap.
The last stage is the easy one and the one we are most apt to forget. We have to remove the emulsion of water and dirty suds. This, of course, is done with a rinse of clear water. As the rinse water runs down the drain, it carries with it the dissolved grease and, sticking to the grease is the dirt.