How do detergents and soap work

Washing effect of detergents and soaps

You will need these materials for this:

  • 4 Erlenmeyer flasks (200 ml) or beakers
  • Glass stir bar
  • 2 funnels
  • Filter paper
  • Water, soapy water, soot

First you put 50 ml of water and a little soot in each of two Erlenmeyer flasks. Alternatively, you can use two normal beakers. In one of the two vessels you add an additional 10 ml of soap solution. Once you have thoroughly stirred both mixtures, the experiment can begin. To do this, run both solutions through a filter.

The filtrate without the soap solution is colorless and the soot remains as a residue in the filter paper. The mixture with the soap solution still contains soot even after you have filtered it and is not colorless, but grayish in color. Why is that?

The fact that parts of the soot have found their way through the filter paper is due to the dispersibility of surfactants. The surfactants are contained in the soap solution. The soot is first surrounded (emulsified) by the surfactants, so that the fat-loving parts of the surfactants are aligned with the carbon black and the water-loving parts with the water. At the same time, the soot is broken up (dispersed) by the soap so that it can pass through the pores of the filter paper. With water alone, the soot does not get through the filter. The emulsifying and dispersing properties of surfactants are the most important prerequisites for washing with soaps and detergents.