|MadSci Network: Chemistry|
Gene, This is a chemistry question that requires a chemical answer. There are many 'common sense' answers that don't do the problem justice. First, soap is made by heating animal fat (triglyceride fats) with lye (sodium hydroxide). The lye (NaOH) gives up its OH group and separates the glycerine from its three fatty acid molecules. This produces one molecule of glycerol and three ionically-bonded molecules of sodium stearate (soap). This sodium salt will give up its sodium ion to a water solution. The stearate will subsequently precipitate if it comes in contact with an ion that wants to hold on to it more strongly. Calcium and magnesium are the usual culprits when 'hard water' is used in the shower. The resulting calcium and/or magnesium stearate make the ever- popular 'bathtub ring'. Sodium stearate (soap) is comprised of a single sodium ion attached to a single stearate ion. This combination is soluble. Calcium and magnesium each have two places to form bonds and each of these metallic ions can combine with two stearate ions. This resulting molecule is insoluble in water and is rinsed away in the shower spary. This same 'bathtub ring' is an ingredient in many candy-like products. Look on the ingredients list of say, Tic Tacs and you will see magnesium stearate! They synthesize the product and do not scrape it from the inside of their bathtubs after the Saturday night cleanup. Now enter the soft water. You have removed the calcium and magnesium ions from the water and have replaced them with sodium. There is no tendency to remove the sodium from the sodium stearate (soap) and therefore, no tendency to form an inosluble compound. The surface of your skin has enough electrical charges in the form of amino acids, to cause the stearate ion to lightly cling to it. The soft water has a much reduced ability to combine with the soap film on your body and therefore, it is much more difficult to rinse off. The answer? Use much less soap and accept the less than clean rinse characteristics, or change to a synthetic detergent (a 'syndet') in the form of a liquid body wash. Adjust the amount of this synthetic cleaner and you should get much better results from your shower experience. I hope this helps.
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