|MadSci Network: Chemistry|
One of the characteristics of the molecules which comprise soap is a free end which is particularly attractive to fat molecules. Soap scum is just a combination of the two. In order to remove soap scum, most cleaners contain two elements: an alkaline compound (usually sodium hydroxide or a related compound) and a mild abrasive. Since sodium hydroxide is fairly hard on the skin, that is why most bathtub/sink cleaning agents recommend you wear rubber gloves when you use them. Ammonia is also an effective scum remover, because it is capable of dissolving fat, which comprises the bulk of soap scum, but again, it's very hard on your skin and you ought to use gloves wyhen you clean with it. (If you use lots and lots of soap, you won't find soap scum - the reason it appears is because people use enough soap to remove fat from their bodies/dishes, but not enough to actually react with all the fat. So a lot of fat is set free, but has nowhere to go but the sides of the sink!)
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