How is a non-soap bar different from soap? | HowStuffWorks
In fact, one of the reasons non-soap products were developed was that soap doesn't do so well in hard water. Water is considered to be hard when it has minerals such as calcium and magnesium in it. The ingredients in soap react to those calcium and magnesium ions, and, as a result, they can leave behind a film on your skin and on your tub.