Washing ones hands in salt or soap:
It is forbidden to wash ones hands in salt and certainly with soap or other fats.
The reason for this is: because [the material] dissolves in ones hand and is as if one has created a new substance on Shabbos which resembles a forbidden action, as was explained in “The Laws of melting snow and ice” [Halacha 4] that it is forbidden to crush snow and hail for this reason. [This prohibition is referred to as Molid.]
Other Opinions: [However] according to those which [held there] that the reason for the prohibition of crushing snow and hail is because of a decree made [to safeguard one from coming to] squeeze fruits which are designated for their juices as snow and hail are likewise designated for their liquids as was explained there [in Halacha 19], [then according to them] soap and other fat which are not designated as liquids are permitted to be [even] initially crushed [when washed in water].
The Final Ruling: One is to be stringent like the former opinion.
Summary of using soap:
It may not be dissolved with ones hands. Thus it is forbidden to wash ones hands or other body parts with a block of soap.