The prevalent view among recent Halachic authorities, including Chacham Bentzion Abba Shaul (Jerusalem, 1924-1998), Rabbi Moshe Feinstein (Russia-New York, 1895-1986) and Chacham Ovadia Yosef, maintains that one should refrain from using bar soap on Shabbat. Although there is a minority view taken by the "Pachad Yitzchak" permitting the use of bar soap on Shabbat, most authorities require that one use specifically liquid soap. Those who are lenient in this regard need not be censured for their conduct, given that they have an authority on whom to rely, but it is far more preferable not to use bar soap on Shabbat.
It should be noted that Chacham Bentzion Abba Shaul requires adding water to dilute the liquid soap before Shabbat in order to render it permissible for use on Shabbat. Undiluted liquid soap is thick enough that one would transgress the prohibition of Memarei'ach – smoothening a thick substance – by smearing it on his skin on Shabbat. One must therefore add some water to smoothen the soap before Shabbat, and he may then use the liquid soap on Shabbat without concern.
Summary: One should not use bar soap on Shabbat. Liquid soap may be used on Shabbat, provided that one first dilute it somewhat before Shabbat by adding some water.
