We will spend the next few days discussing the laws of Shabbos as they pertain to hygiene.
The issue: mema’rei’ach/ smoothing, nolad/ creating something new
The solution: One may not use a bar of soap on Shabbos due to the prohibition of smoothing. Liquid soap may be used on Shabbos. The way to distinguish between a liquid and solid is if when the bottle is turned on its side, the soap moves along with it. There are some who are stringent and do not use foam soap because it creates something new but most are lenient.