Q It appears to me that when I use bar soap exclusively, the shower drain gets clogged, while it stays open when I use liquid soaps. Why would this be?
A Plumbers would agree with this observation, especially when the water supply is "hard", meaning that it contains chemicals, usually calcium or magnesium, that react with those in bar soap to yield scum.
Hard and soft soaps are each produced differently. In a process called saponification, an alkali is used to cause changes in animal or vegetable fats, leaving behind soap and glycerol.
If the alkali is sodium hydroxide, the result is a hard soap, relatively difficult to dissolve. Potassium hydroxide, on the other hand, produces an easily soluble soft soap.
Many shower gels are not technically soap but detergents, usually made from a petroleum base rather than animal or vegetable fats. They tend not to form drain-clogging scum.
To dissolve soap scum, plain vinegar, a mild acid and hot water are often sufficient - there's rarely a need for stronger drain cleaners.