This post does not ask a question but gives information on how to avoid the formation of soap scum or soap rings on bathtubs in the first place. It was prompted by recent questions on how to remove soap scum from bathtubs.
What's the difference between a "soap" and a "detergent".
The answer is that soap is made from natural materials like plant oils (palm oil and olive oil most commonly) whereas detergents are man-made from chemicals in laboratories.
One of the drawbacks of using soap is that hardness ions in the water will cause soap molecules to form soap scum, and that requires extra cleaning to keep bathroom sinks and tubs clean. Soap scum, in fact, is soap that has simply lost it's solubility in water.
Detergents, on the other hand, are made from chemicals and can be formulated not to be affected by those hardness ions. Consequently, detergents don't lose their solubility in water, and will never form a soap scum ring. This is precisely the reason why you might have a soap scum ring in your bathtub and your bathroom sink, but you'll never see a soap scum ring in your kitchen sink. In fact, about the only time you'll ever see a soap scum ring in a kitchen sink is when people use bar soap in that kitchen sink.
So, to avoid soap scum, avoid the use of bar soap.
Use shampoo and cream rinse in your hair (as those are both detergents) and use a detergent to clean your body instead of soap. Mr. Clean and Fantastik are both very good detergents. Dish washing detergents are formulated to remove cooking oils and grease, but can also be used to clean your skin. Judy apply the detergent to a damp sponge and use it to clean your skin. Or apply it to a very gentle Loofa sponge or white Scotchbrite pad and exfoliate dead skin cells at the same time.
Anyhow, in my position as landlord, I can't tell tenants not to use soap, and to bathe with Mr. Clean, so I have to remove soap scum anyhow. However, for people that are looking for a way to prevent soap scum formation, this is one way to do it.
Also, there is some technology in knowing HOW to clean a surface with soap or detergent.
Both soap and detergent molecules consist of long hydrocarbon chains with one end being polar (that is, with an apparant charge) that causes the end of the molecule to be attracted to water molecules (which are also polar). It's that polar end on the soap molecule that makes it soluble in water. The hydrocarbon chain isn't soluble in water, and so when soap dissolves in water it forms microscopically tiny "balls" called "miscelles".
Because only one end of the soap molecule is attracted to water, these soap miscelles form balls with the polar ends on the outside, and all the hydrocarbon chains collected together on the inside. (The above diagram shows a blob of cooking oil or grease on the inside of the inside of the miscelle, but it really should take up the entire inside volume of the miscelle.
In order to use any soap or detergent effectively, you have to SCRUB the surface to be cleaned with diluted soap or detergent. That scrubbing with soap miscelles causes them to break apart on the surface you're cleaning so that the hydrocarbon ends of the soap or detergent molecules penetrate into the oil or grease on the surface being cleaned. Thus, during cleaning, the soap molcules form a PLANE on the oil or grease with the hydrocarbon chains dissolved in the oil or grease and the polar ends sticking out into the water.
Further scrubbing causes the oil or grease on the surface being cleaned to be scrubbed off that surface and into the water so that the soap molecules dissolved in it once again form a spherical structure with the oil or grease encapsulated on the inside of the miscelle. That full miscelle not only suspends the oil or grease in water so that it can be washed away easily with water, but it prevents the soil inside the miscelle from being redeposited on the surface being cleaned.
So, it's not enough to simply lay in a tub of soapy water. In order for soap or detergent to work, scrubbing is ESSENTIAL. You don't have to scrub hard, but you do have to scrub to both break up the soap miscelles on the surface to be cleaned and scrub the surface to get the soap molecules back into the water so they can reform those spherical miscelles with the soil encapsulated inside them.
There, now you know how to wash your hands.