Question

How can soap make soft water from hard water? What objections are there to using soap for this purpose?

Answer 1

The only way I know of is to use enough soap to precipitate all of the "hard" stuff in hard water; namely, the calcium and magnesium salts (primarily) with added amounts of iron, manganese, etc. This forms a soap scum which floats on top of the water. Frankly, I've never heard of using soap to "make water soft." There may be other ways of which I'm unaware.