What is hard water and why can’t soap be washed in it?

The hardest salts, calcium and magnesium, do not mix with the ingredients in the soap. They tend to form a soapy scum and very little suds. Thus, the harder the water, the more soap is required to foam.

Since soap is essentially sodium stearate, if you try to use soap in hard water (containing calcium carbonate), you get a reaction that forms calcium stearate. This process is also called sweeping.