Hard water is water that does not produce a slippery feeling between the hands during washing.
Distilled water or rain water are very soft water: That means they make a very soft feeling during washing.
*  Add hard water to one vial and very soft water to a second vial.
*  Add a drop of soap solution to each of the samples.
*  Close by stoppers and shake the vials for a while.
Observations: On the left side of the photo: Hard water produces a white precipitation and no foam.
On the right side of the photo: Soft water remains clear, there is a lot of foam.