No Suds! (water hardness)

This is an easy demonstration if you live in a place like the Midwest where hard water is readily available. Acquire some empty 20 oz. soda bottles and fill them halfway: one with hard water, one with deionized water. Add the same amount of soap to both bottle (several drops of inexpensive pine cleaner has worked for me, Ivory soap shavings also work well) and close the lids tightly on the bottles. Shake both bottles about 50 times. Immediately after shaking, the hard water will have more foam on its surface, but this foam quickly breaks, leaving only foam on the deionized (DI) water. The lack of foam on the hard water is due the calcium and/or magnesium ions in the water. These ions bind to the soap molecules in the water, rendering them useless. One result of this is that little or no foam forms on the surface of the water.

References:

Soriano, David S.; Draeger, Jon A. A water treatment experiment (chemical hardness) for nonscience majors. J. Chem Educ. 1993 70 414.

Birch, E. John H. Hardness in water- a demonstration. J. Chem Educ. 1949 26 196.