We were once visited by a salesman who tried to sell us a water filtration system. He had a metal brief case full of chemicals, so naturally I looked forward to his presentation.By means of a precipitation reaction, he first demonstrated that our tap water contained a certain degree of hardness caused by magnesium (Mg+2) and calcium (Ca+2) ions. All natural water contains those ions to some varying degree. Our water had 120 ppm of the stuff, which is equivalent to about 120 mg of ions for every litre of water. These ions interfere with the action of soap. To demonstrate this he filled an Erlenmeyer flask with 120 ml of my tap water and a second flask with the same volume of tap water filtered with a resin-containing column that he pulled out of his metal brief case. The resin exchanges sodium ions for Mg+2 and Ca+2, which bind to a negative ion on the resin (SO3-2), effectively softening the water. Each sample then received 5 drops of his soap. The softened water and soap, upon mixing, created a persistent foam, whereas tap water and his soap produced only a fraction of foam. Moreover, the foam quickly subsided, and the remaining solution was mirkier than the one at the bottom of the softened water.
All of this begs three questions:
(1) What happened at the molecular level during his soap demonstration?
(2) Why was his demonstration misleading and not entirely relevant to everyday washing?
(3) Why is removing Ca+2 and Mg+2 from tap water not necessarily a good idea?
(1) Bar soap and liquid hand soap often contain a mixture of sodium stearate (C18H35O2Na) and sodium palmitate. Unfortunately, in the presence of either calcium or magnesium ions, stearate and palmitate will form precipitates, which no longer act as soap molecules. For example:
2 C18H35O2Na(aq) + Ca+2(aq) --> 2 Na+1(aq) + Ca(C18H35O2)2 (s)
When either the stearate or palmitate were added to the treated water, there was no calcium to interfere with the soap ions.
(2) His presentation falsely suggested that all soaps are alike. In reality, detergents contain either sodium alkylbenzenesulfonate, which in the presence of Ca+2 and Mg+2, does not precipitate to the same extent as bar soap. Older types of detergents contain phosphates, which will “fish out” the problematic calcium and magnesium ions (by forming insoluble magnesium or calcium phosphate). After some dubious excuses, the salesman agreed to repeat his experiment with my own detergent, and the differences were far less dramatic. There was only slightly less foam produced when the detergent was added to my untreated tap water.
(3) Drinking water should not be free of Ca+2 and Mg+2. Both of these are essential ions for the human body. Calcium ions are needed for bone formation and teeth. It also plays a role in blood clotting, muscle contraction and relaxation, nerve transmission, and enzyme(ATPase) activation. Magnesium is also needed for bones and teeth. It is a coenzyme in carbohydrate and protein metabolism and helps nerves and muscles function.