Soap, Candy, and Bathtub Ring
Earlier in this chapter water hardness was mentioned in association with dissolved
calcium. Water hardness is manifested by the reaction of the dissolved calcium with soap
and refers to the formation of insoluble calcium salts of the soap anions. Sodium stearate
(see above), is a form of soap, and it reacts with calcium ion,
2CH 3 (CH 2 ) 16 CO 2 Na + Ca2+ → {CH 3 (CH 2 ) 16 CO 2 } 2 Ca(s) + 2Na+ (7.10.1)to form a solid precipitate of calcium stearate, {2CH 3 (CH 2 ) 16 CO 2 } 2 Ca. This material
does not work as a cleaning agent and deposits on bathtubs as infamous “bathtub ring.”
Some sugarless candies have calcium stearate as the main ingredient, mixed with artificial
sweeteners, flavors, and food coloring.
7.11. Heavy Metal Water Pollutants
The heavy metals are those metals of relatively higher atomic numbers. Some
heavy metals are considered among the most troublesome and toxic water pollutants.
The heavy metals of most concern in water are addressed briefly here.
Cadmium, Cd, is widely used in metal plating and in making small batteries,
such as those used in some cameras. Cadmium is very toxic, damaging red blood cells,
kidney tissue, and testicular tissue. Cadmium can enter water from industrial pollution
sources.
Lead, Pb (from its Latin name of plumbum), is arguably the most common heavy
metal pollutant because of its widespread use in industry and in the manufacture of
lead storage batteries as well as its former uses as a leaded additive to gasoline, as a
pigment in white house paint, and as an anticorrosive primer applied prior to painting
steel. Exposure to lead causes a number of adverse health effects and is suspected of
causing mental retardation in exposed children. Lead was widely used in plumbing, and
its use in solder to join together copper water pipe was banned only recently.
The most tragic modern incident of poisoning from mercury, Hg, occurred in the
Minamata Bay area of Japan from 1953 through 1960 when people consumed seafood
from the bay, which had been polluted by drainage of mercury wastes from a chemical
plant. The total number of cases of mercury poisoning reported was 111 and there were
43 deaths and 19 cases of congenital birth defects in babies whose mothers had eaten
the contaminated seafood. Hazardous methylated forms of mercury are discussed under
organometallic compounds below.
Arsenic, As, is a metalloid (among the elements bordering metals and nonmetals in
the periodic table), but its environmental and toxicological effects are much like those
of heavy metals. Toxic arsenic has been employed in hundreds of dastardly murder plots
over the years. Arsenic can cause both chronic poisoning over a long period of time and
acute poisoning from the ingestion of as little as 100 mg of the element. Before the advent
of more modern pesticides, arsenic compounds were used in huge quantities to kill pests
Chap. 7. Water, The Ultimate Green Solvent 173