Section 9.2: Water Treatment
Research This: What Is in Your Water?, page 434
A. Answers may vary. Sample answer: One biological contaminant mentioned in the report was
Escherichia coli.
B. Table 1 Inorganic and Organic Substances in Water
Substance in treated
water
Concentration
MAC
fluoride
0.06 mg/L
1.5 mg/L
nitrate
0.32 mg/L
10.0 mg/L
carbon tetrachloride
< 0.2 µg/L
5.0 µg/L
benzene
< 0.2 µg/L
5.0 µg/L
The concentrations of inorganic and organic compounds in treated water are well below
the MAC.
C. Answers may vary. Sample answer: I found the number of substances present in water to be
surprising.
D. Answers may vary. Sample answer: The concentration of substances in the water was well
below the MAC. Other characteristics such as pH and clarity were also within acceptable limits.
Therefore, the water should be safe to consume.
E. Answers may vary. Sample answer: According to the product label on one brand of bottled
water brand, the product contains no fluoride, nitrate, or toxic metals such as copper and lead.
The tap water sample did not contain copper or lead, but it did contain an extremely low
concentration of mercury — well below the MAC. Therefore, both water samples are safe to
drink. However, the lack of fluoride in the bottled water may be of concern to some concerned
about the health of their teeth.
Mini Investigation: Testing a Cationic Exchange Resin, page 435
A. The treated water sample was less hard than the original because it took fewer drops of EDTA
to reach the endpoint.
B. The indicator was necessary to provide a visual indication of when the hardness ions in the
water had completely reacted with EDTA.
C. To control the variables in this activity, identical volumes of water, indicator, and buffer
solution were used.
Section 9.2 Questions, page 436
1. The three categories of water contaminants are physical contaminants, which includes
suspended solids such as sand, chemical contaminants, which includes agricultural chemicals
such as pesticides, and biological contaminants, which includes bacteria.
2. Sanitation and the quality of drinking water are often related because poor sanitation results in
the contamination of drinking water.
3. (a) Perc is a chemical contaminant.
(b) Grease consists of fats and oils, which are non-polar substances. Since perc dissolves grease,
perc must also be non-polar.
(c) Since water is polar and perc is non-polar, the mixture should separate into a two-phase
mixture. Since the density of perc is greater than the density of water, perc would be the bottom
layer of the mixture while water would float on top.
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Chapter 9: Solutions and Their Reactions
9.2-1
(d) Since oil is less dense than water, oil floats on water. This allows for oil to be skimmed off
the surface or soaked up by absorbent materials. Since the density of perc is greater than the
density of water, perc will sink to the bottom of a lake. This means that a perc spill is more
challenging than an oil spill to clean up.
4. It is important that the MAC of water contaminants be reconsidered from time to time as our
understanding of the properties and health risks of different contaminants increases.
5. Given: cppm = 0.010 ppm
Vwater = 270 L
Required: mass of lead, mlead
Analysis: cppm =
msolute
msolution
! 106
Solution:
Step 1. Convert 1.0 L of water to mass.
The density of water is 1.0 g/mL.
mwater =
1.0 g
1.0 mL
!
1000 mL
! 270 L
1L
mwater = 2.7 ! 105 g
Step 2. Rearrange the equation and substitute values.
cppm ! mwater
mlead =
106
0.010 ! 2.7 ! 105 g
=
106
mlead = 2.7 ! 10 –3 g
Statement: The mass of lead in 270 L is 2.7 × 10–3 g or 2.7 mg.
6. (a) Floc is a white fluffy precipitate used to remove suspended particles in the water treatment
process. As floc particles form, suspended solid contaminants in water stick to the floc particles
and sink to the bottom of the settling tank.
(b) Two chemical alternatives to using chlorine include using ozone or chlorine dioxide. The use
of ultraviolet light is a non-chemical alternative to chlorine.
(c) Ammonia is added to the water because it stabilizes the chlorine so that it remains dissolved
in the water for longer periods of time.
7. Answers may vary. Sample answer: A simple test to determine if the tap water is hard is to
add soap and mix it up. If a curd-like precipitate forms with the soap, the water is hard.
8. (a) Ca2+(aq) + CO32−(aq) → CaCO3(s)
(b) An appropriate carbonate compound is sodium carbonate or washing soda, which is highly
soluble.
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Chapter 9: Solutions and Their Reactions 9.2-2
9. (a) Water softener is necessary in communities that get their water from underground sources
because the water is hard. When the water flows through the rock, minerals dissolve in the water.
(b) It is necessary to regenerate the resin in a home water-softening system to keep the system
working properly. The resin works by removing calcium and magnesium ions in the water with
“soft” ions such as sodium or potassium ions in the resin. Once the soft ions are depleted, the
resin no longer functions. Regenerating the resin replaces these ions.
10. EDTA reacts with calcium ions, which prevents them from precipitating with detergent
anions.
11. During a boil-water advisory, boiling the water eliminates biological contaminants such as
bacteria and viruses in the water. However, boiling water does not eliminate chemical
contaminants unless the heating causes the chemicals to decompose into simpler and safer
substances. Similarly, boiling water cannot remove physical contaminants such as silt.
12. The hard-water scale from the inside of a kettle consists of mostly carbonate compounds such
as calcium carbonate. The scale can be removed by soaking the coils of the kettle in vinegar.
The acetic acid in vinegar reacts with calcium carbonate in the following reaction:
HC2H3O2(aq) + CaCO3(s) → CO2(g) + H2O(l) + Ca(C2H3O2)2(aq)
13. (a) Trihalomethanes that are found in water include chloroform, bromodichloromethane, and
bromoform. Chloroform is the most common trihalomethane found in water. It is a possible
carcinogen and there is a link between reproductive effects and levels of trihalomethanes.
(b) According to Health Canada, the health risk of not disinfecting drinking water far outweighs
the risk of trihalomethanes produced in the treatment process. Furthermore, provided the levels
of trihalomethanes fall within government guidelines, the benefits of chlorine use outweigh the
risks. However, effective alternatives, such as ozonolysis, should be considered to further reduce
the risk.
14. As hair, toenails, and fingernails grow, they incorporate toxins such as arsenic and selenium.
As a result, the composition of hair or nails provides a “snapshot” of the toxins present in the
body. Because hair and nails grow so slowly, they provide a record of exposure over a long
period of time. The same toxins in the body may be quickly metabolized and excreted from the
body. Furthermore, providing samples of nail clippings is far more convenient and less painful
than providing a blood sample.
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Chapter 9: Solutions and Their Reactions 9.2-3