HOW AND WHY DO SUBSTANCES
DISSOLVE IN WATER?
Learning Targets:
*I can explain aqueous solutions and the role of water as the
universal solvent.
*I can investigate factors that affect the rate at which water
functions as the universal solvent.
*I can describe the role of surfactants as solubility agents.
*I can explain the saying “like dissolves like” and relate this to
biomagnification of water contaminants in the food chain.
*I can describe an example of human impact
(biomagnification of pesticides) on ecological systems.
To
understand water quality, you need to
know
WHAT
WHY
dissolves in water,
it dissolves, and
HOW
to specify the concentration of the
resulting solution.
AQUEOUS SOLUTIONS
 SOLVENT-
A substance (usually a liquid) capable of
dissolving one or more pure substances.
 SOLUTE-
Solid, liquid or gas that is dissolved in a solvent.
 SOLUTION-
A homogeneous (looks the same throughout)
mixture of a solvent and one or more solutes.
 AQUEOUS
 SOLUBLE-
SOLUTION- Solution in which water is the solvent.
will dissolve
 INSOLUBLE-
will NOT dissolve
BASIC VOCABULARY
BECAUSE WATER IS SUCH A GOOD
SOLVENT, IT . . .
 is
practically NEVER 100% pure.
 contains
impurities.
 dissolves
tiny amounts of substances when it flows over
rocks and minerals.
 can
dissolve minerals containing ions that are toxic like
arsenic and fluoride.
 can
dissolve tiny amounts of gases in the air like O2 and CO2
 When
it rains, some pollutant gases are VERY soluble and
get dissolved in the rain water—SO2, Sulfur dioxide and NO2,
Nitrogen dioxide and forming acidic solutions—”Acid rain”.
HUMANS CONTRIBUTE TO SUBSTANCES
DISSOLVED IN WATER SOURCES
When
we wash clothes, we add detergent and
particles that washed off dirty clothes.
When
we flush toilets, we add liquid and solid
wastes.
Our
urban streets add solutes to
rainwater/storm run-off.
Our
agricultural practices add fertilizers and
other soluble compounds.
WHAT DOES WATER BEING A GOOD SOLVENT
MEAN FOR OUR DRINKING WATER?
In
order to decide whether drinking water is
“good” or “bad”—potable or non-potable, you
have to know what substances are in it and
HOW MUCH.
Amounts
of dissolved substances are expressed
as CONCENTRATION.
CONCENTRATION-
the ratio of the amount of
solute to the amount of solution.
SOLUTE CONCENTRATIONS IN
AQUEOUS SOLUTIONS
 Example:
Sweet Tea –If one teaspoon of sugar dissolved in 1 cup of
tea--or 3 teaspoons of sugar dissolved in 3 cups of tea, the
concentration is the same for both --1tsp sugar per cup of tea
4
Ways to Express Concentration:
 Percent
(%)
 Parts
per million (ppm)
 Parts
per billion (ppb)
 Molarity
(M)
 Parts
per hundred
 Medical
“Normal saline” solution given
intravenously is 0.9% solution by mass. 0.9
grams of sodium chloride (NaCl) in 100 grams
of solution. ( 99.1 grams of distilled water )
 Rubbing
alcohol/Antiseptic isopropyl alcohol is
70% aqueous solution by volume. 70 ml of
isopropyl alcohol in every 100 ml of solution.
PERCENT (%)
 Used
for low concentrations, typical in drinking water
 Water
that contains 1 ppm of calcium ions contains the equivalent of
1 gram of calcium (in the form of the calcium ion) dissolved in 1
million grams of water.
 Drinking
water contains naturally occurring substances present in the
parts per million range.
 US
standards set acceptable limits. For example: for nitrate ion,
found in some well water in agricultural areas is 10 ppm; the limit for
the fluoride ion is 4 ppm.
 Measuring
Liters.
1
1 million grams of water is not convenient so converted to
ppm of any substance = 1 mg solute dissolved in 1 L water
 Mg/L
used by water utilities to report the minerals and other
substances dissolved in tap water.
PARTS PER MILLION (ppm)
 Contaminants
are dangerous at much lower concentrations
than parts per million and are reported in parts per billion
(ppb).
 ppb
represents extremely small amounts—Example 1 ppb is
like a few centimeters on the circumference of the Earth!!
 Mercury
is a dangerous contaminant that is monitored in
water. The acceptable limit for mercury in drinking water is 2
ppb.
1
ppb is measured in more convenient terms as 1
microgram of solute in 1 liter of water.

1 microgram = 1 x 10 -6 g or 0.000001g
PARTS PER BILLION (ppb)
 Molarity
(M) – a unit of concentration represented
by the number of moles (mol) of solute present in 1
liter of solution.
M
= moles of solute
liter of solution
 Molar
mass of NaCl is 58.5 g (Na-23 + Cl-35.5),
therefore, 1 mol of NaCl has a mass of 58.5 g.
 By
dissolving 58.5 g of NaCl in some water and then
adding enough water to make exactly 1.00 L of
solution, we would have prepared a 1.00 M NaCl
aqueous (aq) solution.
MOLARITY (M)
THE OCEAN—AN AQUEOUS SOLUTION
WITH MANY IONS
 97%
of all water on our planet is found in the oceans.
 This
source contains much more that simple table salt
(NaCl)
 When
salt crystals dissolve in water, the polar H2O
molecules are attracted to the sodium ion (Na+ ) and
the Chlorine ion (Cl-). Over time, the ions are
separated and then surrounded by water molecules.
 NaCl
(s)  Na+(aq) + Cl- (aq)
 Some
ionic compounds dissolve easily others do NOT.
SOLUBILITY
The
sizes and charges of the ions determine
how strongly the ions are attracted to water
molecules.
Landmasses
on Earth are largely composed of
minerals—ionic compounds.
Most
have extremely low solubility in water—
like limestone-CaCO3.—Or everything would
dissolve and end up in the oceans!!
SOLUBILITY RULE:
“LIKE DISSOLVES LIKE”
 Polar
covalent compounds—like sugar-also dissolve in water.
 When
sucrose (sugar) dissolves in water, the molecules
disperse evenly among the H2O molecules. The molecules do
not separate into ions.
 Solubility
happens when an attraction exists between the
solvent molecules and the solute molecules.
 POLAR
molecules are soluble in other POLAR molecules!
 Hydrocarbon
in water.
 We
molecules in oil are NONPOLAR and are insoluble
cannot use water to wash off grease and oil.
SURFACTANTS
Compounds
that help polar and nonpolar
compounds to mix.
Contain
both polar and nonpolar groups.
Polar
groups dissolve in the water while the
nonpolar ones are able to dissolve in the
grease.
Examples:
soap, detergent
NONPOLAR SOLVENTS