Chapter 11
Water and Solutions
Water
• The universal solvent. It has the ability to dissolve many
molecules.
• In living systems these molecules can then be transported
from one place to another by diffusion or by some kind of a
circulatory system.
• Liquid water has a higher density than ice (solid water). Ice
thus floats on the surface of liquid water. Fish and other
organisms can then live below the ice in natural bodies of
water.
• Water has a very high specific heat. Therefore, large bodies
of water can moderate temperature by absorbing great
amounts of heat.
• Water has a very high latent heat. This means that a great
amount of heat is needed to evaporate water. This is what
occurs when people perspire, we get rid of a lot of heat
from our bodies, thus enabling us to withstand high
temperatures.
Bonds in the Water Molecule
• The H-O bonds in the water molecule are polar
covalent. The oxygen atom attracts the electrons
more than the hydrogen atoms because it is
more electronegative.
• Since the molecule has a “V” shape:
O
Negative end
H
H Positive end
• The oxygen end of the molecule is therefore
more negative and the hydrogen end of the
molecule is more positive.
• The molecule is said to be polar and thus
possesses a dipole (a negative and a positive
end).
Hydrogen Bonding
• Due to the dipole moment in the water molecule
water has many unusual characteristics.
• Hydrogen bonding occurs whenever a H atom is
bonded to an O, N, or F atom, since these are
the most electronegative elements. The
electrons are pulled very close to the O, N, or F
atom and therefore the O, N, or F atom becomes
partially negative whereas the H atom bonded to
it becomes partially positive.
• A dotted line is used to represent a hydrogen
bond, as opposed to the solid line which is used
for a single bond.
Hydrogen Bonding
• The partially negative oxygen of one water
molecule attracts and is hydrogen bonded
to a partially positive hydrogen of another
water molecule. This happens throughout
the water molecules present in a sample
of water.
Effects of Hydrogen Bonding
• Due to the hydrogen bonding in the water
molecules water exhibits all the unusual
properties that it is known for and which
ultimately permit life on earth to exist.
• Due to the great attraction of water molecules for
one another the boiling point and melting points
of water are very high for such a small molecule,
the molecules like to stay as close to one
another as possible.
• Other small molecules like water tend to be
gases at room temperature and do not become
solids unless the temperature is extremely low,
much lower than 0oC.
Hydrogen Bonding in Ice
Hydrogen bonding occurs in solid water (ice), but it occurs as part of a very
organized crystalline structure in the forms of hexagons. When the ice melts
the molecule collapses from its highly organized crystal structure to a more compact
structure which permits greater approach of the hydrogen bonded H and O atoms.
This is why liquid water is more dense than ice.
Dissolving Process
• There are two types of mixtures:
• Heterogeneous mixtures, like oil and vinegar, or sand
and water, are those where two or more distinct layers
exist. The two substances are not soluble in one another.
• Homogeneous mixtures, like table salt in water or sugar
in water, are those where there is only one layer, since
the substances are soluble in one another. A
homogeneous mixture is a solution.
• A homogeneous mixture that is in the liquid state can be
recognized because it is clear (transparent). It can be
colored or colorless.
Solutions
• A solution is made up of a solute and a solvent.
• The solvent is the substance which is present in
the higher amount. The solute is present in the
lesser amount.
• Solutions can be in the gas, liquid or solid states,
although the most common are in the liquid
state.
• Solutions in the liquid state can have a gas, a
liquid, or a solid dissolved in a liquid.
• Air is a gaseous solution made up of 78% N2,
21% O2, .9% Ar, and other miscellaneous gases.
Therefore N2 is the solvent.
Solutions
• Aqueous solutions are those in which the solvent is water. Aqueous
comes from aqua, the latin word for water. The solute can be a solid,
a liquid, or a gas.
• A solution becomes saturated when the maximum amount of solute
is dissolved in the solvent.
• A solution which has less than the maximum amount of solute
dissolved is unsaturated.
• If more than the maximum amount of solute is added to a solvent
than can dissolve in it at a certain temperature, then the excess will
settle on the bottom of the container (in the case of a solid solute)
and the solution itself will be saturated, since only the maximum that
can dissolve will be dissolved.
• For example, if the maximum solubility of substance “A” in water is
20 grams of A per 100 grams of water:
Adding 15 grams of A to 100 grams of water-Unsaturated
Adding 20 grams of A to 100 grams of water-Saturated
Adding 30 grams of A to 100 grams of water-Saturated
Adding 20 grams of A to 150 grams of water-Unsaturated
Adding 10 grams of A to 50 grams of water-Saturated
Degrees of solubility
• Miscible substances:
When there is not limit to how much of a
solute can dissolve in a solvent.
• Mixtures of gases are always miscible.
• Like dissolves like: Two polar substances
tend to dissolve in one another. Two non
polar substances tend to dissolve in one
another. Water and ethyl alcohol are both
polar and are miscible in one another.
Degrees of Solubility
• Water is a polar substance, CCl4 (carbon
tetrachloride) is a non-polar substance, since
even though the C-Cl bonds are polar the
molecule has a symmetrical geometry, so it does
not have a more positive or a more negative
end.
• CCl4 will dissolve greases and oils, which are
non polar.
• Soap consists of a molecule which has a part
that is polar and a part that is non polar. Oils and
grease will not dissolve in water alone. However,
soap can be dissolved in water through its polar
end and it will in turn dissolve oils and grease
and is therefore used for cleaning.
Geometry of CCl4
Cl
Cl
C
Cl
Cl
The less electronegative atom, the C, is in the middle
symmetrically surrounded by Cl atoms in the form of a
tetrahedron. Therefore the molecule does not have a
more positive or more negative end and it is nonpolar.
Dissolving Ionic Compounds in
Water
• Many ionic compounds, like NaCl, are
soluble in water because the Na+ is
attracted to the partially negative O of the
water molecule. The Cl- is attracted to the
partially positive H of the water molecule.
• This is called hydration, since the ions
become surrounded by water molecule.
Dissolving Ionic Compounds in
Water
• There is also the effect of the great attraction that
positive and negative ions in an ionic compound have for
one another.
• For an ionic compound to be soluble in water the
attraction of the water molecules and the ions has to be
greater than that of the ions for one another.
• Saturation occurs because there are less water
molecules available to compete for the ions. It is like a
tug of war.
• Ionic compounds that have a greater attraction for its
ions than for water are said to be “insoluble” in water.
Usually some miniscule amount will dissolve when the
compound is considered insoluble. Often this will be
ionic compounds which have ions that have +2 and -2
charges or higher. Most ionic compounds that contain at
least one ion with a +1 or -1 charge are soluble in water.
Ionic Compounds dissolved in
Water
• NaCl(s)
CaCO3(s)
H2O
H2O
Na+(aq) + Cl-(aq)
CaCO3(s)
Concentration of Solutions
• Concentration refers to the amount of
solute which is dissolved in a given
amount of solvent.
• In a qualitative manner when the
concentration is high the solution is said to
be concentrated. When the concentration
is low the solution is said to be dilute.
Concentration of Solutions
• In a quantitative manner one can express
concentration of solutions in many ways.
One of the ways that are used are:
• % by volume= volume of solute x 100
volume of solution
• An example is rubbing alcohol, which can
be, for example, 12% by volume. This
means that there are 12 mLof isopropyl
alcohol per 100 mL of aqueous solution.
Concentration of Solutions
• % by mass=mass of solute x 100
mass of solution
• The mass can be in grams or any other mass
unit in the metric system. It could also be in
weight units in the English system, like ounces.
% by weight is equivalent to % by mass.
• Hydrogen Peroxide, H2O2 is sold as 3% by
weight, which means that there are 3 ounces of
H2O2 per 100 ounces of aqueous solution.
2 parts solute x 100 = 2%
100 parts soln
2 mL solute
100 mL soln
2 g solute
100 g soln
x 100 = 2%
x 100 = 2%
The % by mass of salts in salt water (salinity) is:
35 g salts
x 100 = 3.5 %
1000 g soln
Solubility of Ionic Compounds
• Solubility is defined as the maximum amount of
a solute that will dissolve in a given solvent at a
specified temperature. When you reach this
maximum amount of solute the solution is said to
be saturated.
• For example, adding sugar to tea. If you add too
much sugar, some will not dissolve because the
solution becomes saturated.
• For most solutions solubility increases with
temperature. There are exceptions, however.
Solubility of Gases
• Gases are the opposite of the majority of ionic
compounds when it comes to the relationship
between solubility and temperature.
• The higher the temperature the lower the
solubility of gases in a solution.
• If you open up a soda can at room temperature
more of the dissolved carbon dioxide gas will
escape and the soda will become flat faster than
if you open it after just taking it out of the
refrigerator. This is because the carbon dioxide
gas is less soluble in water at the higher
temperature.
Electrolytes
• Electrolytes are substances or solutions which
conduct electricity. Often these will be aqueous
solutions.
• In order to conduct electricity there has to be a
flow of electrons through the solution.
• In order for there to be a flow of electrons
through a solution there have to be ions in the
solution.
• The more ions there are the stronger the
electrolyte will be.
Only ionic compounds
Or acids dissolved in
water will produce ions in
solution and electricity can
be then conducted through the
Solution.
Non Electrolytes
• All covalent compounds except for acids are non
electrolytes, since no ions are present in
solution.
• Water is a covalent compound, so it is a non
electrolyte, so are all alcohols and most other
substances which are liquids or gases at room
temperature.
• Acids are unique, since they are covalent
compounds but they ionize when dissolved in
water. All acids are soluble in water.
Aqueous Solutions of Acids
• HCl(g)
H2O
H+(aq) + Cl-(aq)
hydrogen ion
• This is the same as:
• HCl(g)
H2O
H3O+(aq) + Cl-(aq)
hydronium ion
The hydrogen ion (H+) is the same thing as the hydronium
(H3O+) ion.
• The hydrochloric acid, HCl is ionized or dissociated in
water, so hydrochloric acid is an electrolyte. The name
hydrochloric acid already implies that the acid, HCl or
hydrogen chloride, is dissolved in water.
Boiling Point
• Occurs when the pressure of the vapor escaping
from a liquid is equal to the atmospheric
pressure.
• The pressure exerted by the vapor is called the
vapor pressure.
• The normal boiling point of a liquid is the boiling
point at 1 atm of pressure, which is the pressure
at sea level.
• The normal boiling point of water is 100oC, or
212oF.
Pure
Solvent
Solution
The rate of evaporation, and thus the vapor pressure, is less for
a solution than for a solvent in the pure state. The greater the
solute concentration, the less the vapor pressure. Therefore,
the higher the boiling point.
Boiling Point of Solutions
• It doesn’t matter what the substance is,
the more particles of solute are present,
the more the boiling point will be
increased, because boiling occurs from
the surface and the presence of solute
particles decreases the rate of particles
escaping to the gas state. They hinder the
boiling process.
• This is called boiling point elevation
The more solute particles there
are, the higher the boiling point.
For ionic compounds each ion counts
as a particle, so the effect is greater.
Freezing Point
• Freezing occurs when the kinetic energy of
molecules has been reduced sufficiently so the
molecules can come together, forming the
crystal structure of the solid.
• The freezing point of water at 1 atm is 0oC or
32oF.
• In aqueous solutions, the presence of solute
particles interferes with the water molecules as
they attempt to form the six-sided hexagonal
crystalline structure.
• The temperature has to be below the freezing
point of the solvent for the solution to freeze.
Freezing Point of Seawater
• The freezing point of seawater is lower
than that of water.
• The ice which forms does not contain any
solute particles, it is pure water.
• As some ice forms the remaining water
contains a higher solute concentration, so
the freezing point is further decreased.
• This is called freezing point depression.
Practical Uses of Freezing Point Depression
and Boiling Point Elevation
• Salt is spread on snow to lower the freezing
point of water, which is what snow is made up of.
CaCl2, calcium chloride, is often used for this
purpose. This avoids the roads to become
slippery after the snow melts.
• Ethylene glycol is antifreeze and is added to
radiator water to lower the freezing point and
raise the boiling point of water. This is protection
against temperatures that are too high or too
low.
Acids, Bases, and Salts
• Salts are ionic compounds. The hardness
of water is related to ionic compounds or
salts dissolved in water.
• The acidity of soils determines how well
plants grow.
• Acid rain is a by product of industry and
automobiles. This is harmful for living
organisms.
Acids, bases and salts
• Acids have sour
tastes
• Changes litmus
paper from blue to
red
• Acids react with
metals releasing
hydrogen gas
• Acids neutralize
bases forming water
and salt
Acids, bases and salts
• Bases have bitter taste
(caffeine)
• Bases turn litmus
paper blue
• Basic solutions feel
slippery on skin.
• Bases neutralize acids
forming water and salts
Acids, bases and salts
• Are there any similarities in the lists
shown before?
Acid - any substance that is a
proton donor when dissolved
in water.
A proton is a hydrogen ion,
H+
Hydronium ion
This is the same as:
H2O
HCl
H+(aq) + Cl-(aq)
Bases
• Many bases are ionic compounds which
contain hydroxide ion (OH-) as the
negative ion.
• They produce OH- when dissolved in
water:
NaOH
Na+(aq) + OH-(aq)
Proton acceptor
A proton is a hydrogen ion (H+)
Acids, bases and salts
• Other bases, like ammonia, NH3, will produce the OHonce it is dissolved in water, since a hydrogen ion gets
transferred from a water molecule to the ammonia
molecule.
H+OHHydroxide ion
Base - Any substance that is a proton
acceptor when dissolved in water.
Proton acceptor
Acids, bases and salts
• What happens when acids and bases
mix?
The H+ from the acid and the OH- from the base react to form water:
H+ (aq) + OH-(aq)
H2O(l)
This is called neutralization
Strong Acids
• A strong acid is one that ionizes (dissociates) 100% in aqueous
solution.
• HNO3
H2O
H+(aq) + NO3-(aq)
100%
• Acids can be recognized because the formula begins with “H”.
• Common strong acids include:
HCl hydrochloric acid
HBr hydrobromic acid
HI
hydroiodic acid
H2SO4 sulfuric acid
HNO3 nitric acid
Weak Acids
• Weak acids only partially ionize (dissociate) in aqueous
solution. They only produce a small percentage of
hydrogen ions (H+).
H2O
• HF
not 100%
H+(aq) + F-(aq)
• A double arrow indicates an equilibrium, so the reaction
occurs both ways, to the right and to the left.
• The arrow to the left is longer because there is more of
the non ionized acid present than of the ions at any
given time.
Weak Acids
• Common weak acids include:
HC2H3O2 acetic acid
H2CO3 carbonic acid
• HC2H3O2
H 2O
H+(aq) + C2H3O2-(aq)
In the case of acetic acid, only about 1%
of the acid molecules ionize in aqueous
solution.
Strong Bases
• Strong bases are those that completely (100%)
ionize (dissociate) in aqueous solution.
NaOH H2O
Na+(aq) + OH-(aq)
100%
• Common strong bases include:
NaOH sodium hydroxide, common name lye, which
is used to make soap.
KOH potassium hydroxide
Also the other Group IA and Group IIA hydroxides
except for Be(OH)2 and Mg(OH)2.
Weak Bases
• Weak bases only partially ionize
(dissociate) in aqueous solution.
Mg(OH)2
Mg2+(aq) + 2OH-(aq)
not 100%
NH4+(aq) + OH-(aq)
NH3(aq) + H2O
not 100%
The pH Scale
• The strength of acids and bases is
measured using the pH scale.
• A neutral substance, like water, which is
neither an acid nor a base, has a pH of 7.
• Acidic solutions (acids dissolved in water)
have a pH lower than 7.
• Basic solutions (bases dissolved in water)
have a pH higher than 7.
Salts
• When an acid reacts with a base a salt (ionic
compound, except bases) is formed. Water is
also formed:
HCl + NaOH
acid base
2HNO3 + Ca(OH)2
acid base
NaCl + H2O
salt
water
Ca(NO3)2 + 2H2O
salt
water
• Neutralization always occurs 100% between an
acid and a base even if they are weak.
Importance of Salts
• Salts are important in the diet, since we need
them as electrolytes and as a source of certain
elements. These are minerals.
• Plants also require the elements that can be
provided by certain salts and are added to plants
as fertilizers. For example, K, N (supplied as
nitrates, NO3-), and P (supplied as phosphates,
PO43-).
• Not all salts are soluble in water. Insoluble salts
is what gives rise to hard water that ends up
forming rings in your bathtub because when
mixed with soap insoluble salts are formed.
Practice Exercises
• p. 294-296 Applying the Concepts:
# 1, 2, 3, 4, 6, 7, 9, 10, 12, 13, 14, 15, 16,
17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27
Review Chapter 11
•
•
•
•
•
•
•
•
Water-polar molecule, undergoes
H-bonding.
Unusual characteristics of water
molecule.
Heterogeneous and
Homogeneous (Solutions)
mixtures.
Saturated, unsaturated, and
miscible solutions.
Like dissolves like: Polar solvents
like water dissolve polar
molecules and ionic compounds.
Non polar solvents, like Carbon
Tetrachloride dissolve nonpolar
solutes, like grease.
Soap-Polar and non polar ends,
dissolves in water and in oils and
greases.
How ionic compounds dissolve in
water.
Concentrated vs. dilute. % by
mass and % by volume.
•
•
•
Solubility-maximum amt. of solute
that can dissolve in a solvent at a
given temperature.
Solubility of ionic compounds
usually increases with increasing
T, solubility of gases decreases
with increasing T.
Electrolytes: contain ions in
solution. Includes ionic
compounds dissolved in water and
also acids dissolved in water.
Covalent compounds, including
water, are non electrolytes.
Review for Chapter 11 (cont.)
•
•
•
•
•
Ionization of strong and weak acids in
aqueous solution (100% for strong
acids, equilibrium for weak acids).
(Strong acids are sulfuric, nitric,
hydrochloric, hydrobromic, and
hydroiodic acids. The rest are weak
acids.)
Ionization of strong and weak bases in
aqueous solution.(100% for strong
bases, equilibrium for weak bases).
(Strong bases are the group IA (alkali
metals) combined with hydroxide ion.
What is vapor pressure-The pressure
exerted by the vapor that evaporates
from a liquid on the surface of the
liquid. Vapor Pressure increases with
temperature.
What is boiling point. The temperature
at which the vapor pressure of a liquid
is equal to the atmospheric pressure.
Boiling point elevation and freezing
point depression in solutions. (The
boiling point increases, the freezing
point decreases).
• Properties of acids and bases.
• How ammonia, NH3 is a base
because it reacts with water to
form NH4+ and OH- in aqueous
solution.
• Neutralization reactions: Acid +
Base react to produce salt
(ionic compound) plus water.
• The pH scale: 0-14. 7 is
neutral, like water. Less than7
is acidic, higher than 7 is
basic.
• The importance of salts: They
provide electrolytes to living
organisms, including plants,
animals and humans and they
are a source of some essential
elements.