Chapter 8
Solutions, Acids, and Bases
8.1 Formation of Solutions
• DISSOLVING
• Recall that a solution is a homogeneous mixture of two or
more substances.
– Solute- substance whose particles are dissolved in a substance.
– Solvent-The substance in which the solute dissolves.
• These can be solids, liquids, or gases. Most familiar is
liquids.
3 WAYS TO FORM SOLUTIONS
• Substances can dissolve in liquids in three ways: dissociation,
dispersion, and ionization
– DISSOCIATION OF IONIC COMPOUNDS
• The particles that hold solutes to each other and solvents to
each other must be overcome. Ex: NaCl is an ionic
compound. Water is a polar molecule, and is attracted to
the salt. The sodium and chlorine ions are pulled into
solution one by one.
• Dissociation-The process in which an ionic compound
separates into ions as it dissolves.
When an ionic
compound dissolves
in water, the
charged ends of
water molecules
surround the
oppositely charged
ions.
3 WAYS TO FORM SOLUTIONS
– DISPERSION OF MOLECULAR COMPOUNDS
• Ex: Sugar and water are both polar molecules, so they
attract one another. When the attractions between them
are great enough to overcome the attractions holding the
sugar molecule to the surface of the crystal, The sugar
molecule breaks free, and is pulled into solution by the
water molecules.
• Dispersion-Breaking into small pieces that spread
throughout the water.
Saliva dissolves the
sugar in hard candy by
dispersion. As water
molecules collide with
sugar crystals,
attractions develop
between the water
molecules and sugar
molecules at the
surface of the solid.
3 WAYS TO FORM SOLUTIONS
– IONIZATION OF MOLECULAR COMPOUNDS
• When HCl dissolves in water the hydrogen atom gets
pulled away from chlorine and forms a hydronium ion.
• Ionization-Neutral molecules gain or lose electrons.
This is different from the previous two because it is a
chemical change. The solution that results contains new
substances.
Properties of Liquid Solutions
• Only three physical properties of a solution
will differ from those of its solute and solvent.
They are conductivity, freezing point, and
boiling point.
– CONDUCTIVITY
• When NaCl is dissolved in water, the ion are free to
move. This will conduct an electric current. NaCl not
dissolved will not conduct a current.
Properties of Liquid Solutions
– FREEZING POINT AND MELTING POINT
• MgCl2 is used in winter to melt ice and snow. It
dissociates into its ions. The structure of ice is
disrupted by the ions and cannot freeze.
• Coolant is added to cars to raise the boiling point and
prevent the car from overheating.
The presence of solute
particles affects how a
solvent freezes. A Pure
water freezes in a
hexagonal pattern. B In
water “salted” with
MgCl2, the dissociated
Mg2+ and Cl− ions disrupt
the formation of ice
crystals.
HEAT OF SOLUTION
• During the formation of a solution, energy is
either released or absorbed. Like chemical
reactions, this is described as endothermic or
exothermic.
• In order for a solution to form, bonds must be
broken, and new attractions formed.
– Breaking attractions requires energy
– The formation of attractions releases energy
• The difference in these energies is known as the
heat of solution.
FACTORS AFFECTING RATES OF
DISSOLVING
• Like rates of chemical reactions, rates of
dissolving depend on the frequency and
energy of collisions that occur between very
small particles.
• Factors that affect the rate of dissolving
include surface area, stirring, and
temperature.
• Surface Area
– When a sugar cube dissolves in water, the
dissolving takes place at the surfaces of the cube.
– The greater the surface area of a solid solute, the
more frequent the collisions are between solute
and solvent particles.
– More collisions result in a faster rate of dissolving
• Stirring
– Stirring moves
dissolved particles
away from the
surface of the
solid, and allows
for more collisions
between solute
and solvent
particles.
– Stirring will make it
dissolve faster.
The rate of
dissolving can
be increased by
reducing the
particle size of
the solute, by
stirring, and by
heating the
solvent.
• Temperature
– Increasing the temperature of a solvent causes its
particles to move faster, on average.
– As a result, both the number of collisions and the
energy of these collisions with solute particles
increase.
– The solute goes into solution more quickly.
– Increase in temp. will speed up the rate of
dissolving.
8.2 Solubility & Concentration
• Solubility- The maximum amount of a solute
that dissolves in a given amount of solvent at
a constant temperature
• Solubilities are usually expressed in grams of solute per
100 grams of solvent at a specified temperature
At a given
temperature,
different
solutes have
different
solubilities in
water.
3 Ways Solutions are Classified
• Solutions are described as saturated,
unsaturated, or supersaturated, depending on
the amount of solute in solution.
– Saturated Solutions - contains as much solute as
the solvent can hold at a given temperature
• The solvent is “filled” with solute. If you add more
solute, it will not dissolve.
3 Ways Solutions are Classified
– Unsaturated Solution - A solution that has less than the
maximum amount of solute that can be dissolved
– Supersaturated Solution is one that contains more solute
than it can normally hold at a given temperature.
• Supersaturated solutions are very unstable. If even a tiny crystal
of the solute falls into a supersaturated solution, the extra solute
can rapidly deposit out of solution
• If you heat a solvent up, more solute may dissolve. Then you then
carefully cool the solvent back without jarring it, you may be able
to keep the extra solute in solution.
In the photo sequence,
a single crystal of
sodium acetate,
NaC2H3O2, is added to a
supersaturated solution
of sodium acetate in
water. The excess solute
rapidly crystallizes out
of the solution.
Factors Affecting Solubility
• Three factors that affect the solubility of a
solute are the polarity of the solvent,
temperature, and pressure
Generally, a
solute is more
likely to dissolve
in a “like” solvent
than an “unlike”
solvent.
• Polar and Nonpolar Solvents
– Oil does not dissolve in water because oil
molecules are nonpolar and water molecules are
polar.
– guideline for predicting solubility is “like dissolves
like.”
Soaps and detergents are used to remove grease and oil stains. Soap molecules form attractions
to both polar water molecules and nonpolar oil molecules. As the water flows away, it carries
the oil with it.
• Temperature
– In general, the solubility of solids increases as the
solvent temperature increases
– When a glass of cold water warms up to room
temperature, bubbles form on the inside of the
glass. These bubbles are gases that were dissolved
in the water. They come out of the solution as the
water temperature rises.
• Unlike most solids, gases usually become less soluble
as the temperature of the solvent increases.
• Pressure
– Increasing the pressure on a gas increases its
solubility in a liquid.
– The pressure of CO2 in a sealed 12-ounce can of
soda at room temperature can be two to three
times atmospheric pressure
Concentration of Solutions
• Concentration of a solution-the amount of
solute dissolved in a specified amount of
solution
– Concentration of solution is usually expressed as
% by volume or % by mass.
• Percent by volume = volume of solute x 100
volume of solution
– Useful when dealing with liquids
• Percent by mass = mass if solute x 100
mass of solution
– Useful when solute is a solid
8.3 Properties of Acids and Bases
• Identifying Acids
– Acid- a compound that produces hydronium ions
(H3O+) when dissolved in water.
Properties of Acids
• Sour Taste
– Lemons, grapefruits, limes, and oranges have citric
acid
– Vinegar has acetic acid
– Spoiled dairy products have butyric acid
– NEVER TEST AN ACID BY TASTING IT
Properties of Acids
• Reactivity with Metals
– Acids and metals undergo a single replacement
reaction
•
• Color Changes in indicators
– Indicator – any substance that changes color in
the presence of an acid or base.
• Ex: Blue Litmus paper turns red
Identifying Bases
• Base- a compound that produces hydroxide
ions (OH-) when dissolved in water.
Properties of Bases
• Bitter Taste
– Unsweetened (baking) chocolate
– Cough syrup
• Slippery Feel
– Wet soap
– Wet rocks
• Color Changes in Indicators
– Red Litmus paper turns blue in a base
– Phenolphthalein turns red
– Hydrangeas turn blue in acid soil, pink in basic soil
Neutralization and Salts
• The reaction between an acid and a base is
called neutralization
– Negative ions in an acid combine with the positive
ions in a base to produce an ionic compound
called a salt.
The pH Scale
• pH scale – a number scale from 0 to 14 used to
describe the concentration of hydronium ions in a
solution.
– A low pH tells you that the concentration of H+ ions is
high. (acid)
– A high pH tells you that the concentration of H+ ions is
low. (base)
– pH lower than 7 = acidic (0-6)
– pH higher than 7 = basic (8-14)
– pH of 7 = neutral