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Chapter 8
Substances, Mixtures, and
Solubility
I. Substances
A. Atoms and Elements
• A substance is matter that has the same
fixed composition and properties; i.e. an
element
• A substance can not be broken down into
simple parts by physical processes
• Only a chemical process can change a
substance into one or more new substances
B. Compounds
• A compound is made of two or more
elements that are chemically combined in
fixed compositions
- The ratio of compounds is always the same;
i.e. water H20
II. Solutions
• A solution is a
homogeneous
mixture – two or
more substances that
are evenly mixed but
are still not bonded
together; i.e. sugar
water
• Solutions are formed by dissolving
- The substance that dissolves – or seems to
disappear – is called the solute; i.e. the sugar
- The substance that dissolves the solute is
called the solvent and is usually present in
the greatest quantity; i.e. the water
A. Forming Solids from Solutions
• A solute can come back out of its solution
and form a solid
- Crystallization
- Result of a physical change
• A solid can form as a result of a chemical
reaction when some solutions are mixed
- Precipitation
- Result of a chemical change
B. Liquid Solutions
1. Liquid-Gas Solutions
• Carbonated beverages
Solvent – water, other liquid
Solute – carbon dioxide (gas)
2. Liquid-Liquid Solutions
• A smaller amount of one liquid (solute) is
dissolved in a larger amount of another liquid
(solvent)
• Vinegar
Solvent – water
Solute – acetic acid
C. Gaseous Solutions
• A smaller amount of one gas (solute) is
dissolved in a larger amount of another gas
(solvent)
• The air you breathe
Solvent – Nitrogen (78%)
Solute – Other gases i.e. oxygen, carbon
dioxide
D. Solid Solutions
• A solid can be mixed with another solid,
liquid, or gas
• Solid-Solid solutions are called alloys
i.e. steel and brass
III. Solubility
• Water can dissolve many different solutes,
thus chemists call it the universal solvent
• A solution in which water is the solvent is
called an aqueous solution
A. Molecular Compounds
• In a covalent bond, electrons are shared
• Compounds that contain covalent bonds are
called molecular compounds, or molecules
• Include polar (H2O) and nonpolar (H2)
molecules
B. Ionic Bonds
• In an ionic bond, electrons are gained or lost
• The compound formed is called an ionic
compound Na+Cl-
C. How water dissolves ionic compounds
• Water is polar and has positive and negative ends
- The positive parts of water (H+) are attracted to
negative ions (Cl-)
- The negative part of water (O-) is attracted to
positive ions (Na+)
- Water can thus pull apart the different ions of an
ionic compound
D. How water dissolves molecular compounds
• Water does not break molecular compounds
(sugar) apart
• Instead, water moves between different
molecules of sugar, separating them
E. What will dissolve?
• “Like dissolves Like”
• Polar solvents dissolve polar solutes
• Nonpolar solvents dissolve nonpolar solutes
• If a solvent and a solute are not similar, the
solute will not dissolve (oil and water)
F. How much will dissolve?
• Solubility is a measurement that describes
how much solute dissolves in a given amount
of solvent
1. Solubility in Liquid-Solid Solutions
• Increased temperature of solvent may or may
not also increase
- The rate at which the solute dissolves
- The amount of solute that dissolves
2. Solubility in Liquid-Gas Solutions
• Increased pressure can increase the amount
of solute (gas) in a solvent (liquid)
3. Saturated Solutions
• A solution is said to be saturated when the
solvent can not hold any additional solute
• In a liquid-solid solution, the extra solid
solute will settle to the bottom of the
container
• A hot solvent can hold more solute than a
cold solvent can
- When the hot solvent cools, the solute
usually falls out of solution
- If solute stays in the solvent for a period of
time after the solvent cools, it is
supersaturated because it contains more than
the normal amount of solute
G. Rate of Dissolving
• A solute dissolves faster when the solution is
stirred or shaken
• This method increases the rate at which the
surfaces of the solute come into contact with
the solvent
• Increasing the collisions between the solvent
and solute can also increase the rate of
dissolving
H. Concentration
• Tells you how much solute is present
compared to the amount of solvent
• In a concentrated solution, there is more
solute dissolved in a solvent compared to
another solution
• In a dilute solution, there is less solute
dissolved in a solvent compared to another
solution
1. Measuring Concentration
• Described as a percentage of the volume
(liquid) or mass (solid) of the solution that is
made up of the solute
2. Effects of Solute Particles
• Adding solutes to a solvent, decreases the
freezing point of the solute
i.e. Antifreeze
• Adding solutes to a solvent, increases the
boiling point of the solute
IV. Acids
• Release positively charged hydrogen ions, H+,
in water
HCl  H+ + Cl• Hydrogen ions, H+, combine with water, to
form hydronium ions, H3O+
H+ + H2O  H3O+
• Hydronium ions in acidic solutions can carry
the electric charges in a current
- Therefore acids can conduct electricity
V. Bases
• Bases are substances that release negatively
charged hydroxide ions, OH-, in water
NaOH  Na+ + OH• Bases can thus accept hydrogen ions, H+
H3O+ + OH-  2H2O
VI What is pH?
• pH is related directly to its concentrations of
hydronium ions (H3O+) and hydroxide ions
(OH-)
• Acidic solutions (pH 0-6) have more
hydronium ions (H3O+)
• Basic solutions (pH 8-14) have more
hydroxide ions (OH-)
• Neutral solutions (pH 7) have equal numbers
of hydronium (H3O+) and hydroxide (OH-) ions
A. pH Scale
• 1 pH unit represents a tenfold change in the
acidity of a solution
Example, if one solution has a pH of 1 and a
second solution has a pH of 2, the first
solution is ten times more acidic than the
second
• To find the difference in pH strength, use the
following calculation
10n where n = the difference between pHs
pH3 – pH1 = 2
102 = 100 x more acidic
B. Strengths of Acids and Bases
• The strength of an acid is related to how
easily hydrogen ions, H+, are released
- A strong acid (pH = 1) releases more
hydrogen ions than a weak acid (pH = 6)
• The strength of a base is related to how easily,
hydroxide ions, OH-, are released
- A strong base (pH = 14) releases more
hydroxide ions than a weak base (pH = 8)
C. Indicators
• Compounds that react with acidic and basic
solutions and produce certain colors,
depending on the solution’s pH
D. Neutralization
• A reaction of an acid with a base
• The properties of both the acid and base are
diminished, or neutralized
• Produces a water and a salt in most cases
HCl + NaOH  H2O + Na+Cl-