CHEMISTRY SECTION V
Chemistry
2014-2015
CLASSIFYING SUBSTANCES
Many of the substances we know about
can be sorted into general categories
according to their properties, such as
solubility (the ability to dissolve) melting
point (the temperature at which a
substance melts) and conductivity (the
ability to conduct electricity).
Some substances, like sugar, sodium
chloride, and ammonium sulfate,
dissolve in water to make an aqueous
solution (a term we learned a few weeks
ago). Gold, however, does not dissolve in
water (but aqua regia, which we learned
about in our first unit, can dissolve gold).
A substance that dissolves in water is said to
be soluble in water, and one that does not
dissolve is said to be insoluble. Therefore
sodium chloride is soluble in water, while gold
is insoluble in water.
Oil and
water are
insoluble.
The liquid on the left is insoluble in water.
The liquid on the right is soluble in water.
The property of conductivity is related to a
substance’s ability to conduct electricity.
Electrical conductivity requires the movement
of ions or electrons.
 Copper wire is an excellent conductor; the human
body can also act as a conductor. Rubber is a poor
conductor.
 Electrical conductivity can be tested by setting up a
simple electrical circuit—if the substance being tested
is part of the circuit, it will either lead to the flow or
blockage of an electrical current. This is easy to test if
a light bulb is also part of the circuit; if the bulb lights
up, the substance conducts electricity.
T YPES OF SUBSTANCES
We can classify substances into four
categories using these properties.
Ionic
Molecular covalent
Metallic
Network covalent
IONIC
Ionic substances are soluble in water, conduct
electricity, and have high melting points.
Examples: sodium chloride, copper sulfate
Types of atoms: ionic compounds (a metal and
a nonmetal, or polyatomic ions) in aqueous
solution
Type of bond: Ionic
Electrons are transferred
Description: brittle solids
Ex. sodium chloride, which can
form large “halite” crystals like
the one on the right
Copper sulfate (left) and cobalt chloride
(right). Ionic compounds contain a metal
and a nonmetal, and/or polyatomic ions.
Many are white crystals, but some are
colored.
MOLECULAR COVALENT
 Molecular covalent substances are soluble in water,
do not conduct electricity, and have low melting
points.
 Examples: table sugar/sucrose, C 12 H 22 O 11 ; carbon
dioxide, CO 2
 Types of atoms: nonmetals; often C, H, and O
 Type of bond: molecular covalent
 Electrons are shared between atoms in a molecule.
 Often liquids or gases, sometimes solids
In a covalent bond, electrons are
shared, like in the bond between two
oxygen atoms on the right.
Molecular covalent molecules often
contain carbon, hydrogen, and/or
oxygen. Look at the three molecular
covalent compounds below—they
contain these elements.
METALLIC
Metallic substances are insoluble in water,
conduct electricity, and have high melting
points.
Examples: gold, copper, aluminum
Types of atoms: metals
Type of bond: metallic
Free-flowing “sea of electrons”
Bendable, malleable, shiny solids
Malleable—can be
hammered into a thin sheet
Metals can often be bent,
pressed, hammered,
and/or stretched into wires
Remember—some of the electrons in metals are freemoving; they are no longer anchored to atoms, but they can
move around the entire metal
NETWORK COVALENT
Network covalent substances are insoluble in
water, do not conduct electricity, and have
high melting points.
Examples: sand/glass (silicon dioxide),
diamonds (one form of carbon)
Types of atoms: nonmetals
Type of bond: network covalent
Electrons are shared throughout substance
Extremely hard solids
Sand, quartz, and glass
are all silicon dioxide,
SiO2. Note that it
contains two nonmetals
bonded together in a
large network.
PACKET 3 EXAMPLE 1: PREDICTING
PROPERTIES
Predict whether the following substances will dissolve
in water, and whether they will conduct electricity.
Lead Lead is a metal—look at the properties of metals
soluble / insoluble
conduct / not conduct
Potassium bromide
soluble / insoluble
“Potassium bromide”—contains a
metal and a nonmetal; ionic
conduct / not conduct
Ionic substances only conduct electricity
when they are dissolved in water
BONDING
Chemists call the attraction that holds atoms
together a chemical bond. Several types of
bonds exist, and they all involve electrons in
some way.
We’ve seen that most substances can be
divided into four categories based on their
physical properties. These categories can be
explained by different models of bonding. You
can see these in the table on the previous
page.
Carbon dioxide exists as
separate CO2 molecules;
silicon dioxide exists as a
network of silicon and
oxygen.
BONDS AND PROPERTIES
Some properties are directly related
to the type of bonds the atoms in the
substances have. Therefore, it is
possible to match the bonding with
the physical properties observed in
different substances.
Conduction requires the movement of
charged particles.
Ionic substances in aqueous solution contain
free-moving cations and anions, so they
conduct electricity.
Metallic substances conduct electricity
because their valence electrons are free to
move within the solid.
Network covalent substances and molecular
covalent substances do not contain ions or
transfer electrons, so they do not conduct
electricity.
Melting point depends on the
attractive forces between the
particles.
The higher the melting point, the
stronger the attractive forces.
Packet 3 Example 2: Identifying Types of Bonds
Determine the type of bond in each of the following
substances. Then decide the physical properties each
substance would have.
Substance
Magnesium
chloride,
MgCl2
Rubbing
alcohol,
C 3H8O
Type of
Bond
Ionic
(metal +
nonmetal)
Molecular
covalent
(C, H, and
O)
Soluble in
water?
Yes
Yes
Conducts
electricity?
Electrons
are…
Yes—when
dissolved
in water
Transferred
No
Shared
between
atoms
Description
Brittle
solids
Liquid
ELECTROPLATING METALS
Most metals are dug out of the ground as
ionic compounds (ores); in other words, they
cannot be found in nature in their pure forms.
Through the ages, people have struggled to
extract the pure metals from these ores; some
are easier to purify than others.
This is a piece of gold ore—
obviously, it’s not pure gold. Gold
and other metals can be extracted
from ores using a variety of
methods.
ALUMINUM
 Despite being the third most common element in
Earth’s crust, aluminum was one of the most difficult
metals for scientists to isolate. It was first purified
in 1827 by a German chemist named Friedrich
Wöhler. Processing aluminum was still difficult and
inefficient, making it more expensive per ounce than
gold for quite some time.
This is bauxite, the
ore aluminum can
be extracted from.
 Napoleon III famously let only his
favorite guests use his aluminum
cutlery, while the rest had to use
gold.
 The Washington Monument was
also capped with aluminum
which (at the time) was as
expensive as silver.
 It was not until the advent of
electrolysis (running an electric
current through aluminum ore),
that the isolation of aluminum
became more efficient, therefore
dropping the price of aluminum.
ELECTROPLATING
 Electricity can be used to
extract metal from compounds
by “giving” electrons back to
metal ions, which converts
them to neutral metal atoms.
This process is called
electroplating.
 Ex. Copper metal can be extracted
from a copper sulfate solution by
running an electrical current through
the solution.
ELECTROPLATING WITH GOLD