Ch 12 The Nature of Solids
In a solid, particles are packed tightly,
frequently in a highly organized pattern. In
the solid state, the particles don’t
“translate,” or move around, but they do still
vibrate and rotate about fixed points or axes.
Intermolecular forces are responsible for the bonds that exist between the
particles. It is these bonds that keep them from translating. Generally
speaking, the stronger the bonds, the harder it is to get the substance to melt.
Q: What does it take to get a solid to melt?
A: As you heat a solid, you give the particles energy. This energy causes the
particles to vibrate morr and morr, increasing their kinetic energy.
Eventually, they gain enough kinetic energy to break the bonds between them,
and they start to slide around.
DEF: Melting Point (m.p.) = the temperature at which the
particles in a solid gain enough kinetic energy to break their
intermolecular bonds, and begin to slide around, or melt.
Ionic compounds tend to have very high melting points. Why?
EX: m.p. of NaCl = 801 0C
m.p. of HCl = - 112 0C
Q: What is the opposite of melting?
A: Freezing.
Q: How does the freezing point of a substance compare to
the melting point?
A: They are the same! What governs whether something continues to freeze
or continues to melt is whether you continue to subtract or add energy by
continuing to subtract or add heat, respectively.
Solid
melting
→
←

freezing
liquid
Q: Do all solids melt?
A: Nope!! Some things never melt! For example, diamonds never melt. They
simply vaporize at about 3700 0C. Iodine and dry ice sublime at room
temperature, though you could force them to melt by first solidifying them,
and then appropriately pressurizing them. Wood never melts, no matter what
you do! Technically, glass! Morr about this in a moment!
DEF: Crystal = atoms, ions, or molecules that are arranged in
an orderly, 3-D, repeating pattern.
DEF: Crystal lattice = the specific internal pattern for a
crystal.
There are 7 basic crystal lattices, or shapes.
They are: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal,
and rhombohedral. See old AW overhead. ☺
Q: Are all solids crystals?
A: Nope! Solids may be amorphous internally.
DEF: Amorphous solid = solid that lacks internal shape (not a
crystal).
EX: plastic, asphalt, rubber, super-cooled liquids (glass)
DEF: Super-cooled liquid = amorphous substance that cools
to a (seemingly) rigid state, but technically still flows very
slowly.
DEF: Allotropes = 2 or morr forms of the same substance
arranged differently internally.
Ex: Crystalline and amorphous SiO2, as shown below, are allotropes of each
other! They have the same chemical formula, but their internal arrangement
is different.
Ex: Diamond, graphite, buckyball, coal, and graphene are all allotropes of
carbon. Only coal is amorphous. Let’s look at the 3-D models for some.
Graphene
buckyball
diamond
graphite
Coal
Bonus Point Opportunity – 5 pts! Build a 3-D crystal model.
See me for specific assignment of your crystal!
Now, let’s take a look at something called a Heating – Cooling Curve. See old
AW Transparency!
For ANY Substance, the Heating – Cooling Curve generally looks like this:
Heat (Joules)