higher boiling point than CH 4 (which just has London dispersion).
Note that when ionic substances change phase, bonds between the individual
atoms are actually broken. When covalent substances change phase, the bonds
between the individual atoms remain in place: the forces that connect the
molecules to other molecules are what break apart.
Solids, Liquids, and Gases
The relationship between a substance’s average kinetic energy and the strength
of its intermolecular forces is responsible for determining if the substance will be
a solid, a liquid, or a gas. In a solid, a substance’s intermolecular forces are much
stronger than the average kinetic energy of its molecules. As a result, molecules
are restricted in their ability to move about. These strong intermolecular forces
permit molecules to merely vibrate in place. This gives a solid its definite size
and shape.
In a liquid, intermolecular forces are still more significant than the kinetic energy
of molecules. However, molecules in a liquid have enough kinetic energy to
move past each other. This allows for the liquid’s ability to flow. Despite being
able to move about, molecules in a liquid are still confined within the sample.
The relationship between intermolecular forces and molecular kinetic energies is
vastly different in a gas. Molecules in a gas are so energetic that they easily
overcome intermolecular attraction. Gas molecules spread about to fill the
volume of whatever container they are in.
What’s a Network Solid?
For the SAT Chemistry Subject Test, you may need to know about something
called a network solid. No, it has nothing to do with television. Network solids
are covalently bonded substances that do not consist of individual molecules.
Instead they consist of atoms joined to form molecules that attract each other
through intermolecular forces. So, in a sense, the substance is one giant
molecule. For this reason, network solids are sometimes called macromolecular
substances. Since covalent bonds are much stronger than intermolecular forces,
network solids are extremely hard to melt. Diamond (pure carbon network) and
quartz (SiO 2 network) are examples of network solids.