Video Transcript
Which of the following properties
can generally be used to differentiate a molecular solid from a covalent
network? (A) Electrical conductivity, (B)
brittleness, (C) color, (D) melting point, or (E) chemical reactivity.
This question is asking us to find
a key difference between simple molecular structures made up of a set number of
covalent bonds and covalent network structures made up of a continuous network of
covalent bonds. In order to find the property that
differentiates these two, we need to find the property that is both distinct between
the two and consistent within each type. In other words, we wanna make sure
that all simple molecules consistently have a certain physical property, and we
wanna make sure that all covalent network solids have the opposite physical
property.
Let’s take a look at the choices
one by one. Unfortunately, there’s no
consistent color pattern to simple molecules or covalent networks. There are a variety of colors of
substances of each type, so we can’t rely on color to differentiate these two types
of solids.
We can also eliminate brittleness
as an answer. While covalent networks are
brittle, simple molecules with covalent bonds can be brittle or they can be more
soft and flexible. Since some simple molecules are
brittle, we can’t use brittleness to differentiate between them and covalent
networks.
There’s no difference in electrical
conductivity as well. Neither simple molecules nor
covalent networks allow for electricity to conduct through the substance as there
are no free charged particles to flow. Since there is no difference here,
we can eliminate (A) as an answer.
There’s not a clear pattern for
choice (E), chemical reactivity, either. Covalent network solids are
generally unreactive, while simple molecules can be reactive or not reactive. For example, the molecular solid
glucose, C6H12O6, is relatively reactive, whereas the molecular solid dry ice, the
solid form of CO2, is relatively unreactive. So, we can eliminate choice
(E).
The last remaining choice, and the
correct choice, is choice (D) melting point. Molecular solids have low melting
points, while covalent network solids have quite high melting points in
comparison. Substances consisting of simple
molecules are held together by weak intermolecular forces. In the diagram here, the strongest
intermolecular force is the hydrogen bonds holding together the water molecules
pictured.
On the other hand, network solids
are held together by relatively stronger covalent bonds. These bonds are an example of
intramolecular forces or forces within the molecule. The weaker forces in simple
molecules lead to a lower boiling point because it takes less energy to separate the
particles of the substance. On the other hand, the strong
covalent bonds found in network solids rigidly hold together the atoms of the
substance. It takes a lot more energy to
loosen the atoms held in this rigid way. So, these substances have
relatively high melting points.
Looking at some examples confirms
this pattern. Molecular substances like propane
and water tend to have melting points below room temperature, whereas covalent
network solids like diamond and graphite have melting points in the sweltering
thousands of degrees. It’s likely impractical to reach
this temperature with a simple laboratory setup. So, when comparing the melting
points of different substances, it’s easiest to use their referenced values from a
textbook.
The property that can generally be
used to differentiate a molecular solid from a covalent network is the melting
point.
