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- What are the objects around us
- The properties of matter
The properties of matter
Let us now look at what we have learned about chemical
bonds, intermolecular forces and the kinetic theory of matter, and see whetherthis can help us to understand some of the macroscopic properties of materials.
-
Melting point
Melting point
- The temperature at which a
solid changes
its phase or state to become a
liquid . The
process is called melting and the reverse process (change in phase from liquidto solid) is called
freezing .
In order for a solid to melt, the energy of the particles
must increase enough to overcome the bonds that are holding the particlestogether. It makes sense then that a solid which is held together by strong
bonds will have a
higher melting point
than one where the bonds are weak, because more energy (heat) is needed to breakthe bonds. In the examples we have looked at metals, ionic solids and some
atomic lattices (e.g. diamond) have high melting points, whereas the meltingpoints for molecular solids and other atomic lattices (e.g. graphite) are much
lower. Generally, the intermolecular forces between molecular solids are
weaker than those between ionic and
metallic solids.
-
Boiling point
Boiling point
- The temperature at which a
liquid changes
its phase to become a
gas . The process is
called evaporation and the reverse process is called condensation
When the temperature of a liquid increases, the average
kinetic energy of the particles also increases and they are able to overcomethe bonding forces that are holding them in the liquid. When boiling point is
reached,
evaporation takes place and some
particles in the liquid become a gas. In other words, the energy of theparticles is too great for them to be held in a liquid anymore. The stronger the
bonds within a liquid, the higher the boiling point needs to be in order tobreak these bonds. Metallic and ionic compounds have high boiling points while
the boiling point for molecular liquids is lower.The data in
[link] below may help you to understand some of
the concepts we have explained. Not all of the substances in the table aresolids at room temperature, so for now, let's just focus on the
boiling points for each of these substances. What do
you notice?
Substance |
Melting
point (
) |
Boiling point (
) |
Ethanol (C
2 H
6 O) |
-
114,3 |
78,4 |
Water |
0 |
100 |
Mercury |
-38,83 |
356,73 |
Sodium chloride |
801 |
1465 |
The melting and boiling
points for a number of substances
You will have seen that substances such as ethanol, with relatively weak
intermolecular forces, have the lowest boiling point, while substances withstronger intermolecular forces such as sodium chloride and mercury, must be
heated much more if the particles are to have enough energy to overcome theforces that are holding them together in the liquid.
Forces and boiling point
The table below gives the molecular formula and the boiling point
for a number of organic compounds called
alkanes (more on these compounds in grade 12). Refer
to the table and then answer the questions that follow.
Organic compound |
Molecular
formula |
Boiling point (
C) |
Methane |
CH
4 |
-161.6 |
Ethane |
C
2 H
6 |
-
88.6 |
Propane |
C
3 H
8 |
-45 |
Butane |
C
4 H
10 |
-0.5 |
Pentane |
C
5 H
12 |
36.1 |
Hexane |
C
6 H
14 |
69 |
Heptane |
C
7 H
16 |
98.42 |
Octane |
C
8 H
18 |
125.52 |
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Source:
OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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