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 (
)
-
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. See the
section below for a further exercise on boiling point.
Density and viscosity
Density and viscosity is not in CAPS - Included for Completeness
Density
Density is a measure of the mass of a substance per
unit volume.
The density of a solid is generally higher than that of a liquid
because the particles are held much more closely together and therefore thereare more particles packed together in a particular volume. In other words, there
is a greater mass of the substance in a particular volume. In general, densityincreases as the strength of the intermolecular forces increases.
Viscosity
Viscosity is a measure of how resistant a liquid is to
flowing (in other words, how easy it is to pour the liquid from one container toanother).
Viscosity is also sometimes described as the 'thickness' of a fluid.
Think for example of syrup and how slowly it pours from one container into
another. Now compare this to how easy it is to pour water. The viscosity ofsyrup is greater than the viscosity of water. Once again, the stronger the
intermolecular forces in the liquid, the greater its viscosity.
It should be clear now that we can explain a lot of
the
macroscopic properties of matter (i.e.
the characteristics we can see or observe) by understanding their
microscopic structure and the way in which the atoms
and molecules that make up matter are held together.
Exercise: 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 (
)
Methane
-161.6
Ethane
-
88.6
Propane
-45
Butane
-0.5
Pentane
36.1
Hexane
69
Heptane
98.42
Octane
125.52
Data from: http://www.wikipedia.com
Draw a
graph to show the relationship between the number of carbon atoms in eachalkane and its boiling point. (Number of carbon atoms will go on the x-axis and
boiling point on the y-axis).
Describe what you see.
Suggest a reason for what you have observed.
Why was it enough for us to use 'number of carbon atoms'
as a measure of the molecular weight of the molecules?
Questions & Answers
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Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
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you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
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