THE CORPUSCULAR NATURE OF MATTER AND ITS
PHYSICAL STATES
In this unit we are going to study the matter from a microscopic point of view
using the kinetic theory. We will understand the properties of the different matter
states and their behavior. We are also going to study the changes of state using the
kinetic theory.
1. Kinetic theory of matter
Kinetic theory is a physical theory about the microscopic constitution of matter.
Although this theory is very simple, it can explain a lot of experimental facts, such as
the different states of matter, its main physical properties, the diffusion of gases, etc.
It could be summed up according to the following statements:

Matter is composed by tiny particles (that we usually represent as spheres),
called atoms or molecules.

Particles which form matter have a continuous and random motion which
depends on its thermal energy. In other words, the greater the temperature, the
faster its motion.

As the particles are closer, they will be more attracted, and as the distance
increases, the interaction will decrease.
Now we are going to explain the main features and physical properties of the states of
matter, according to its microscopic structure.
1
a) Solid State
The particles are arranged in an ordered structure. Each particle is as close to the
others as possible and is placed at the same distance of everyone. As the distance
between particles is the shortest, the attractive interaction is great, so these particles
cannot leave its place nor become closer to their neighbours. That’s why solids have a
permanent and characteristic shape and are incompressible.
This also explains why usually solids have high density, because the particles are
very close to each other and do not take up a lot of space.
b) Liquid State
Liquids are formed by particles which are relatively close, but they are free to move
from a position to another one. As the particles are continuously moving, the shape of a
liquid can change, which explain that liquids conform to the shape of its container.
Although they are in motion, particles in a liquid are very close to the others and they
are still attracted very tightly, so they are incompressible and as one of them go out,
the others follow the first and the liquid can flow as a whole set.
Compared to the solid state the particles are not so close. This explains why
liquids usually have less density than solids
c) Gas State
Gases are formed by particles which are far away from the others, so there is
almost no attractive interaction amongst them. They are moving very fast in straight
lines, colliding with other particles and with the walls of the container. That’s why gases
can fill up the whole volume of their container or even they can scape if there is a hole
in the container. As the average distance between the particles is great compared to
their own size, they can approach and remain closer when they are forced by a
pressure, so gases are compressible. This great distance between particles also
explains the very low density of gasses.
2
2. Effect of temperature and the changes of state
One of the statements of the kinetic theory says that the greater the temperature,
the higher the energy of the particles and consequently the faster the motion of them. If
the temperature is high enough matter will change from one state to another. We are
going to explain the changes of state using the kinetic theory starting from the solid
state.
As a solid is heated, it reaches to a point where the kinetic energy of its particles is
higher than attractive interactions among them, so the particles can leave their own
places in the solid and move to different locations. This is the moment when the
change of state to liquid is produced.
Equally when a liquid is heated, it reaches to a point where the kinetic energy of its
particles is greater than the attractive interactions, so particles start to scape from the
liquid, and the liquid become gas.
Melting
Solidification
Vaporization
Condensation
Add Energy = Heat
Remove Energy = Cool Down
We are going to study a particular example when we heat a solid (ice) at a
constant rate. We can distinguish 5 different sections.
Section 1 (heating solid): Heat energy goes into the vibration motion of the
particles, increasing their kinetic energy. Since temperature depends on the
kinetic energy, the temperature of the solid increases. The temperature can
increase until it reaches the melting point of the solid. In this case, that is 0 oC.
3
Section 2 (melting): At 0 oC, any heat added to the solid goes into partially
breaking intermolecular bonds (attraction forces of the particles). The heat does
not increase the kinetic energy of the molecules, so the temperature remains
constant. As long as there is solid present, the temperature cannot increase. We
say that the solid and liquid are in equilibrium if they are both present at the same
time.
Section 3 (heating liquid): Heat energy, as in section 1, goes into the motion of
the particles. Now we are in the liquid state, so the heat energy increases the
speed of the particles. As we continue heating the kinetic energy increases, so the
temperature of the liquid increases. The temperature can increase until it reaches
the boiling point of the liquid. In this case, that is 100 oC.
Section 4 (boiling): At 100 oC, any heat added to the liquid goes into completely
breaking intermolecular bonds (attraction forces of the particles). Again like in
section 2, the heat does not increase the kinetic energy of the molecules, so the
temperature remains constant. As long as there is liquid present, the temperature
cannot increase. We say that the liquid and solid are in equilibrium if they are both
present at the same time.
Section 5 (heating gas): The heat, again, goes into the kinetic energy of the
particles. The kinetic energy increases, so the temperature of the gas
increases. The temperature can increase indefinitely, or until the substance
decomposes.
4
3. Gas Laws and the kinetic theory
In the 18th and 19th centuries various scientist study the behavior of gasses.
They analyzed three magnitudes pressure (P), temperature (T) and volume (V)
and they found the relationship amongst them. In each experiment they keep
constant one of the magnitudes (P, V or T), they varied another one and studied
the effect of this variation in the third one. After all their studies they established
the gas laws.
3.1 Boyle-Mariotte’s Law
Simultaneously Robert Boyle in
England and Edmé Mariotte in France
studied the variation of the volume of a
gas if you change the pressure, keeping
constant
the
temperature.
They
concluded that, for the pressure and
volume of a gas, when one value
increases the other decreases if you
keep
constant
the
temperature.
Mathematically it can be expressed:
where K is a constant or
We can use the kinetic theory to explain the Boyle’s law, keeping in mind
a very important thing, which is that the pressure depends on the collisions of
the particles with the walls of the container:
If we keep the same temperature the kinetic energy of the particles will
not change, in other words the will move at the same speed. So, if we have a
certain amount of gas in a container, and we reduce the volume, the particles
will collide more times with the walls (since there is less space to move),
increasing this way the pressure. Another way of looking at this is that as the
pressure increases it drives the particles together. These compacted particles
now occupy less volume.
3.2 Gay Lussac’s Law
At the beginning of the 19th century the French chemist Joseph-Louis
Gay-Lussac studied the variations of the pressure of a gas when you heat it,
keeping constant the volume of the container. After his studies he ended up
with the following conclusion: If you keep the volume constant, when you heat a
gas the pressure increase and vice versa.
5
T is the temperature in Kelvin
Using the kinetic theory we can also explain the Gay Lussac’s Law. At
constant volume, if you heat a gas the speed of the particles will increase. This
fact results in more collisions and thereby greater pressure to the container.
3.3 Charles’ Law
Also at the beginning of the 19th
century, another French scientist,
Jacques-Alexander
Charles
was
studying the behavior of the volume of
a gas when you heat it, keeping
constant the pressure. He found that,
when you increase the temperature the
volume increase, if you keep constant
the pressure, and vice versa.
T is the temperature in Kelvin
Using the kinetic theory we can also understand very well this fact.
According to Charles' law, gases will expand when heated. The temperature of
a gas is really a measure of the movement of the particles. As the temperature
increases, the particles will move faster and will make more collisions with the
container. But remember that the pressure must remain constant. The only way
to do this is by increasing the volume.
6