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Simple Kinetic Molecular Model of Matter


Title: Simple Kinetic Molecular Model of Matter Author: chiakp Last modified by: gem Created Date: 6/5/2010 7:22:14 AM Document presentation format – PowerPoint PPT presentation

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Title: Simple Kinetic Molecular Model of Matter

Simple Kinetic Molecular Model of Matter
Name ________________ Class _________________
Index ________________
  • Learning Objectives
  • compare the properties of solids, liquids and
  • infer from Brownian motion experiment the
    evidence for the movement of molecules.
  • describe qualitatively the molecular structure
    of solids, liquids and gases, relating their
    properties to the forces and distances between
    molecules and to the motion of the molecules.
  • describe the relationship between the motion of
    molecules and temperature.
  • explain the pressure of a gas in terms of the
    motion of the molecules.
  • recall and explain the following relationships
    using the kinetic model (stating of the
    corresponding gas laws is not required)
  • a change in pressure of a fixed mass of gas at
    constant volume is caused by a change in
    temperature of the gas.
  • a change in volume of a fixed mass of gas at
    constant pressure is caused by a change in
    temperature of the gas.
  • a change in pressure of a fixed mass of gas at
    constant temperature is caused by a change in
    volume of the gas.
  • use the relationships stated above to solve
    problems (qualitative treatment would suffice).

  • The kinetic molecular model of matter describes
    matter as being made up of molecules in
    continuous , random motion.

What is Brownian Motion?
  • Brownian motion is the random motion of a
    molecule, or other very light object. It is
    observed by reflections of light when the object
    is big enough to reflect the light, as a speck of
    dust in a light beam, either in air or in water.
  • We will see random movement of specks of light
    with the use of a microscope.The speck of dust in
    air is being struck at random by molecules of
    air, and keeps changing direction because of

Modern equipment to observe Brownian motion
Brownian motion
We will see random, erratic and haphazard
movement of the specks of light. The molecules
bombard the air particles or dust at high speeds.
Heat, temperature and the motion of molecules are
all related. Temperature is a measure of the
average kinetic energy of the molecules in a
material. Heat is the energy transferred between
materials that have different temperatures.
Increasing the temperature increases the
translational motion of molecules.
When heat is added to a substance, the molecules
and atoms vibrate faster. As atoms vibrate
faster, the space between atoms increases. The
motion and spacing of the particles determines
the state of matter of the substance. The end
result of increased molecular motion is that the
object expands and takes up more space. Mass of
the object remains the same, however. Solids,
liquids and gases all expand when heat is added.
When heat leaves all substances, the molecules
vibrate slower. The atoms can get closer which
results in the matter contracting. Again, the
mass is not changed.
Pressure in Gases
From the kinetic theory of gases, a gas is
composed of a large number of molecules that are
very small relative to the distance between
molecules. The molecules of a gas are in
constant, random motion and frequently collide
with each other and with the walls of any
The molecules possess the physical properties of
mass, momentum, and energy. The momentum of a
single molecule is the product of its mass and
velocity, while the kinetic energy is one half
the mass times the square of the velocity. As the
gas molecules collide with the walls of a
container, as shown on the right, the molecules
impart momentum to the walls, producing a force
perpendicular to the wall. The sum of the forces
of all the molecules striking the wall divided by
the area of the wall is defined to be the
The Pressure Law The pressure law states that for
a constant volume of gas in a sealed container
the temperature of the gas is directly
proportional to its pressure. This can be easily
understood by visualising the particles of gas in
the container moving with a greater energy when
the temperature is increased. This means that
they have more collisions with each other and the
sides of the container and hence the pressure is
The graph below shows the pressure of a fixed
mass of gas at constant volume is directly
proportional to the absolute temperature in
Kelvin. p/T constant or (p1/T1) (p2/T2) Where
T1 and T2 are the absolute temperatures, before
and after the change respectively.
Boyle's Law
Boyle's Law states that for a given mass of gas,
at a constant temperature, the value of pressure
multiplied by the volume is a constant. As a
mathematical equation, Boyle's law is Where P
is the pressure (Pa), V the volume (m3) of a gas,
and k1 (measured in joules) is the constant from
this equationit is not the same as the constants
from other equations.
Graph of Volume against pressure
Boyle's Law formula
P1V1P2V2 P3V3
Charless Law An inflatable pool float may seem
quite firm as it sits on a deck in the hot sun. 
However, minutes after you toss to float into the
cold pool, the same float may seem
under-inflated.  You may suspect that the float
has developed a slow leak, but that may not be
the most likely explanation for the apparent loss
of air pressure.  It may be that Charles's law is
Charles's law, discovered by Jacques Charles,
states that the volume of a quantity of gas, held
at constant pressure, varies directly with the
Kelvin temperature.  
Gases expand as they are heated and they contract
when they are cooled.  In other words, as the
temperature of a sample of gas at constant
pressure increases, the volume increases.  As the
temperature goes down, the volume decreases as
well.  The mathematical expression for Charles's
law is shown below V1/T1 V2/T2 Remember that
Charles's law calculations must be done in the
Kelvin scale.
Example 1 The volume of a fixed mass of gas in a
cylinder is decreased at a constant temperature.
Why does the pressure exerted by the molecules
of the gas increase?
Solution As the volume decreases, the gas
molecules strike the cylinder walls more often.
Example 2 The figure on the right shows some air
trapped by a layer of mercury. When the beaker of
water is heated, explain what happens to the
layer of mercury.
Solution As the temperature increases, the
kinetic energy of the air molecules increases.
These molecules will strike the mercury and the
walls of the test tube with greater frequency.
The pressure of the trapped air increases and
pushes up the layer of mercury.
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