STATES OF MATTER AND THE KINETIC MOLECULAR THEORY
05 FEBRUARY 2013
Lesson Description
In this lesson we:
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Identify different states of matter
Revise the processes required to change state
Explain why matter exists in different states by using the Kinetic-molecular theory
Introduce the concept of Inter-molecular Forces
Key Concepts
States of Matter
There are three common sates of matter, namely solid, liquid or gas. There are not the only states of
matter.
What is a solid?
A solid is a form of matter where the substance has a fixed shape and volume.
What is a liquid?
A liquid is a form of matter where the substance has a fixed volume but takes on the shape of the
container in which it is placed. You can pour a liquid.
What is a gas?
A gas is a form of matter where the substance has a no fixed shape or volume. A gas takes the shape
and volume of the container in which it is placed.
We refer to the characteristics of states of matter as macroscopic properties of matter. Macroscopic
properties also include properties such as temperature, pressure, conductivity, mass, shape and
volume.
Changes in States of Matter
A sample of a substance can change state from solid to liquid to gas. These processes are called
physical changes. No new substance is formed. The processes are reversible dependent on
temperature.
Solid to Liquid: This process is called melting. The melting point of a substance is the temperature at
which this process takes place. This is always the same for the same substance.
Liquid to Solid: This process is called freezing. The freezing point of a substance is the temperature
at which this process takes place. The freezing point and melting point are the same temperature.
Liquid to Gas: Liquids can change to gas at any temperature above the melting point. This process
is called evaporation. Evaporation takes place at the surface of a liquid. When a substance reaches a
certain temperature and can no longer remain a liquid, we observe bubbles forming inside the liquid.
This change from liquid to gas is called boiling. The boiling point of liquids is dependent on
atmospheric pressure.
Gas to Liquid: The process is called condensation.
Gas to Solid: This process is called sublimation.
Kinetic-Molecular Theory
To explain the existence of states of matter and changes of phase, we use a microscopic model of
matter.
The kinetic theory of matter states that the matter is made up of tiny particles that move all the time.
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All matter is made up of tiny, invisible, moving particles. The particles can be atoms,
molecules or ions.
Whether matter is a solid, liquid or gas, the particles move all the time. The higher the
temperature, the faster the particles will move.
There are forces of attraction between particles of matter
Gas phase:
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The average distance between the particles is much greater than the distance in liquids or
solids.
Because of the big distances the force of attraction between the particles are very small.
The molecules move all the time. Because they collide and change direction all the time we
find a random motion.
A gas will fill the whole container it is kept in.
The average kinetic energy is directly proportional to the temperature of the gas.
Liquid phase
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The particles are quite close together, but the force of attraction is smaller than in a solid, but
greater than in a gas.
The particles have more freedom of movement than in a solid but less than in a gas.
Liquids can flow past each other.
The average kinetic energy is directly proportional to the temperature of the liquid.
Solid phase:
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The particles attract each other strongly with intermolecular forces.
The particles are arranged in a fixed way.
Particles can vibrate.
The particles are arranged in a regular way and form beautiful crystals.
The average kinetic energy is directly proportional to the temperature of the solid.
Demonstration
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Investigate what happens when ice melts
Start with ice in a glass beaker. Use a thermometer to read the temperature every minute.
Draw a graph of temperature and time.
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Investigate what happens when boiling water cools
Start with a beaker of boiling water. Use a thermometer to read the temperature every minute.
Draw a graph of temperature and time.
Questions
Question 1
Draw a sketch to represent molecules of substance undergoing the following changes:
a.)
b.)
c.)
Freezing
Evaporation
Sublimation
Question 2
During an investigation into water and its properties, Precious took some ice cubes, placed them in a
beaker and heated it over a burner. She took the temperature over regular intervals and drew a graph
from the results. The graph is shown below.
Graph of temperature against time for the heating of ice cubes
D
E
Temp
C
0
B
C
A
Time (minutes)
In terms of increase in temperature, forces between molecules, distance between molecules, average
kinetic energy between molecules, phase change, increase in potential energy and boiling point
explain what is happening
a.)
between points A and B
b.)
at point B
c.)
between points B and C
d.)
Are the phase changes that occur physical or chemical changes? Explain
e.)
If you keep heating the water beyond point E, will you be able to separate the water vapour
into hydrogen and oxygen? Explain.
Question 3
When cornflower is mixed with water it forms an unusually type of substance. Can you classify it as
solid or liquid?
Links
States of Matter
http://phet.colorado.edu/en/simulation/states-of-matter-basics
Heating Curve of Water
http://www.kentchemistry.com/links/Matter/HeatingCurve.htm
Cornflower Monster
http://www.thenakedscientists.com/HTML/content/kitchenscience/garage-science/exp/a-cornflourmonster/