2. THERMAL PHYSICS
2.1 Kinetic Particle Model of Matter
2.1.1 States of Matter
Properties of Solids, Liquids & Gases
The Three States of Matter
The three states of matter in terms of shape and volume
Solids
Solids have a definite shape and a definite volume
o Solids cannot flow and are not compressible
Liquids
Liquids have no definite shape but do have a definite volume
o Liquids are able to flow to take the shape of a container but they are not
compressible
Gases
Gases have no definite shape and no fixed volume
o Gases can flow to take the shape of their container and are highly compressible
Changes of State
When a substance changes state, the number of molecules in that substance doesn't change and so
neither does its mass
The only thing that changes is its energy
Changes of state are physical changes and so they are reversible
Melting & Freezing
Melting occurs when a solid turns into a liquid (e.g. ice to water)
Freezing occurs when a liquid turns into a solid
Boiling & Condensing
Boiling occurs when a liquid turns into a gas
o This is also called evaporating
Condensing occurs when a gas turns into a liquid
,Exam Tip
It is very useful to think about water when learning the changes of state, because we are all used
to the idea that solid water (ice) melts to become liquid water (water!) and boils to become
gaseous water (water vapour).
However, do remember that all substances undergo the same changes between the three states,
but at different temperatures. So while water will help you to remember the names, don't refer to
water in your answers unless it has been specified.
2.1.2 Molecular Matter
Arrangement & Motion of Particles
In a solid:
o The molecules are very close together and arranged in a regular pattern
o The molecules vibrate about fixed positions
In a liquid:
o The molecules are still close together (no gaps) but are no longer arranged in a
regular pattern
o The molecules are able to slide past each other
In a gas:
The molecules are widely separated - about 10 times further apart in each
direction
The molecules move about randomly at high speeds
The Forces & Distances between Molecules
The forces between particles affect the state of matter
This is because the magnitude of the forces affects the relative distances and motion of
the particles
This affects the ability of the substance to
o Change shape
o Change volume
o Flow
, The particles that make up matter include
o Atoms
o Molecules
o Ions
o Electrons
Intermolecular Forces and Motion of Particles
Solids
The molecules in a solid are held in place by strong intermolecular forces
o They only vibrate in position
o The distance between them is fixed
o This gives the solid its rigid shape and fixed volume
Liquids
The molecules in a liquid have enough energy to overcome the forces between them
They are still held close together
o The volume of the liquid is the same as the volume of the solid
Molecules can move around (by sliding past each other)
o This allows the liquid to change shape and flow
Gases
The molecules in a gas have more energy and move randomly at high speeds
o The molecules have overcome the forces holding them close together
Because of the large spaces between the molecules
o The gas can easily be compressed and is also able to expand
o Gases flow freely
Worked Example
Two states of matter are described below. Identify each of the states of matter.
Substance 1
o molecules are spaced very far apart
o molecules move very quickly at random
o molecules move in a straight line
Substance 2
o molecules are quite closely packed together
o molecules move about at random
o molecules do not have fixed positions
Substance 1
Step 1: Identify the distances between the molecules
o The molecules are spaced far apart
This can only describe a gas
Step 2: Identify the motion of the molecules
o The molecules move quickly, at random and in a straight line
This confirms that substance 1 is a gas
Substance 2
Step 1: Identify the distances between the molecules
o The molecules are closely packed
This could describe either a solid or a liquid
Step 2: Identify the motion of the molecules
o The molecules move at random and do not have fixed positions
This confirms that substance 2 is a liquid
Temperature & Energy of Particles
The amount of pressure that a gas exerts on its container is dependent on the temperature
of the gas
o This is because particles gain kinetic energy as their temperature increases
, As the temperature of the gas decreases, the pressure on the container also decreases
In 1848, Mathematician and Physicist, Lord Kelvin, recognised that there must be a
temperature at which the particles in a gas exert no pressure
o At this temperature they must no longer be moving, and hence not colliding with
their container
This temperature is called absolute zero and is equal to -273 °C
At absolute zero, or -273 °C, particles will have no net movement. It is therefore not possible to
have a lower temperature
The unit kelvin is written as K
o Note that there is no degree as with Celsius, which is written °C
Exam Tip
Once you start thinking about the motion of molecules, atoms, electrons and ions affecting their
kinetic energy this topic will become much easier to remember.
An increase in temperature means an increase in internal energy. With more energy the molecules
can move about more as they get more kinetic energy. Once they are moving more they can break
their bonds and spread apart, which we experience as a change of state.
Changes of state are caused, basically, by simple conservation of energy and energy transfers,
which you should already be comfortable with from Years 7 and 8.
2.1.3 Temperature & Pressure
Motion of Particles in a Gas
Molecules in a gas are in constant random motion at high speeds
Random motion means that the molecules are travelling in no specific path and undergo
sudden changes in their motion if they collide:
o With the walls of its container
o With other molecules
Pressure in a gas is caused by the collisions with the surface (walls) of the container
Random motion of gas molecules in a container
Pressure & Force of Particles in a Gas
EXTENDED
2.1 Kinetic Particle Model of Matter
2.1.1 States of Matter
Properties of Solids, Liquids & Gases
The Three States of Matter
The three states of matter in terms of shape and volume
Solids
Solids have a definite shape and a definite volume
o Solids cannot flow and are not compressible
Liquids
Liquids have no definite shape but do have a definite volume
o Liquids are able to flow to take the shape of a container but they are not
compressible
Gases
Gases have no definite shape and no fixed volume
o Gases can flow to take the shape of their container and are highly compressible
Changes of State
When a substance changes state, the number of molecules in that substance doesn't change and so
neither does its mass
The only thing that changes is its energy
Changes of state are physical changes and so they are reversible
Melting & Freezing
Melting occurs when a solid turns into a liquid (e.g. ice to water)
Freezing occurs when a liquid turns into a solid
Boiling & Condensing
Boiling occurs when a liquid turns into a gas
o This is also called evaporating
Condensing occurs when a gas turns into a liquid
,Exam Tip
It is very useful to think about water when learning the changes of state, because we are all used
to the idea that solid water (ice) melts to become liquid water (water!) and boils to become
gaseous water (water vapour).
However, do remember that all substances undergo the same changes between the three states,
but at different temperatures. So while water will help you to remember the names, don't refer to
water in your answers unless it has been specified.
2.1.2 Molecular Matter
Arrangement & Motion of Particles
In a solid:
o The molecules are very close together and arranged in a regular pattern
o The molecules vibrate about fixed positions
In a liquid:
o The molecules are still close together (no gaps) but are no longer arranged in a
regular pattern
o The molecules are able to slide past each other
In a gas:
The molecules are widely separated - about 10 times further apart in each
direction
The molecules move about randomly at high speeds
The Forces & Distances between Molecules
The forces between particles affect the state of matter
This is because the magnitude of the forces affects the relative distances and motion of
the particles
This affects the ability of the substance to
o Change shape
o Change volume
o Flow
, The particles that make up matter include
o Atoms
o Molecules
o Ions
o Electrons
Intermolecular Forces and Motion of Particles
Solids
The molecules in a solid are held in place by strong intermolecular forces
o They only vibrate in position
o The distance between them is fixed
o This gives the solid its rigid shape and fixed volume
Liquids
The molecules in a liquid have enough energy to overcome the forces between them
They are still held close together
o The volume of the liquid is the same as the volume of the solid
Molecules can move around (by sliding past each other)
o This allows the liquid to change shape and flow
Gases
The molecules in a gas have more energy and move randomly at high speeds
o The molecules have overcome the forces holding them close together
Because of the large spaces between the molecules
o The gas can easily be compressed and is also able to expand
o Gases flow freely
Worked Example
Two states of matter are described below. Identify each of the states of matter.
Substance 1
o molecules are spaced very far apart
o molecules move very quickly at random
o molecules move in a straight line
Substance 2
o molecules are quite closely packed together
o molecules move about at random
o molecules do not have fixed positions
Substance 1
Step 1: Identify the distances between the molecules
o The molecules are spaced far apart
This can only describe a gas
Step 2: Identify the motion of the molecules
o The molecules move quickly, at random and in a straight line
This confirms that substance 1 is a gas
Substance 2
Step 1: Identify the distances between the molecules
o The molecules are closely packed
This could describe either a solid or a liquid
Step 2: Identify the motion of the molecules
o The molecules move at random and do not have fixed positions
This confirms that substance 2 is a liquid
Temperature & Energy of Particles
The amount of pressure that a gas exerts on its container is dependent on the temperature
of the gas
o This is because particles gain kinetic energy as their temperature increases
, As the temperature of the gas decreases, the pressure on the container also decreases
In 1848, Mathematician and Physicist, Lord Kelvin, recognised that there must be a
temperature at which the particles in a gas exert no pressure
o At this temperature they must no longer be moving, and hence not colliding with
their container
This temperature is called absolute zero and is equal to -273 °C
At absolute zero, or -273 °C, particles will have no net movement. It is therefore not possible to
have a lower temperature
The unit kelvin is written as K
o Note that there is no degree as with Celsius, which is written °C
Exam Tip
Once you start thinking about the motion of molecules, atoms, electrons and ions affecting their
kinetic energy this topic will become much easier to remember.
An increase in temperature means an increase in internal energy. With more energy the molecules
can move about more as they get more kinetic energy. Once they are moving more they can break
their bonds and spread apart, which we experience as a change of state.
Changes of state are caused, basically, by simple conservation of energy and energy transfers,
which you should already be comfortable with from Years 7 and 8.
2.1.3 Temperature & Pressure
Motion of Particles in a Gas
Molecules in a gas are in constant random motion at high speeds
Random motion means that the molecules are travelling in no specific path and undergo
sudden changes in their motion if they collide:
o With the walls of its container
o With other molecules
Pressure in a gas is caused by the collisions with the surface (walls) of the container
Random motion of gas molecules in a container
Pressure & Force of Particles in a Gas
EXTENDED
