Kinetic Theory
The properties of solids, liquids and gases can be explained by kinetic theory.
Kinetic theory states that matter is made of tiny particles that move all the
time.
The main points of the theory are;

All matter is made of tiny, invisible, moving particles.

The particles move all the time. The higher the temperature the faster
they move.

Heavier particles move more slowly than light ones at a given temperature.
DIFFUSION
This is the mixing of atoms or molecules due to their continuous and random
motion. e.g. Mixing of bromine vapour and air
The random movement of a gas particle (due to collision) is shown here:
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Rate of diffusion
Experiments using gases diffusing in a tube are used to examine the motion
of the particles. Ammonia and hydrochloric acid particles react when they
meet and white smoke (ammonium chloride) forms. Diffusion is slow and the
reaction does not occur for several minutes as both collide with air particles
in the tube.
As the ammonia travels farthest along the tube we know that:


The ammonia particles move faster than hydrochloric acid particles.
The ammonia particles are lighter than the hydrochloric acid particles.
ATOMIC THEORY
Atomic theory assumes that all elements are made up of "atoms". If you
were to divide a lump of an element into smaller and smaller pieces you would
eventually come to a piece that could not be divided any further - a single
ATOM of the element. Atoms are therefore very small.
Definition: An atom is the smallest particle of an element that can exist on
its own or take part in a chemical change.
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Molecules
All elements are made up of atoms. In some gaseous elements (e.g. argon)
single atoms move around freely. But in other gaseous elements, single atoms
cannot exist on their own at ordinary temperatures: in these elements the
free-moving particles consist of pairs of atoms.
The two atoms forming a pair (a MOLECULE) are joined together by a
chemical "bond". This is the case with hydrogen (H2), oxygen (O2) and
nitrogen (N2). Such substances are said to be diatomic.
Some substances consist of ions joined together but we will consider these
in detail later.
Solids, Liquids and Gases
Solids
Liquids
•Have a fixed shape
•Take the shape of
•Take the shape of
•Can’t be compressed
their container
their container
•Particles close
•Can’t be compressed
•Can be compressed
together in a regular
•Particles close
•Particles widely
pattern called a
together but a little
spaced in random
lattice
disordered
order
•Particles vibrate
•Particles move
•Particles moving
around a fixed point
freely
very fast.
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Gases
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Changing states
Most substances can exist in all three states, but not all at once! The state of a
substance depends on the temperature, so changes of state happen by heating
and cooling. The changes between the different states are summarised below.
.
The graph above shows what happens to the temperature of a substance as it
changes state.
1) Starting from the solid state, the temperature increases steadily
2) When the temperature reaches the melting point for that particular
substance the particles in the solid start separate and a liquid is formed. The
temperature does not change because all the energy being supplied is being used
to separate the particles from each other. This is the melting point and is shown
by the first flat section of the graph.
3) Once the entire solid has turned into a liquid then the energy being supplied
increases the speed of the particles. Now the graph shows the temperature of
the liquid rising steadily.
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4) The temperature of the liquid continues to rise until it reaches the boiling
point for that particular substance. Here, the particles in the liquid begin to
separate and a gas if formed. The temperature does not change during boiling
because the energy being supplied is used to separate the particles in the liquid.
This is the boiling point and is shown by the second flat section of the graph.
5) Once all the liquid has turned into a gas then the energy being supplied makes
the particles in the gas move with increasing speed. Now the graph shows the
gas becoming steadily hotter.
These processes are all easily reversible simply by cooling down.
In a pure substance changes of state happen at fixed temperatures (except by
evaporation). The BOILING POINT of a liquid is the temperature at which it
quickly changes into a gas. The MELTING POINT of a solid is the temperature
it changes into a liquid.
The freezing point of a substance is the same as its melting point. In other
words, the temperature at which a solid changes into a liquid (melting) is the
same at which the liquid changes back into a solid (freezing). For example, solid
ice melts into liquid water at 0 °C, and liquid water freezes into ice at 0 °C.
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Other substances melt and boil at different temperatures depending on the
strength of the forces of attraction which must be overcome. Oxygen melts at
-219°C and boils at -183°C. This tells us that not much heat energy needs to be
applied to overcome the forces of attraction between the molecules. However
diamond (a form of carbon) melts at 3550°C and boils at 4832°C meaning a huge
amount of energy is needed.
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Topic 1:States of matter
Summary questions
1
2
3
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4
5
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6
7
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