6.1
Particle theory
1.
Matter is anything that has mass and takes up space.
2.
All matter is made up of particles. The idea of particles is summarized in the
particle theory. The main points of the particle theory are as follows:

All matter is made up of particles.

Particles are very small.

There are empty spaces between particles.

Particles are moving all the time.

Different matter is made up of different particles.

Different particles have different sizes and masses.
3.
Scientists have found evidence from experiments to support the particle theory.
4.
When 50 cm3 of water and 50 cm3 of alcohol are mixed, the total volume is less
than 100 cm3.
water
particle
alcohol
particle
+
alcohol
water
The evidence from the above experiment supports the following points of the
particle theory:
(1) All matter is made up of particles.
(2) There are empty spaces between particles.
(3) Different matter is made up of different particles.
(4) Different particles have different sizes and masses.
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5.
Atoms are the basic types of particles making up matter. For example, gold and
iron are made up of gold atoms and iron atoms respectively.
iron
gold
gold atom
6.
iron atom
A molecule consists of two or more atoms joined together. For example, a water
molecule contains two hydrogen atoms and one oxygen atom. A carbon dioxide
molecule contains one carbon atom and two oxygen atoms.
carbon atom
hydrogen atom oxygen atom
water molecule
water
E
oxygen atom
carbon dioxide
molecule
carbon dioxide (inside)
7.
Particles of different substances have different sizes and masses.
8.
A chemical formula shows the types and the numbers of the atoms that a
molecule consists of. The chemical formula of a water molecule is H2O.
oxygen atom
water
molecule
O
H
represented by the
chemical formula
symbol of hydrogen
symbol of oxygen
omit ‘1’ here as there is
only one oxygen atom
in one molecule
H
hydrogen atoms
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6.2
9.
Particle model for the three states of matter
Matter can exist in three states: solid, liquid or gas.
10. The table below shows the properties of solids, liquids and gases.
Have fixed shape?
Have fixed volume?
Can they be compressed?
Solids



Liquids



Gases



11. Scientists use the particle model to describe the arrangements and movements of
particles in the three states of matter.
The particle model
The particles
 are close together and arranged
regularly.
Solid
 cannot move from one position
to another, but can only vibrate
about fixed positions.
The particles
 are close together but arranged
irregularly.
Liquid
 can move around each other.
The particles
 are far apart.
 can move freely in all directions.
Gas
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6.3
Dissolving
12. When a solid dissolves in water, the particles of the solid separate and mix with
the water particles.
13. We can use the particle theory to describe the process of dissolving sugar in
water.
sugar
dissolving
sugar particle
water particle
dissolving
Sugar particles separate and
mix with the water particles.
Sugar particles and water particles
are moving. They soon become
evenly spread out.
14. When sugar dissolves in water, the volume of the sugar solution formed is
slightly less than the total volume of the sugar and water before dissolving.
sugar particle
water particle
Based on the particle theory, there are spaces between particles. When sugar
dissolves in water, the sugar particles separate. The smaller water particles fit
into the spaces between the bigger sugar particles. Therefore, the volume
changes in dissolving.
15. The total number of the sugar particles and water particles remains the same in
dissolving. Therefore, the mass of the sugar solution is equal to the total mass of
the sugar and the water before dissolving. Mass is conserved in dissolving.
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6.4
Thermal expansion and contraction
16. When the temperature is higher, the particles move faster. When the temperature
is lower, the particles move slower.
17. The diagram below shows the change in the spacing of the particles in a solid, a
liquid and a gas when temperature changes.
Solid
heating
cooling
Liquid
heating
cooling
Gas
heating
cooling

When a substance is heated, its particles move faster. Therefore, the
particles are further apart and the substance as a whole increases in size, i.e.
it expands on heating. This is called thermal expansion.

When a substance is cooled, its particles move slower. Therefore, the
particles are closer together and the substance as a whole decreases in size,
i.e. it contracts on cooling. This is called thermal contraction.
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18. The following are some daily applications of thermal expansion and contraction.
Thermometer
Thermometers contain a liquid such as coloured
alcohol. The liquid expands or contracts when
the temperature changes. The liquid level
changes and the temperature is shown.
liquid (coloured alcohol)
Fire sprinkler
A fire
breaks out
glass tube (filled
with a red liquid)
A fire sprinkler has a glass tube filled
Water flows out from an
with a liquid. When there is a fire, the
opening connected to water
heat from the fire causes the liquid to
pipes in the ceiling.
expand. This breaks the glass tube.
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E
6.5
Gas pressure
19. Gas pressure is produced when gas particles hit the walls of the container
holding the gas.
gas pressure
gas particle
The gas pressure is produced in the
basketball when the gas particles
inside constantly hit its inner walls.
20. Gas pressure changes with temperature.
21. When the temperature of a gas increases, the gas particles move faster. They hit
the container walls harder and more frequently. As a result, the gas pressure
increases.
bottle
gas pressure
temperature
increases
gas particle
22. When the temperature of a gas decreases, the gas particles move slower. They hit
the container walls less hard and less frequently. As a result, the gas pressure
decreases.
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23. When the volume of a gas decreases, the gas particles hit the container walls
more frequently. As a result, the gas pressure increases.
air particle
plunger
air particle
syringe
lower gas pressure
plunger
reduce the volume
of the gas
push
push
higher gas pressure
24. The table below shows how gas pressure changes with the temperature and the
volume of a gas.
Temperature increases
Volume decreases
How does the speed of the
particles of the gas change?
increases
remains unchanged
Do the particles hit the
container wall more frequently?
yes
yes
How does gas pressure change?
increases
increases
25. When objects on the Earth are hit by the air particles in the atmosphere, a
pressure is produced. This pressure is called the atmospheric pressure or air
pressure.
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6.6
Density
26. Density is the mass per unit volume of a substance. Its unit is g/cm3 or kg/m3.
density (g/cm3) =
mass (g)
volume (cm3)
27. Iron has a density of 7.9 g/cm3. That means 1 cm3 of iron has a mass of 7.9 g.
28. Iron has a higher density than aluminium so an iron block has a greater mass than
an aluminium block of the same volume.
aluminium (1 cm3)
iron (1 cm3)
29. The table below shows how to calculate the densities of some blocks.
Aluminium
block
1 cm 1 cm
1 cm
2 cm
Volume (cm3)
Density (g/cm3)
2.7
111
=1
2.7
= 2.7
1
63.2
222
=8
63.2
= 7.9
8
90.4
124
=8
90.4
= 11.3
8
2 cm
Iron block
2 cm
1 cm
Mass (g)
2 cm
Lead block
4 cm
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30. The following shows how to calculate the density of a stone with irregular shape.
measuring
cylinder
Mass of the stone = 30 g
Volume of the stone = (70 – 50) cm3 = 20 cm3
Density of the stone
=
stone
mass (g)
30 g
3
3 =
3 = 1.5 g/cm
volume (cm )
20 cm
31. An object floats in a liquid if its density is lower than that of the liquid. For
example, the density of ice (0.9 g/cm3) is lower than that of water (1 g/cm3).
Therefore, ice floats in water.
iceberg
sea
32. An object sinks in a liquid if its density is higher than that of the liquid.
33. The diagram below shows a density column containing four different
substances with different densities.
Substance
Density (g/cm3)
oil
0.9
sugar syrup
1.48
water
1.0
sugar syrup
iron block
7.9
iron block
oil
water
34. When a substance of higher density (e.g. steel) is made into a shape containing a
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large volume of air, its density is lowered and it can float in water.
ship body
Much of the space inside a ship
body is air so that its overall
density is lower than that of water.
E 35. The table below summarizes the effect of temperature change on the density of a
substance.
Temperature increases
Temperature decreases
Do the particles move faster or
slower?
faster
slower
Do the particles move closer
together or further apart?
further apart
closer together
Does the volume of the
substance increase or decrease?
increases
decreases
decreases
increases
Does
the
density
of
the
substance increase or decrease?
E 36. A hot air balloon rises because the density of the air inside decreases when the
air is heated.
air particle
the air inside
is heated
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