6 Matter as particles
6
Matter as particles
6. 1
Matter
1
Matter is anything which has mass and takes up space.
2
There are three states of matter: solid state, liquid state and gas state.
3
Shape
Volume
Solid
fixed
fixed
Liquid
not fixed, takes up the shape of the
container
fixed
Gas
not fixed, takes up the shape of the
closed container
not fixed, takes up the volume of the
closed container
The change of states of matter:
temperature increases
Boiling
Melting
Solid
Liquid
Freezing
Gas
Condensation
temperature decreases
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Melting, freezing and boiling take place at fixed temperatures. Take water as an example:

The melting point of ice is 0℃. Ice melts into water at 0℃.

The boiling point of water is 100℃. Water boils to steam at 100℃.

The freezing point of water is 0℃. Water freezes to become ice at 0℃.
6.2
The particle theory of matter
1
Pollen grains are hit randomly by water
particles from all directions, so the pollen
grains move about randomly in all
directions.
2
The motion of the pollen grains is called
water particle
pollen grain
Brownian motion.
3
The particle theory can be summarized as follows:
i
All matter is made up of particles.
ii
Different substances are made up of different particles.
iii
Particles are tiny.
iv
Particles are moving all the time.
v
There are spaces between particles.
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Evidence for the particle theory:
a
The drop of concentrated Ribena spreads out
in the water when it is added to the bottom
of a test tube.
b
The brown gas moves in a direction to
fill both jars.
air
brown
gas
cover
plate
water
Ribena
 supports points i & iv of the theory
 supports points i, ii & iv of the theory
c
The volume of the water and alcohol
mixture is smaller than the sum of the
volumes of the two liquids.
water
d
Add a few drops of perfume into a
balloon and tie its opening. We can still
smell the perfume.
particle of
rubber sheet
perfume
particle
+
alcohol
outside of balloon
 supports points i, ii, iii & v of the theory
inside of balloon
 supports all the points of the theory
5
Different substances are made up of different particles of different sizes.
6
Atoms are the smallest units of matter.
water particle
oxygen
atom
hydrogen
atom
gold atom
A water particle is made up of one oxygen
atom and two hydrogen atoms.
Gold is made up of gold atoms.
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6 Matter as particles
6.3
1
The particle model
The particle model can be used to illustrate the arrangement and motion of particles in solids,
liquids and gases.
Solid
Liquid
Gas
regular pattern
irregular pattern
irregular pattern
Motion of
particles
vibrate about a fixed
position
move around easily
move freely in all
directions
Spaces between
particles
small
small
large
Shape
fixed
not fixed
not fixed
Volume
fixed
fixed
not fixed
Arrangement
of particles
6 .4
Gas pressure
1
Hitting of gas particles on a surface exerts a gas pressure on the surface.
2
We can measure gas pressure by a Bourdon gauge. The unit of gas pressure is pascal (Pa).
3
Gases can be compressed because there are large spaces between gas particles.
4
When a gas in a closed container is compressed, gas particles hit the container walls more
frequently and thus the gas pressure increases.
5
Atmospheric pressure is the gas pressure exerted by the air particles in the atmosphere.
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Illustration of the power of the atmospheric pressure:
Magdeburg hemispheres
rubber tubing
(to vacuum pump)
The number of particles
inside the hemispheres
decreases.
The gas pressure inside the
hemispheres becomes lower
than the atmospheric
pressure.
The hemispheres are
difficult to be pulled apart.
The greater pressure outside
makes the two hemispheres
hold each other firmly.
Air is drawn out of the
hemispheres.
6 .5
Density of matter
1
The density of an object is its mass per unit volume. The unit of density is g/cm3 or kg/m3.
2
For objects of the same volume, the larger the mass, the higher the density.
 lead cube
1

cm3,
 plastic foam cube
1 cm3, 0.02 g
11.3 g
density of lead (11.3 g/cm3) ＞ density of plastic foam (0.02 g/cm3)
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mass
volume
3
Denisty 
4
If the density of an object is higher than that of a liquid, it sinks in the liquid. If the density of
an object is lower than that of a liquid, it floats in the liquid.
5
A steel ship can float in the sea because:
air
The density of air is much lower than
that of steel.

The space in the steel ship holds a
large volume of air.

The overall density of the steel ship is
lower than that of sea water.
6 .6
Thermal expansion and contraction
1
Most substances expand when heated and contract when cooled.
2
In general, gases expand more than liquids and liquids expand more than solids.
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3
Explaining thermal expansion and contraction in solids using the particle theory:
temperature rises
temperature drops
Thermal expansion
4
Thermal contraction
The temperature of a substance increases.
The temperature of a substance drops.


The particles gain energy and
move faster.
The particles lose energy and their
movement slows down.


The spaces between the particles
become larger.
The spaces between the particles
become smaller.


The substance expands.
The substance contracts.
How a hot air balloon rises:
The air inside the balloon is heated.

The spaces between the air particles become larger.

The volume of the air inside the balloon increases.

air is heated
The density of air inside the balloon becomes lower
than that outside the balloon.
air particle

The balloon rises.
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Thermal expansion and contraction of materials may cause materials to crack and break.

Solutions to problems caused by thermal expansion and contraction:
Example
6
Explanation
Gaps are left on
road surface
Roads may not crack easily when
the material for building the roads
expands or contracts
Power cables are
slack
The cables may not break easily
when temperature drops
Applications of thermal expansion and contraction in daily life:
a
Liquid-in-glass thermometers
(e.g. alcohol thermometer)
Alcohol thermometer 

When the temperature rises, the liquid inside the glass expands. The liquid level
goes up.

When the temperature drops, the liquid contracts. The liquid level falls.
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b
Thermostats

Some thermostats are installed with bimetallic strips.

The working principle of a bimetallic strip in controlling the temperature in an
electric iron:
 In the bimetallic strip, the
 When the bimetallic strip is heated
brass strip expands more than
the iron strip when heated.
above a certain temperature, the strip
bends downwards. The circuit breaks
and the electric iron is turned off.
brass
contact points
bimetallic strip
heater
iron
 When the electric iron cools
 The circuit becomes closed. The
down, the strip straightens.
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electric iron is turned on and is
heated up again.
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