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1.1 Everything is made of particles

Made of particles
Rock, air, and water look very different. But they have one big thing in
common: they are all made of very tiny pieces, far too small to see.
For the moment, we will call these pieces particles.
In fact everything around you is made of particles – and so are you!

Particles on the move

In rock and other solids, the particles are not free to move around. But in
liquids and gases, they move freely. As they move they collide with each
other, and bounce off in all directions.
So the path of one particle, in a liquid or gas, could look like this:

from
here to
here

  All made of particles!

The particle moves in a random way, changing direction every time it hits
another particle. We call this random motion.

Some evidence for particles

There is evidence all around you that things are made of particles, and
that they move around in liquids and gases. Look at these examples.

Evidence outside the lab

1  Cooking smells can spread out into the street. This 2  You often see dust and smoke dancing in the air, in
is because ‘smells’ are caused by gas particles mixing bright sunlight. The dust and smoke are clusters of
with, and moving through, the air. They dissolve in particles. They dance around because they are being
moisture in the lining of your nose. bombarded by tiny particles in the air.

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Evidence in the lab

air air
particle particle
bromine particles
bromine particles
and air particles
and air particles
water water now fully mixednow fully mixed
particle particle
particles from particles from
the crystal mixthe crystal mix
among the among the
bromine bromine
water particleswater particles
particle particle
the crystal the crystal

1  Place a crystal of potassium manganate(VII) in a 2  Place an open gas jar of air upside down on an open
beaker of water. The colour spreads through the water. gas jar containing a few drops of red-brown bromine.
Why? First, particles leave the crystal – it dissolves. The colour spreads upwards because particles of
Then they mix among the water particles. bromine vapour mix among the particles of air.

Diffusion
In all those examples, particles mix by colliding with each other and
bouncing off in all directions. This mixing process is called diffusion.
The overall result is the flow of particles from where they are more
concentrated to where they are less concentrated, until they are evenly
spread out.

So what are these particles?

The very smallest particles, that we cannot break down further by
chemical means, are called atoms.
 
In some substances, the particles are just single atoms. For example
argon, a gas found in air, is made up of single argon atoms.
 
In many substances, the particles consist of two or more atoms joined
together. These particles are called molecules. Water, bromine, and the
gases nitrogen and oxygen in air, are made up of molecules.
 
In other substances the particles consist of atoms or groups of atoms
that carry a charge. These particles are called ions. Potassium
manganate(VII) is made of ions.
You’ll find out more about all these particles in Chapters 2 and 3.

‘Seeing’ particles   This image was taken using a

We are now able to ‘see’ the particles in some solids, using very powerful tunneling electron microscope.
microscopes. For example the image on the right shows palladium atoms The white blobs are palladium atoms,
sitting on carbon atoms. In this image, the atoms appear over 70 million the blue ones are carbon. (The colour
times larger than they really are! was added to help us see them.)

Q
1 The particles in liquids and gases show random motion. 3 Bromine vapour is heavier than air. Even so, it spreads
What does that mean, and why does it occur? upwards in the experiment above. Why?
2 Why does the purple colour spread when a crystal of 4 a What is diffusion?  b  Use the idea of diffusion to
potassium manganate(VII) is placed in water? explain how the smell of perfume travels.

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1.2 Solids, liquids, and gases

What’s the difference?
It is easy to tell the difference between a solid, a liquid and a gas:

A solid has a fixed shape and a fixed A liquid flows easily. It has a fixed A gas does not have a fixed volume
volume. It does not flow. Think of volume, but its shape changes. It or shape. It spreads out to fill its
all the solid things around you: their takes the shape of the container container. It is much lighter than
shapes and volumes do not change. you pour it into. the same volume of solid or liquid.

Water: solid, liquid and gas

Water can be a solid (ice), a liquid (water), and a gas (water vapour or
steam). Its state can be changed by heating or cooling:
thermometer
shows 100 °C
thermometer water vapour
shows 0 °C water vapour (invisible)
steam
(visible)
water
boiling water

ice cubes melting

heat heat

1  Ice slowly changes to water, 2  When the water is heated its 3  Soon bubbles appear in the
when it is put in a warm place. temperature rises, and some of it water. It is boiling. The water
This change is called melting. changes to water vapour. This vapour shows up as steam.
The thermometer shows 0 °C until change is called evaporation. The thermometer stays at 100 °C
all the ice has melted. So 0 °C is The hotter the water gets, the while the water boils off. 100 °C is
called its melting point. more quickly it evaporates. the boiling point of water.

And when steam is cooled, the opposite changes take place:

cool below 100 °C cool below 0 °C freezes or solidifies

steam condenses to form water
to form ice

You can see that:

 condensing is the opposite of evaporating
 freezing is the opposite of melting
 
the freezing point of water is the same as the melting point of ice, 0 °C.

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Other things can change state too

It’s not just water! Nearly all substances can exist as solid, liquid and gas.
Even iron and diamond can melt and boil! Some melting and boiling
points are given below. Look how different they are.

Substance Melting point / °C Boiling point / °C

oxygen –219 –183
ethanol –15 78
sodium 98 890
sulfur 119 445
iron 1540 2900
diamond 3550 4832

Showing changes of state on a graph

Look at this graph. It shows how the temperature changes as a block of   Molten iron being poured out at an
ice is steadily heated. First the ice melts to water. Then the water gets iron works. Hot – over 1540 °C!
warmer and warmer, and eventually turns to steam:

Heating curve for water

150 water
vapour
125
water boiling getting
hotter
100
Temperature (°C)

75

50 ice water warming up

melting (some evaporation occurs)
25

0
ice warming up
25
0 1 2 3 4 5 6 7 8 9 10
Time (minutes)

A graph like this is called a heating curve.

Look at the step where the ice is melting. Once melting starts, the
temperature stays at 0 °C until all the ice has melted. When the water
starts to boil, the temperature stays at 100 °C until all the water has turned
to steam. So the melting and boiling points are clear and sharp.   Evaporation in the sunshine …

Q
1 Write down two properties of a solid, two of a liquid, and 5 Look at the heating curve above.
two of a gas. a A bout how long did it take for the ice to melt, once
2 Which word means the opposite of: melting started?
a boiling?     b melting? b How long did boiling take to complete, once it started?
3 Which has a lower freezing point, oxygen or ethanol? c Try to think of a reason for the difference in a and b.
4 Which has a higher boiling point, oxygen or ethanol? 6 See if you can sketch a heating curve for sodium.

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1.3 The particles in solids, liquids, and gases

How the particles are arranged
Water can change from solid to liquid to gas. Its particles do not change.
They are the same in each state. But their arrangement changes.
The same is true for all substances.

State How the particles are arranged Diagram of particles

Solid
The particles in a solid are arranged
in a fixed pattern or lattice.
Strong forces hold them together.
So they cannot leave their positions.
The only movements they make are
tiny vibrations to and fro.
Liquid

The particles in a liquid can move

about and slide past each other.
They are still close together, but not
in a lattice. The forces that hold them
together are weaker than in a solid.

Gas
The particles in a gas are far apart,
and they move about very quickly.
There are almost no forces holding
them ­together. They collide with each
other and bounce off in all directions.

Changing state
Melting  When a solid is heated, its particles get more energy and vibrate
more. This makes the solid expand. At the melting point, the p­ articles vibrate
so much that they break away from their positions. The solid turns liquid.

heat heat energy at

energy melting point

solid the vibrations get larger a liquid is formed

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Boiling  When a liquid is heated, its particles get more energy and move
faster. They bump into each other more often, and bounce further apart. This
makes the liquid expand. At the boiling point, the particles get enough energy
to overcome the forces between them. They break away to form a gas:

heat heat energy at

energy boiling point

the particles get enough

slow-moving particles the particles energy to escape
in liquid move faster

Evaporating  Some particles in a liquid have more energy than others.

Even well below the boiling point, some have enough energy to escape The kinetic particle theory !
and form a gas. This is called evaporation. It is why ­puddles of rain dry Look at the key ideas you have met:
up in the sun.
 A substance can be a solid, a
liquid, or a gas, and change from
How much heat is needed? one state to another.
The amount of heat needed to melt or boil a substance is different for  It has different characteristics in
every substance. That’s because the particles in each substance are each state. (For example, solids
different, with different forces between them. do not flow.)
The stronger the forces, the more heat energy is needed to overcome  The differences are due to the
them. So the higher the melting and boiling points will be. way its particles are arranged,
and move, in each state.
Reversing the changes Together, these ideas make up the
You can reverse those changes again by cooling. As a gas cools, its kinetic particle theory.
particles lose energy and move more slowly. When they collide, they do (Kinetic means about motion.)
not have enough energy to bounce away. So they stay close, and form a
liquid. On further cooling, the liquid turns to a solid.
Look at this diagram for water:

on heating, the particles gain energy

melts as it warms up, some evaporates;

ice (solid) water (liquid) steam (gas)
at 0 °C the rest boils at 100 °C

freezes (solidifies) as you cool it below 100 °C, the water

ice water steam
at 0 °C vapour begins to condense or liquify

on cooling, the particles lose energy and move more slowly;

as they get closer together the forces of attraction take over

Q
1 Using the idea of particles, explain why: 3 Oxygen is the gas we breathe in. It can be separated from
a you can pour liquids  b  solids expand on heating the air. It boils at –219 8C and freezes at –183 8C.
2 Draw a diagram to show what happens to the particles, a In which state is oxygen, at:  i  0 8C?  ii  –200 8C?
when a liquid cools to a solid. b How would you turn oxygen gas into solid oxygen?

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1.4 A closer look at gases

What is gas pressure?
When you blow up a balloon, you fill it with air particles. They collide
with each other. They also hit the sides of the balloon, and exert pressure
on it. This pressure keeps the balloon inflated.
In the same way, all gases exert a pressure. The pressure depends on the
temperature of the gas and the volume it takes up, as you’ll see below.

When you heat a gas

  The harder you blow, the greater the

pressure inside the balloon.

The particles in this gas are moving . . . the particles take in heat energy
fast. They hit the walls of the and move even faster. They hit the
container and exert pressure on walls more often, and with more
them. If you now heat the gas . . . force. So the gas pressure increases.

The same happens with all gases:   In a pressure cooker, water vapour
When you heat a gas in a closed container, its pressure increases. (gas) is heated to well over 100 °C. So it
That is why the pressure gets very high inside a pressure cooker. is at high pressure. You must let a
pressure cooker cool before you open it!
When you squeeze a gas into a smaller space
plunger
plunger
pushed
pushed
in in

gas gas
particles
particles gas gas compressed
compressed
intointo a smaller
a smaller
volume
volume

There is a lot of space between the … like this. Now the particles are
particles in a gas. You can compress in a smaller space – so they hit the
the gas, or force its particles closer, walls more often. So the gas
by pushing in the plunger … pressure increases.

The same thing is true for all gases:

When a gas is compressed into a smaller space, its pressure increases.
All gases can be compressed. If enough force is applied, the particles can
be pushed so close that the gas turns into a liquid. But liquids and solids
cannot be compressed, because their particles are already very close   When you blow up a bicycle tyre,
together. you compress air into the inner tube.

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The rate of diffusion of gases

On page 7 you saw that gases diffuse because the particles collide with
other particles, and bounce off in all directions. But gases do not all
diffuse at the same rate, every time. It depends on these two factors:
1 The mass of the particles 
The particles in hydrogen chloride gas are twice as heavy as those in
ammonia gas. So which gas do you think will diffuse faster? Let’s see:
 
Cotton wool soaked in ammonia solution is put into one end of a long
tube (at A below). It gives off ammonia gas.
 
At the same time, cotton wool soaked in hydrochloric acid is put into
the other end of the tube (at B). It gives off hydrogen chloride gas.
 
The gases diffuse along the tube. White smoke forms where they meet:
A B
  The scent of flowers travels faster in
a warm room. Can you explain why?

cotton wool soaked glass white smoke cotton wool soaked

in ammonia solution tube forms here in hydrochloric acid

The white smoke forms closer to B. So the ammonia particles have

travelled further than the hydrogen chloride particles – which means they
have travelled faster.
The lower the mass of its particles, the faster a gas will diffuse.
That makes sense when you think about it. When particles collide and
bounce away, the lighter particles will bounce further.
The particles in the two gases above are molecules. The mass of a
molecule is called its relative molecular mass. So we can also say:
The lower its relative molecular mass, the faster a gas will diffuse.

2 The temperature 
When a gas is heated, its particles take in heat energy, and move faster.
They collide with more energy, and bounce further away. So the gas   The faster a particle is moving when
diffuses faster. The higher the temperature, the faster a gas will it hits another, the faster and further it
diffuse. will bounce away. Just like snooker balls!

Q
1 What causes the pressure in a gas? 5 a Why does the scent of perfume spread?
2 Why does a balloon burst if you keep on blowing? b  Why does the scent of perfume wear off faster in warm
3 A gas is in a sealed container. How do you think the weather than in cold?
pressure will change if the container is cooled? 6 Of all gases, hydrogen diffuses fastest at any given
Explain your answer. temperature. What can you tell from this?
4 A gas flows from one container into a larger one. 7 Look at the glass tube above. Suppose it was warmed a little
What do you think will happen to its pressure? in an oven, before the experiment. Do you think that would
Draw diagrams to explain. change the result? If so, how?

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Checkup on Chapter 1
Revision checklist Questions
Core curriculum Core curriculum
Make sure you can … 1 A large crystal of potassium manganate(VII) was
 give two examples of evidence, from the lab, that placed in the bottom of a beaker of cold water, and
matter is made of particles left for several hours.
 explain what diffusion is, and how it happens
 name the three states of matter, and give their
physical properties (hard, fixed shape, and so on)
 describe, and sketch, the particle arrangement in cold water
each state
 describe how a substance changes state when you
heat it, and explain this using the idea of particles
 explain, and use, these terms:
crystal of potassium manganate(VII)
melt boil evaporate condense
melting point boiling point freezing point a Describe what would be seen:
 sketch, and label, a heating curve i after five minutes  ii  after several hours
 explain why a gas exerts a pressure b Explain your answers using the idea of particles.
 explain why the pressure increases when you: c Name the two processes that took place during
– heat a gas the experiment.
– push it into a smaller space
2 Use the idea of particles to explain why:
Extended curriculum a solids have a definite shape
Make sure you can also … b liquids fill the bottom of a container
 describe an experiment to show that a gas will c you can’t store gases in open containers
diffuse faster than another gas that has heavier d you can’t squeeze a sealed plastic syringe that is
particles completely full of water
 say how, and why, the temperature affects the rate e a balloon expands as you blow into it.
at which a gas diffuses
3 B
 elow is a heating curve for a pure substance. It
shows how the temperature rises over time, when
the substance is heated until it melts, then boils.

a What is the melting point of the substance?
b What happens to the temperature while the
substance changes state?
c The graph shows that the substance takes longer
to boil than to melt. Suggest a reason for this.
d How can you tell that the substance is not water?
f Sketch a rough heating curve for pure water.
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4 A cooling curve is the opposite of a heating curve. Extended curriculum

It shows how the temperature of a substance 7 You can measure the rate of diffusion of a gas
changes with time, as it is cooled from a gas to a using this apparatus. The gas enters through the
solid. Here is the cooling curve for one substance: thin tube:
H2 air
hydrogen plug of
gas (H2) in porous plaster
0

10

20
water rising
30
in tube

a W
 hat is the state of the substance at room water 40

temperature (20 °C)?
b Use the list of melting and boiling points on
page 9 to identify the substance. The measuring tube is sealed at the top with a plug
c Sketch a cooling curve for pure water. of porous plaster. Air and other gases can diffuse in
and out through the tiny holes in the plug.
5 Using the idea of particles explain why:
a the smell of burnt food travels through the house The water rises in the measuring tube if the chosen
b when two solids are placed on top of each other, gas diffuses out through the plug faster than air
they do not mix diffuses in. Air is mainly nitrogen and oxygen.
c pumping up your bike tyres gives a smooth ride a W  hen you use hydrogen gas, the water rises in
d smokers can cause lung damage in other people the measuring tube. Why?
e heating a gas in a closed container will increase b What does this tell you about the rate of diffusion
its pressure of hydrogen, compared to the gases in air?
f a liquid is used in a car’s breaking system, to c Explain your answer to b. Use the term mass!
transfer the pressure from the brake pedal d The molecules in carbon dioxide are heavier
g poisonous gases from a factory chimney can than those in nitrogen and oxygen.
affect a large area.  So what do you think will happen to the water
6 a Which of these are examples of diffusion? in the measuring tube, when you use carbon
i a helium-filled balloon rising in air dioxide? Explain your answer.
ii a hydrogen-filled balloon deflating, due to 8 Gas Formula Relative atomic or
gas passing through the skin molecular mass
iii the smell of perfume from a person methane CH4 16
standing on the other side of a room helium He 4
iv sucking a drink from a bottle, using a straw
oxygen O2 32
v an ice lolly turning liquid when it is left out
nitrogen N2 28
of the freezer
chlorine Cl2 71
vi the tea in the cup changing colour when
you add milk, without stirring Look at the table above.
vii a light, coloured gas, spreading down a Which two gases will mix fastest? Explain.
through a gas jar b Which gas will take least time to escape from a
viii a blue crystal forming a blue solution, when gas syringe?
it is left sitting in a glass of water c Would you expect chlorine to diffuse more
ix spraying paint from a spray can. slowly than the gases in air? Explain.
b For one of the examples of diffusion, draw a d An unknown gas diffuses faster than nitrogen,
diagram showing the particles before and after but more slowly than methane. What you can
diffusion has taken place. say about its relative molecular mass?

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