The States of Matter and the Particle Theory
The essentials of life — food, water, and air — are examples of substances that
occur i n the three different states of matter: solid, liquid, and gas. As you learned
in earlier science smdies:
• Solid is the state of matter of a substance that has a definite shape and volume
(for example, a sugar cube).
• Liquid is the state of matter of a substance that has a definite volume, but no
definite shape (for example, water).
• Gas is the state of matter of a substance that has neither a definite shape nor a
definite volume (for example, oxygen).
By using the particle theory, you can explain why liquids and gases flow but solids
do not.
Gas
According to the particle theon/,
the particles of a gas will
completely fill a container. If you
removed the stopper in this flask,
what would happen to the gas?
Liquid
This liquid is held in a
container. You have probably
noticed that liquids take the
shape of the container in which
they are placed.
Solid
This solid cube of sugar does
not need a container to keep
its shape.
The five major points of the particle theory of matter are:
1 . A l l matter is made up of very small particles.
2. A l l particles in a pure substance are the same. Different substances are made
of different particles.
3. There is space between the particles.
4. The particles are always moving. As the particles gain energy, they move faster.
5. The particles in a substance are attracted to one another. The strength of the
attractive force depends on the type of particle.
Viscosity: Tliiclc or Thin?
111
Changes of State
As you may recall from earlier smdies, die process of melting is an example of a
change of state, which occurs when the physical state of a substance is transformed
into another state. Figure 4.3 shows changes of state. The change from solid to
liquid is called melting, and the change from liquid to gas is called vaporization.
These changes of state occur when the substance is heated and the particles of the
substance gain energy. I f you were to cool the substance, the reverse changes of
state would occur because the particles lose energy. The change from gas to liquid
is called condensation, and the change from liquid to solid is called freezing.
An unusual change of state occurs when a solid turns into its gaseous state
without going through the liquid state. This change of state is known as
sublimation. An example of sublimation occurs when dry ice is used at a rock
concert — a chunk of frozen carbon dioxide (a solid) gains energy and gives off a
thick cloud of fog (carbon dioxide gas). Figure 4.4 shows this change of state. The
change from a gas directiy to a solid is called sublimation as well. An example of
this occurs when frost forms on windows on bitterly cold days (water vapour in the
air loses energy rapidly and forms snowy ice).
Pause&
Reflect
How do raindrops form?
If clouds contain water
vapour and remain suspended in ttie sky, try to
explain what you tfiink
happens inside the clouds
to form rain. Using the
word "condensation,"
write an explanation in
your Science Log. You
may draw a sketch to
illustrate your answer.
mmCOMMECT
What if you wanted to
describe the change of
Figure 4.3
state from liquid to solid
Clianges of state
for a metal, such as copper, or a non-metal, such
as wax? Neither one of
these substances can be
said to "freeze" back into
a solid. What verb would
you use to describe this
change of state? Look for
a clue in Figure 4.3.
nnelting (also called fusion)
SOLID
freezing (also called solidification)
Figure 4.4 Solid carbon dioxide is called dry
ice because it does not become a liquid before
it becomes a gas. Dry ice is much colder than
regular ice, so a fog of condensed water vapour
forms as air surrounding it cools. This result is
often used for special effects in television, film,
and theatre.
PidYnuKnow?
What is the difference between a gas and a vapour? A substance is called
a gas if it exists as a gas at room temperature (for example, oxygen
gas or carbon dioxide gas). The same substance is called a vapour
if it normally exists as a solid or a liquid at room temperature
(for example, water vapour or perfume vapour).
115
Did you know that science fiction script writers often feature
changes of state in
imaginary ways?
"Morphing" has
become one of the
most popular special
effects developed for
science fiction movies.
By means of computergenerated graphics,
characters on screen
appear to metamorphose, or morph, into
someone or something
else. In some movies,
for example, solid
beings appear to morph
into a liquid that can
creep under doors, or
slip through cracks, and
then quickly resolidify.
Morphing has also
become a popular
feature of music videos,
television programs,
Evaporation is slow vaporization. I t occurs over a wide range
of temperatures. A wet towel will
dry even i f the air temperamre is
not high. On a cool day it will
simply take longer for the water
to evaporate from the towel.
Boiling is rapid vaporization.
It occurs at a specific temperamre, called the boiling point.
The boiling point of water is
100°C (at sea level). Similarly,
every substance has its own
freezing point and melting
point. The freezing point of
water, for example, is 0°C (at sea
level). This is the temperamre at
which liquid water freezes. I t is
also the temperamre at which ice
melts — its melting point. Figure
4.5 illustrates the melting point
of two other substances: paraffin
Figure 4.5 Every substance has its own melting point.
(wax) and silver. The picmres show how the properties of a substance, such as the
melting point, can have valuable uses. When normally sofid substances are melted,
the liquid can be poured into moulds of various shapes. When the substance is
cooled, i t solidifies and takes the shape of the mould. The result can be a wax candle, a silver teapot, and many other products. The processes picmred here make
good use of flowing Hquids and viscosity.
and commercials.
Check Your Understanding
1 . In your own words, what does "to flow" mean?
2 . Classify the following items as fluids or as non-fluids.
shampoo
pencil
honey
thumbtacks
thread
sap
paper
nail polish
blood
air
molten lava
smoke
sugar
namral gas
ash
gravel
hairspray
balloon
perfume
snow
3 . How could you test whether or not a substance is a fluid?
COMMECT
In your Science Log,
4 . A substance has a definite volume but an indefinite shape. Is the substance a
sohd, a liquid, or a gas?
5 . Apply Use the particle theory to explain why ice cubes form in your freezer.
write a short story that
involves "morphing."
Include an illustration of
the morphing process
that occurs in your story.
116
Fantastic Fluids
6 . Thinking Critically Describe a place or a simation in which you could find
water as a solid, a liquid, and a gas all at the same time.
7 . Thinking Critically Which substances could you use to demonstrate "morphing"? Are these substances fluids? For each substance, explain your answer.