M Density and the
Particle Theory
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Figure 5.1 There is very
little space between the
people on a crowded
elevator — they are
densely packed together.
Have you ever been on a crowded elevator? As
Figure 5.1 shows, i t is definitely uncomfortable
when too many people are jammed together tightly, or densely, on an elevator.
Using everyday words, density can be described
as the "crowdedness" of the particles that make up
matter. I n scientific terms, density is the amount of
a substance that occupies a particular space. When
you describe a substance as being "heavy" or
"light," you are referring to the property o f density.
According to the particle theory, different substances have different-sized particles. The size of
the particles determines how many particles can
"fit into" a given space. Therefore, each substance
has its own unique density, based on particle size.
Furthermore, the particle theory suggests that
there is empty space between the particles of matter. Could as many people fit onto an elevator i f
each person were surrounded by a large "spacing
box"? Would larger spaces among the people
increase or reduce the density (crowdedness) of
the people travelling on the elevator? The answer is
shown in Figure 5.2.
Density of Solids, Liquids, and Gases
How is the density of a substance related to the substance's physical state? Imagine
filling a film container with liquid water and another film container with water vapour.
Both liquid water and water vapour are the same substance and therefore have particles of the same size. According to the particle theory, gas particles have more space
between them than do liquid particles. Therefore, the water vapour in the container
would have fewer particles than the liquid water. I t would be reasonable to conclude
that the density of the water vapour is less than the density of liquid water.
PauseSc
Reflect
For tips on how to
make and use a
Science Log, turn to
page 534.
132
Fantastic Fluids
Imagine you were to fill three balloons to the same size: one with plaster, which would harden
solid; the second with water, a liquid; and the third with air, a mixture of gases. In your Science
Log, draw these three balloons. Draw O's or X's to represent the particles and to show how the
particles are spread out inside each balloon. Use the particle theory to try to explain how and why
the density of the substances inside each balloon varies.
DidYouKnow?
O
O
0
Figure 5.2
Increasing the
spaces between
people on an
elevator will
reduce the density.
Could the solid state be
less dense than the liquid
state of the same substance? The answer is
yes. You have probably
seen ice cubes floating in
a glass of water. As solid
ice forms from liquid
water, the ice expands.
This means that there is
more empty space
trapped in a chunk ot ice
than in the same amount
of water Thus, solid ice
is less dense than liquid
water.
How are density and state of matter related to the physical properties of a substance? Solid objects can move easily through liquids and gases. For example, dolphins can leap through the air and then dive back underwater so smoothly that the
activity appears almost effortless, as shown in Figure 5.3. According to the particle
theory, the fluid properties of water and of air allow water particles and air particles to move out ot the way of the firmer, non-fluid bodies of marine animals. V\Tiy
do solid particles tend to hold together while fluid particles tend to move apart?
Figure 5.3 The particles of a fluid move apart easily when a solid object, such as a dolphin, travels
through the fluid.
Density and Buoyancy
133
W h e n an object moves through a fluid, i t pushes
particles apart and moves between them. Particles i n
a solid cannot be pushed apart. To understand why,
imagine that you and a few friends are together. You
want to prevent anyone else from pushing your group
apart and moving between you. What would you do?
First, you would have to stand quite close together.
Then you would probably hold on to each other very
tightiy. I f you do not let go of one another, no one can
move between you. That is what particles i n a solid do.
Attractive forces among the particles of a solid are
stronger than those between fluid particles and thus th(
particles in a solid cannot be pushed apart.
Figure 5.4 The cartoonized weight-lifters represent the tightly
packed particles in the solid plate. The plate can hold its shape
and support materials such as the large rock, because the
weight-lifters are holding on to each other very tightly.
Find Our
Differing Densities
Using your knowledge of the particle theory, what
inferences can you make about the densities of
2. With a partner, pick up each film container
and observe how heavy or how light each
one feels. Decide on the order of the film
different substances?
containers from heaviest to lightest. In your
What You Need S
I
table, record the numbers of the film contain-
3-5 c l a s s sets of numbered film containers (pre-
ers, from heaviest to lightest.
pared by your teacher), completely filled with var3. Predict what the substance inside each con-
ious "mystery substances"
.'^.^tainer might be. Record your predictions.
4. Find out what the substances are by check^
^
^
^
•
'
^
^
^
^
^
^
^
^
ing your teacher's master list. Record the
^
actual substances in the table.
What Did You Find O u t ?
1. Which substances did you predict correctly
(or closely)? Which substances, if any, surprised you?
2. The volumes of the substances were identical
What to Do
1. Make a table with the following headings:
because each container was filled to the top.
Why is it important to keep the volumes
Container
number
Rank order (heaviest
to lightest)
Substance
(your guess)
equal in this activity?
(actual)
3. Make a general statement about the substances, in terms of the density of the
particles.
134
Fantastic Fluids
DidYnuKnow?
"Empty space" does not
mean "air" Empty space
means a separation
between two objects with
nothing between those
objects, not even air particles. Simply because empty
space and air /oo/(the same
does not mean they are the
same! For example, outer
space is mostly empty
space, but it has no air.
Astronauts would suffocate
if they were to venture outside their spacecraft without masks and air tanks,
because there is no oxygen
in outer space.
Figure 5.5 Although liquid particles are sometimes
quite closely packed together, they cannot support
objects in the same way that solids can, because the
particles do not have a strong enough attraction for
each other Thus, liquid particles move apart easily.
I f you were to step onto the surface of a lake, the water
would not support your foot. Instead, your foot would
go right through the water. I n fact, you would
continue to fall through the water, pushing the water
particles out of the way, as shown in Figure 5.5.
Liquids cannot support objects in the same way that
sohds can, because the particles of a liquid move apart
easily, allowing a dense, solid object, such as your foot,
to pass through the liquid. The attractive forces
between liquid particles are not strong enough to prevent your foot from pushing them apart.
Similarly, you cannot walk on air, because gases are
even less dense than solids or liquids. When you move
through air, you are moving through mostly empty
space. You do not have to move as many particles of air
out of the way as you do in*water (see Figure 5.6). This
explains why running through air is much easier and
faster than running through water. In general, gases are
less dense than liquids.
Figure 5.6 When you move through air, you do
not have to move as many particles of air out
of the way as you do water particles in water
Density and Buoyancy
135
As temperature increases, a substance will change from solid, to liquid, to gas.
The particle theory states that the particles of a substance spread out as they gain
energy when heated. Thus, the particles take up more space, which means that the
density of the substance decreases. I t is almost always true that, for each pure substance (for example, gold), the density of its solid state is greater than the density
of its liquid state. The substance's sohd state and Hquid state are, i n tum, denser
than its gaseous state.
In some cases, the densities of two substances can be so different that the liquid
state of one is denser than the soUd state of the other! One example of this is
shown i n Figure 5.7A Many solid metals, such as copper, nickel, and silver, can
float on mercury, one of the densest substances known. A more familiar example
of differing densities is shown iti Figure 5.7B.
Figure 5.7A Liquid mercury is so dense tliat it can
support a solid iron bolt. A layer of oil has been
placed on top of the mercury to prevent vapour
from escaping into the surrounding air.
Figure 5.7B A solid block of wood floats easily
on the surface of liquid water.
Check Your Understanding
1. I n your own words, explain what density means.
2. Explain why solids can support objects more easily than fluids can.
3. What is the only way i n which the density of a pure substance can change?
4. Apply Find some small items in the classroom (for example, pencils or paper
clips) and determine whether these items are denser than water by dropping
them into a container fiiU of water. Organize your observations i n a table.
5. Apply Butter is made by intentionally changing the density of liquid milk.
Find out how this is done and write a brief description. Explain the change i n
terms of the particle theory.
6. T h i n k i n g Critiically The density of molten lava increases as i t cools and
hardens. List other examples of natural changes in density.
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Fantastic Fluids